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L_0050 | telescopes | T_0495 | Humans have been studying the night sky for thousands of years. Knowing the motions of stars helped people keep track of seasons. With this information they could know when to plant crops. Stars were so important that the patterns they made in the sky were named. These patterns are called constellations. Even now, constellations help astronomers know where they are looking in the night sky. The ancient Greeks carefully observed the locations of stars in the sky. They noticed that some of the stars moved across the background of other stars. They called these bright spots in the sky planets. The word in Greek means wanderers. Today we know that the planets are not stars. They are objects in the solar system that orbit the Sun. Ancient astronomers made all of their observations without the aid of a telescope. | text | null |
L_0050 | telescopes | T_0496 | In 1610, Galileo looked at the night sky through the first telescope. This tool allowed him to make the following discoveries (among others): There are more stars in the night sky than the unaided eye can see. The band of light called the Milky Way consists of many stars. The Moon has craters (see Figure 23.10). Venus has phases like the Moon. Jupiter has moons orbiting around it. There are dark spots that move across the surface of the Sun. Galileos observations made people think differently about the universe. They made them think about the solar system and Earths place in it. Until that time, people believed that the Sun and planets revolved around Earth. One hundred years before Galileo, Copernicus had said that the Earth and the other planets revolved around the Sun. No one would believe him. But Galileos observations through his telescope proved that Copernicus was right. | text | null |
L_0050 | telescopes | T_0497 | Galileos telescope got people to think about the solar system in the right way. Modern tools have also transformed our way of thinking about the universe. Imagine this: Today you can see all of the things Galileo saw using a good pair of binoculars. You can see sunspots if you have special filters on the lenses. (Never look directly at the Sun without using the proper filters!) With the most basic telescope, you can see polar caps on Mars, the rings of Saturn, and bands in the atmosphere of Jupiter. You can see many times more stars with a telescope than without a telescope. Still, stars seen in a telescope look like single points of light. They are so far away. Only the red supergiant star Betelgeuse is large enough to appear as a disk. Except for our Sun, of course. Today, astronomers attach special instruments to telescopes. This allows them to collect a wide variety of data. The data is fed into computers so that it can be studied. An astronomer may take weeks to analyze all of the data collected from just a single night! | text | null |
L_0050 | telescopes | T_0498 | A spectrometer is a special tool that astronomers commonly use. Spectrometers allow them to study the light from a star or galaxy. A spectrometer produces a spectrum, like the one shown in Figure 23.11. A prism breaks light into all its colors. Gases from the outer atmosphere of a star absorb light. This forms dark lines in the spectrum. These dark lines reveal what elements the star contains. Astronomers use the spectrum to learn even more about the star. One thing they learn is how hot the star is. They also learn the direction the star is going and how fast. By carefully studying light from many stars, astronomers know how stars evolve. They have learned about the distribution and kinds of matter found throughout the universe. They even know something about how the universe might have formed. To find out what you can expect to see when looking through a telescope, check out | text | null |
L_0051 | early space exploration | T_0499 | Humans did not reach space until the second half of the 20th century. They needed somehow to break past Earths gravity. A rocket moves rapidly in one direction. The device is propelled by particles flying out of it at high speed in the other direction. There are records of the Chinese using rockets in war against the Mongols as early as the 13th century. The Mongols then used rockets to attack Eastern Europe. Early rockets were also used to launch fireworks. | text | null |
L_0051 | early space exploration | T_0500 | Rockets were used for centuries before anyone could explain how they worked. The theory came about in 1687. Isaac Newton (16431727) described three basic laws of motion, now referred to as Newtons Laws of Motion: 1. An object in motion will remain in motion unless acted upon by a force. 2. Force equals mass multiplied by acceleration. 3. To every action, there is an equal and opposite reaction. Which of these three best explains how a rocket works? Newtons third law of motion. When a rockets propulsion pushes in one direction, the rocket moves in the opposite direction, as seen in the Figure 23.12. For a long time, many people believed that a rocket wouldnt work in space. There would be nothing for the rocket to push against. But they do work! Fuel is ignited in a chamber. The gases in the chamber explode. The explosion creates pressure that forces the gases out of one side of the rocket. The rocket moves in the opposite direction, as shown in Figure 23.13. The force pushing the rocket is called thrust. | text | null |
L_0051 | early space exploration | T_0501 | For centuries, rockets were powered by gunpowder or other solid fuels. These rockets could travel only short distances. Around the turn of the 20th century, several breakthroughs took place. These breakthroughs led to rockets that could travel beyond Earth. Liquid fuel gave rockets enough power to escape Earths gravity (Figure 23.14). By using multiple stages, empty fuel containers could drop away. This reduced the mass of the rocket so that it could fly higher. Rockets were used during World War II. The V2 was the first human-made object to travel high enough to be considered in space (Figure 23.15). Its altitude was 176 km (109 miles) above Earths surface. Wernher von Braun was a German rocket scientist. After he fled Germany in WWII, he helped the United States develop missile weapons. After the war, von Braun worked for NASA. He designed the Saturn V rocket (Figure | text | null |
L_0051 | early space exploration | T_0502 | One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. Natural objects in orbit are called natural satellites. The Moon is a natural satellite. Human-made objects in orbit are called artificial satellites. There are more and more artificial satellites orbiting Earth all the time. They all get into space using some sort of rocket. | text | null |
L_0051 | early space exploration | T_0502 | One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. Natural objects in orbit are called natural satellites. The Moon is a natural satellite. Human-made objects in orbit are called artificial satellites. There are more and more artificial satellites orbiting Earth all the time. They all get into space using some sort of rocket. | text | null |
L_0051 | early space exploration | T_0502 | One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. Natural objects in orbit are called natural satellites. The Moon is a natural satellite. Human-made objects in orbit are called artificial satellites. There are more and more artificial satellites orbiting Earth all the time. They all get into space using some sort of rocket. | text | null |
L_0051 | early space exploration | T_0502 | One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. An orbit is a circular or elliptical path around an object. Natural objects in orbit are called natural satellites. The Moon is a natural satellite. Human-made objects in orbit are called artificial satellites. There are more and more artificial satellites orbiting Earth all the time. They all get into space using some sort of rocket. | text | null |
L_0051 | early space exploration | T_0503 | Why do satellites stay in orbit? Why dont they crash into Earth due to the planets gravity? Newtons law of universal gravitation describes what happens. Every object in the universe is attracted to every other object. Gravity makes an apple fall to the ground. Gravity also keeps you from floating away into the sky. Gravity holds the Moon in orbit around Earth. It keeps Earth in orbit around the Sun. Newton used an example to explain how gravity makes orbiting possible. Imagine a cannonball launched from a high mountain, as shown in Figure 23.17. If the cannonball is launched at a slow speed, it will fall back to Earth. This is shown as paths (A) and (B). Something different happens if the cannonball is launched at a fast speed. The Earth below curves away at the same rate that the cannonball falls. The cannonball then goes into a circular orbit, as in path (C). If the cannonball is launched even faster, it could go into an elliptical orbit (D). It might even leave Earths gravity and go into space (E). Unfortunately, Newtons idea would not work in real life. A cannonball launched at a fast speed from Mt. Everest would not go into orbit. The cannonball would burn up in the atmosphere. However, a rocket can launch straight up, then steer into orbit. It wont burn up in the orbit. A rocket can carry a satellite above the atmosphere and then release the satellite into orbit. | text | null |
L_0051 | early space exploration | T_0504 | The first artificial satellite was launched just over 50 years ago. Thousands are now in orbit around Earth. Satellites have orbited other objects in the solar system. These include the Moon, the Sun, Venus, Mars, Jupiter, and Saturn. Satellites have many different purposes. Imaging satellites take pictures Earths surface. These images are used for military or scientific purposes. Astronomers use imaging satellites to study and make maps of the Moon and other planets. Communications satellites, such as the one in Figure 23.18, are now extremely common. These satellites receive and send signals for telephone, television, or other types of communications. Navigational satellites are used for navigation systems, such as the Global Positioning System (GPS). The largest artificial satellite is the International Space Station. The ISS is designed for humans to live in space while conducting scientific research. | text | null |
L_0051 | early space exploration | T_0505 | Dozens of satellites collect data about the Earth. One example is NASAs Landsat satellites. These satellites make detailed images of Earths continents and coastal areas. Other satellites study the oceans, atmosphere, polar ice sheets, and other Earth systems. This data helps us to monitor climate change. Other long-term changes in the planet are also best seen from space. Satellite images help scientists understand how Earths systems affect one another. Different satellites monitor different wavelengths of energy, as in Figure 23.19. | text | null |
L_0051 | early space exploration | T_0506 | Satellites have different views depending on their orbit. Satellites may be put in a low orbit. These satellites orbit from north to south over the poles. These satellites view a different part of Earth each time they circle. Imaging and weather satellites need this type of view. Satellite may be placed so that they orbit at the same rate the Earth spins. The satellite then remains over the same location on the surface. Communications satellites are often placed in these orbits. | text | null |
L_0051 | early space exploration | T_0507 | The Cold War was between the Soviet Union (USSR) and the United States. The war lasted from the end of World War II in 1945 to the breakup of the USSR in 1991. The hallmark of the Cold War was an arms race. The two nations spared no expense to create new and more powerful weapons. The development of better missiles fostered better rocket technologies. | text | null |
L_0051 | early space exploration | T_0508 | The USSR launched Sputnik 1 on October 4, 1957. This was the first artificial satellite ever put into orbit. Sputnik 1, shown in Figure 23.20, sent out radio signals, which were detected by scientists and amateur radio operators around the world. The satellite stayed in orbit for about 3 months, until it burned up as a result of friction with Earths atmosphere. The launch of Sputnik 1 started the Space Race between the USSR and the USA. Americans were shocked that the Soviets had the technology to put the satellite into orbit. They worried that the Soviets might also be winning the arms race. On November 3, 1957, the Soviets launched Sputnik 2. This satellite carried the first living creature into orbit, a dog named Laika. | text | null |
L_0051 | early space exploration | T_0509 | In response to Sputnik program, the U.S. launched two satellites. Explorer I was launched on January 31, 1958 and Vanguard 1 on March 17, 1958. National Aeronautics and Space Administration (NASA) was established that same year. The race was on! On April 12, 1961, a Soviet cosmonaut became the first human in space and in orbit. Less than one month later May 5, 1961 the U.S. sent its first astronaut into space: Alan Shepherd. The first American in orbit was John Glenn, in February 1962. And on it went. | text | null |
L_0051 | early space exploration | T_0510 | On May 25, 1961, President John F. Kennedy challenged the U.S. Congress: I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him back safely to the Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish. The Soviets were also trying to reach the Moon. Who would win? The answer came eight years after Kennedys challenge, on July 20, 1969. NASAs Apollo 11 mission put astronauts Neil Armstrong and Buzz Aldrin on the Moon, as shown in Figure 23.21. A total of five American missions put astronauts on the Moon. The last was Apollo 17. This mission landed on December 11, 1972. No other country has yet put a person on the Moon. Today, most space missions are done by | text | null |
L_0051 | early space exploration | T_0510 | On May 25, 1961, President John F. Kennedy challenged the U.S. Congress: I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him back safely to the Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish. The Soviets were also trying to reach the Moon. Who would win? The answer came eight years after Kennedys challenge, on July 20, 1969. NASAs Apollo 11 mission put astronauts Neil Armstrong and Buzz Aldrin on the Moon, as shown in Figure 23.21. A total of five American missions put astronauts on the Moon. The last was Apollo 17. This mission landed on December 11, 1972. No other country has yet put a person on the Moon. Today, most space missions are done by | text | null |
L_0051 | early space exploration | T_0511 | Both the United States and the Soviet Union sent space probes to other planets. A space probe is an unmanned spacecraft. The craft collects data by flying near or landing on an object in space. This could be a planet, moon, asteroid, or comet. The USSR sent several probes to Venus in the Venera missions. Some landed on the surface and sent back data. The U.S. sent probes to Mercury, Venus, and Mars in the Mariner missions. Two probes landed on Mars during the Viking missions. The U.S. also sent probes to the outer solar system. These probes conducted fly-bys of Jupiter, Saturn, Uranus, and Neptune. The Pioneer and Voyager probes are now out beyond the edges of our solar system. We have lost contact with the two Pioneer probes. We hope to maintain contact with the two Voyager probes until at least 2020. | text | null |
L_0052 | recent space exploration | T_0512 | While the United States continued missions to the Moon in the early 1970s, the Soviets worked to build a space station. A space station is a large spacecraft. People can live on this craft for a long period of time. | text | null |
L_0052 | recent space exploration | T_0513 | Between 1971 and 1982, the Soviets put a total of seven Salyut space stations into orbit. Figure 23.22 shows the last of these, Salyut 7. These were all temporary stations. They were launched and later inhabited by a human crew. Three of the Salyut stations were used for secret military purposes. The others were used to study the problems of living in space. Cosmonauts aboard the stations performed a variety of experiments in astronomy, biology, and Earth science. Salyut 6 and Salyut 7 each had two docking ports. One crew could dock a spacecraft to one end. A replacement crew could dock to the other end. The U.S. only launched one space station during this time. It was called Skylab. Skylab was launched in May 1973. Three crews visited Skylab, all within its first year in orbit. Skylab was used to study the effects of staying in space for long period. Devices on board were and for studying the Sun. Skylab reentered Earths atmosphere in 1979, sooner than expected. | text | null |
L_0052 | recent space exploration | T_0514 | The first space station designed for long-term use was the Mir space station (Figure 23.23). Mir was launched in several separate pieces. These pieces were put together in space. Mir holds the current record for the longest continued presence in space. There were people living on Mir continuously for almost 10 years! Mir was the first major space project in which the United States and Russia worked together. American space shuttles transported supplies and people to and from Mir. American astronauts lived on Mir for many months. This cooperation allowed the two nations to learn from each other. The U.S. learned about Russias experiences with long-duration space flights. Mir was taken out of orbit in 2001. It fell into the Pacific Ocean. | text | null |
L_0052 | recent space exploration | T_0514 | The first space station designed for long-term use was the Mir space station (Figure 23.23). Mir was launched in several separate pieces. These pieces were put together in space. Mir holds the current record for the longest continued presence in space. There were people living on Mir continuously for almost 10 years! Mir was the first major space project in which the United States and Russia worked together. American space shuttles transported supplies and people to and from Mir. American astronauts lived on Mir for many months. This cooperation allowed the two nations to learn from each other. The U.S. learned about Russias experiences with long-duration space flights. Mir was taken out of orbit in 2001. It fell into the Pacific Ocean. | text | null |
L_0052 | recent space exploration | T_0515 | The International Space Station, shown in Figure 23.24 is a joint project between the space agencies of many nations These include the United States (NASA), Russia (RKA), Japan (JAXA), Canada (CSA), several European countries (ESA) and the Brazilian Space Agency. The International Space Station is a very large station. It has many different sections and is still being assembled. The station has had people on board since 2000. American space shuttles deliver most of the supplies and equipment to the station. Russian Soyuz spacecraft carry people. The primary purpose of the station is scientific research. This is important because the station has a microgravity environment. Experiments are done in the fields of biology, chemistry, physics, physiology and medicine. | text | null |
L_0052 | recent space exploration | T_0516 | NASA wanted a new kind of space vehicle. This vehicle had to be reusable. It had to able to carry large pieces of equipment, such as satellites, space telescopes, or sections of a space station. The new vehicle was called a space shuttle, shown in Figure 23.25. There have been five space shuttles: Columbia, Challenger, Discovery, Atlantis, and Endeavor. A space shuttle has three main parts. You are probably most familiar with the orbiter. This part has wings like At the end of the mission, the orbiter re-enters Earths atmosphere. The outside heats up as it descends. Pilots have to steer the shuttle to the runway very precisely. Space shuttles usually land at Kennedy Space Center or at Edwards Air Force Base in California. The orbiter is later hauled back to Florida on the back of a jet airplane. | text | null |
L_0052 | recent space exploration | T_0516 | NASA wanted a new kind of space vehicle. This vehicle had to be reusable. It had to able to carry large pieces of equipment, such as satellites, space telescopes, or sections of a space station. The new vehicle was called a space shuttle, shown in Figure 23.25. There have been five space shuttles: Columbia, Challenger, Discovery, Atlantis, and Endeavor. A space shuttle has three main parts. You are probably most familiar with the orbiter. This part has wings like At the end of the mission, the orbiter re-enters Earths atmosphere. The outside heats up as it descends. Pilots have to steer the shuttle to the runway very precisely. Space shuttles usually land at Kennedy Space Center or at Edwards Air Force Base in California. The orbiter is later hauled back to Florida on the back of a jet airplane. | text | null |
L_0052 | recent space exploration | T_0517 | The space shuttle program has been very successful. Over 100 mission have been flown. Space shuttle missions have made many scientific discoveries. Crews have launched many satellites. There have been other great achievements in space. However, the program has also had two tragic disasters. The first came just 73 seconds after launch, on January 28, 1986. The space shuttle Challenger disintegrated in mid-air, as shown in Figure 23.27. On board were seven crew members. All of them died. One of them was Christa McAuliffe, who was to be the first teacher in space. The problem was later shown to be an O-ring. This small part was in one of the rocket boosters. Space shuttle missions were put on hold while NASA improved the safety of the shuttles. The second occurred during the takeoff of the Columbia on January 16, 2003. A small piece of insulating foam broke off the fuel tank. The foam smashed into a tile on the shuttles wing. The tile was part of the shuttles heat shield. The shield protects the shuttle from extremely high temperatures as it reenters the atmosphere. When Columbia returned to Earth on February 3, 2003, it could not withstand the high temperatures. The shuttle broke apart. Again, all seven crew members died. The space shuttle will be retired in 2011. All the remaining shuttle missions will be to the ISS. Orion will replace the shuttle. Known as a Crew Exploration Vehicle, Orion is expected to be ready by 2016. | text | null |
L_0052 | recent space exploration | T_0518 | The disasters have caused NASA to focus on developing unmanned missions. Missions without a crew are less expensive and less dangerous. These missions still provide a great deal of valuable information. | text | null |
L_0052 | recent space exploration | T_0519 | Incredible images have come from the Hubble Space Telescope (HST). Even more incredible scientific discoveries have come from HST. The Hubble was the first telescope in space. It was put into orbit by the space shuttle Discovery in 1990. Since then, four shuttle missions have gone to the Hubble to make repairs and upgrades. The last repair mission to the Hubble happened in 2009. An example of a HST image is in Figure 23.28, | text | null |
L_0052 | recent space exploration | T_0520 | We continue to explore the solar system. A rover is like a spacecraft on wheels (Figure 23.29). It can wheel around on the surface. Scientists on Earth tell it where to go. The craft then collects and sends back data from that locations. The Mars Pathfinder studied the red planet for nearly three months in 1997. Two more rovers, Spirit and Opportunity, landed on Mars in 2004. Both were only designed to last 90 days, but have lasted many times longer. Spirit sent back data until it became stuck in January 2010. Opportunity continues to explore Mars. Several spacecraft are currently in orbit, studying the Martian surface and thin atmosphere. | text | null |
L_0052 | recent space exploration | T_0520 | We continue to explore the solar system. A rover is like a spacecraft on wheels (Figure 23.29). It can wheel around on the surface. Scientists on Earth tell it where to go. The craft then collects and sends back data from that locations. The Mars Pathfinder studied the red planet for nearly three months in 1997. Two more rovers, Spirit and Opportunity, landed on Mars in 2004. Both were only designed to last 90 days, but have lasted many times longer. Spirit sent back data until it became stuck in January 2010. Opportunity continues to explore Mars. Several spacecraft are currently in orbit, studying the Martian surface and thin atmosphere. | text | null |
L_0052 | recent space exploration | T_0521 | Budget concerns have impacted NASA in recent years. Many scientists have come together to discuss the goals of the U.S. space program. Some would like to further explore the Moon. Others are interested in landing on Mars. A variety of destinations in the inner solar system may also be visited. Private aerospace companies will play more of a role in the coming years. | text | null |
L_0052 | recent space exploration | T_0522 | How to Discover a New Planet Thousands of planets - ones that look totally different than what were used to, and possibly could support life, exist outside of our solar system. But were only just now starting to find them. In the video below, Ashley takes you behind the simple technique that astronomers have been using to discover these curious new planets. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0053 | planet earth | T_0523 | As you walk, the ground usually looks pretty flat, even though the Earth is round. How do we know this? We have pictures of Earth taken from space that show that Earth is round. Astronauts aboard the Apollo 17 shuttle took this one, called The Blue Marble (Figure 24.1). Earth looks like a giant blue and white ball. Long before spacecraft took photos of Earth from space, people knew that Earth was round. How? One way was to look at ships sailing off into the distance. What do you see when you watch a tall ship sail over the horizon of the Earth? The bottom part of the ship disappears faster than the top part. What would that ship look like if Earth was flat? No part of it would disappear before the other. It would all just get smaller as it moved further away. In the solar system, the planets orbit around the Sun. The Sun and each of the planets of our solar system are round. Earth is the third planet from the Sun. It is one of the inner planets. Jupiter is an outer planet. It is the largest planet in the solar system at about 1,000 times the size of Earth. The Sun is about 1,000 times bigger than Jupiter! (Figure The outer planets in the solar system are giant balls of swirling gas. Earth and the other inner planets are relatively small, dense, and rocky. Most of Earths surface is covered with water. As far as we know, Earth is also the only planet that has liquid water. Earths atmosphere has oxygen. The water and oxygen are crucial to life as we know it. Earth appears to be the only planet in the solar system with living creatures. You can learn more about the planets in the Our Solar System chapter. Some of the different parts of the Earth are our: Since Earth is round, the layers all have the word sphere at the end (Figure 24.3). All of Earths layers interact. Therefore, Earths surface is constantly undergoing change. | text | null |
L_0053 | planet earth | T_0523 | As you walk, the ground usually looks pretty flat, even though the Earth is round. How do we know this? We have pictures of Earth taken from space that show that Earth is round. Astronauts aboard the Apollo 17 shuttle took this one, called The Blue Marble (Figure 24.1). Earth looks like a giant blue and white ball. Long before spacecraft took photos of Earth from space, people knew that Earth was round. How? One way was to look at ships sailing off into the distance. What do you see when you watch a tall ship sail over the horizon of the Earth? The bottom part of the ship disappears faster than the top part. What would that ship look like if Earth was flat? No part of it would disappear before the other. It would all just get smaller as it moved further away. In the solar system, the planets orbit around the Sun. The Sun and each of the planets of our solar system are round. Earth is the third planet from the Sun. It is one of the inner planets. Jupiter is an outer planet. It is the largest planet in the solar system at about 1,000 times the size of Earth. The Sun is about 1,000 times bigger than Jupiter! (Figure The outer planets in the solar system are giant balls of swirling gas. Earth and the other inner planets are relatively small, dense, and rocky. Most of Earths surface is covered with water. As far as we know, Earth is also the only planet that has liquid water. Earths atmosphere has oxygen. The water and oxygen are crucial to life as we know it. Earth appears to be the only planet in the solar system with living creatures. You can learn more about the planets in the Our Solar System chapter. Some of the different parts of the Earth are our: Since Earth is round, the layers all have the word sphere at the end (Figure 24.3). All of Earths layers interact. Therefore, Earths surface is constantly undergoing change. | text | null |
L_0053 | planet earth | T_0523 | As you walk, the ground usually looks pretty flat, even though the Earth is round. How do we know this? We have pictures of Earth taken from space that show that Earth is round. Astronauts aboard the Apollo 17 shuttle took this one, called The Blue Marble (Figure 24.1). Earth looks like a giant blue and white ball. Long before spacecraft took photos of Earth from space, people knew that Earth was round. How? One way was to look at ships sailing off into the distance. What do you see when you watch a tall ship sail over the horizon of the Earth? The bottom part of the ship disappears faster than the top part. What would that ship look like if Earth was flat? No part of it would disappear before the other. It would all just get smaller as it moved further away. In the solar system, the planets orbit around the Sun. The Sun and each of the planets of our solar system are round. Earth is the third planet from the Sun. It is one of the inner planets. Jupiter is an outer planet. It is the largest planet in the solar system at about 1,000 times the size of Earth. The Sun is about 1,000 times bigger than Jupiter! (Figure The outer planets in the solar system are giant balls of swirling gas. Earth and the other inner planets are relatively small, dense, and rocky. Most of Earths surface is covered with water. As far as we know, Earth is also the only planet that has liquid water. Earths atmosphere has oxygen. The water and oxygen are crucial to life as we know it. Earth appears to be the only planet in the solar system with living creatures. You can learn more about the planets in the Our Solar System chapter. Some of the different parts of the Earth are our: Since Earth is round, the layers all have the word sphere at the end (Figure 24.3). All of Earths layers interact. Therefore, Earths surface is constantly undergoing change. | text | null |
L_0053 | planet earth | T_0524 | Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path (Figure 24.4). Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earths gravity pulls the Moon toward Earths center. Without gravity, the Moon would continue moving in a straight line off into space. All objects in the universe have a gravitational attraction to each other (Figure 24.5). The strength of the force of gravity depends on two things. They are the mass of the objects and the distance between them. The greater the objects mass, the greater the force of attraction. As the distance between the objects increases, the force of attraction decreases. | text | null |
L_0053 | planet earth | T_0525 | Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). | text | null |
L_0053 | planet earth | T_0525 | Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). | text | null |
L_0053 | planet earth | T_0525 | Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). | text | null |
L_0053 | planet earth | T_0525 | Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). | text | null |
L_0053 | planet earth | T_0525 | Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8). | text | null |
L_0053 | planet earth | T_0526 | Earths axis is an imaginary line passing through the North and South Poles. Earthsrotation is its spins on its axis. Rotation is what a top does around its spindle. As Earth spins on its axis, it also orbits around the Sun. This is called Earths revolution. These motions lead to the cycles we see. Day and night, seasons, and the tides are caused by Earths motions. | text | null |
L_0053 | planet earth | T_0527 | In 1851, Lon Foucault, a French scientist, hung a heavy iron weight from a long wire. He pulled the weight to one side and then released it. The weight swung back and forth in a straight line. If Earth did not rotate, the pendulum would not change direction as it was swinging. But it did, or at least it appeared to. The direction of the pendulum appeared to change because Earth rotated beneath it. Figure 24.9 shows how this might look. A Turn of the Earth In this video, MIT students demonstrate how a Foucault Pendulum is used to prove that the Earth is rotating. See the video at . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0053 | planet earth | T_0528 | How long does it take Earth to spin once on its axis? One rotation is 24 hours. That rotation is the length of a day! Whatever time it is, the side of Earth facing the Sun has daylight. The side facing away from the Sun is dark. If you look at Earth from the North Pole, the planet spins counterclockwise. As the Earth rotates, you see the Sun moving across the sky from east to west. We often say that the Sun is rising or setting. The Sun rises in the east and sets in the west. Actually, it is the Earths rotation that makes it appear that way. The Moon and the stars at night also seem to rise in the east and set in the west. Earths rotation is also responsible for this too. As Earth turns, the Moon and stars change position in the sky. | text | null |
L_0053 | planet earth | T_0529 | The Earth is tilted 23 1/2 on its axis (Figure 24.10). This means that as the Earth rotates, one hemisphere has longer days with shorter nights. At the same time the other hemisphere has shorter days and longer nights. For example, in the Northern hemisphere summer begins on June 21. On this date, the North Pole is pointed directly toward the Sun. This is the longest day and shortest night of the year in the Northern Hemisphere. The South Pole is pointed away from the Sun. This means that the Southern Hemisphere experiences its longest night and shortest day (Figure 24.11). The hemisphere that is tilted away from the Sun is cooler because it receives less direct rays. As Earth orbits the Sun, the Northern Hemisphere goes from winter to spring, then summer and fall. The Southern Hemisphere does the opposite from summer to fall to winter to spring. When it is winter in the Northern hemisphere, it is summer in the Southern hemisphere, and vice versa. | text | null |
L_0053 | planet earth | T_0529 | The Earth is tilted 23 1/2 on its axis (Figure 24.10). This means that as the Earth rotates, one hemisphere has longer days with shorter nights. At the same time the other hemisphere has shorter days and longer nights. For example, in the Northern hemisphere summer begins on June 21. On this date, the North Pole is pointed directly toward the Sun. This is the longest day and shortest night of the year in the Northern Hemisphere. The South Pole is pointed away from the Sun. This means that the Southern Hemisphere experiences its longest night and shortest day (Figure 24.11). The hemisphere that is tilted away from the Sun is cooler because it receives less direct rays. As Earth orbits the Sun, the Northern Hemisphere goes from winter to spring, then summer and fall. The Southern Hemisphere does the opposite from summer to fall to winter to spring. When it is winter in the Northern hemisphere, it is summer in the Southern hemisphere, and vice versa. | text | null |
L_0053 | planet earth | T_0530 | Earths revolution around the Sun takes 365.24 days. That is equal to one year. The Earth stays in orbit around the Sun because of the Suns gravity (Figure 24.12). Earths orbit is not a circle. It is somewhat elliptical. So as we travel around the Sun, sometimes we are a little farther away from the Sun. Sometimes we are closer to the Sun. Students sometimes think the slightly oval shape of our orbit causes Earths seasons. Thats not true! The seasons are due to the tilt of Earths axis, as discussed above. | text | null |
L_0054 | earths moon | T_0531 | The Moon is Earths only natural satellite. The Moon is about one-fourth the size of Earth, 3,476 kilometers in diameter. Gravity on the Moon is only one-sixth as strong as it is on Earth. If you weigh 120 pounds on Earth, you would only weigh 20 pounds on the Moon. You can jump six times as high on the Moon as you can on Earth. The Moon makes no light of its own. Like every other body in the solar system, it only reflects light from the Sun. The Moon rotates on its axis once for every orbit it makes around the Earth. What does this mean? This means that the same side of the Moon always faces Earth. The side of the Moon that always faces Earth is called the near side. The side of the Moon that always faces away from Earth is called the far side (Figure 24.13). All people for all time have only seen the Moons near side. The far side has only been seen by spacecraft. The Moon has no atmosphere. With no atmosphere, the Moon is not protected from extreme temperatures. The average surface temperature during the day is approximately 107C (225F). Daytime temperatures can reach as high as 123C (253F). At night, the average temperature drops to -153C (-243F). The lowest temperatures measured are as low as -233C (-397F). | text | null |
L_0054 | earths moon | T_0532 | We all know what the Moon looks like. Its always looked the same during our lifetime. In fact, the Moon has looked the same to every person who has looked up at it for all time. Even the dinosaurs and trilobites, should they have looked up at it, would have seen the same thing. This is not true of Earth. Natural processes continually alter the Earths surface. Without these processes, would Earths surface resemble the Moons? Even though we cant see it from Earth, the Moon has changed recently too. Astronauts footprints are now on the Moon. They will remain unchanged for thousands of years, because there is no wind, rain, or living thing to disturb them. Only a falling meteorite could destroy them. | text | null |
L_0054 | earths moon | T_0533 | The landscape of the Moon - its surface features - is very different from Earth. The lunar landscape is covered by craters caused by asteroid impacts (Figure 24.14). The craters are bowl-shaped basins on the Moons surface. Because the Moon has no water, wind, or weather, the craters remain unchanged. The Moons coldest temperatures are found deep in the craters. The coldest craters are at the south pole on the Moons far side, where the Sun never shines. These temperatures are amongst the coldest in our entire solar system. | text | null |
L_0054 | earths moon | T_0534 | When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava. Maria covers around 16% of the Moons surface, mostly on the near side. The maria formed about 3.0 to 3.5 billion years ago, when the Moon was continually bombarded by meteorites (Figure 24.15). Large meteorites broke through the Moons newly formed surface. This caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago. The lighter parts on the Moon are called terrae, or highlands (Figure 24.15). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Suns light. | text | null |
L_0054 | earths moon | T_0534 | When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava. Maria covers around 16% of the Moons surface, mostly on the near side. The maria formed about 3.0 to 3.5 billion years ago, when the Moon was continually bombarded by meteorites (Figure 24.15). Large meteorites broke through the Moons newly formed surface. This caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago. The lighter parts on the Moon are called terrae, or highlands (Figure 24.15). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Suns light. | text | null |
L_0054 | earths moon | T_0535 | There are no lakes, rivers, or even small puddles anywhere to be found on the Moons surface. So there is no running water and no atmosphere. This means that there is no erosion. Natural processes continually alter the Earths surface. Without these processes, our planets surface would be covered with meteorite craters just like the Moon. Many moons in our solar system have cratered surfaces. NASA scientists have discovered a large number of water molecules mixed in with lunar dirt. There is also surface water ice. Even though there is a very small amount of water, there is no atmosphere. Temperatures are extreme. So it comes as no surprise that there has not been evidence of life on the Moon. | text | null |
L_0054 | earths moon | T_0536 | Like Earth, the Moon has a distinct crust, mantle, and core. The crust is composed of igneous rock. This rock is rich in the elements oxygen, silicon, magnesium, and aluminum. On the near side, the Moons crust is about 60 kilometers thick. On the far side, the crust is about 100 kilometers thick. The mantle is made of rock like Earths mantle. The Moon has a small metallic core, perhaps 600 to 800 kilometers in diameter. The composition of the core is probably mostly iron with some sulfur and nickel. We learned this both from the rock samples gathered by astronauts and from spacecraft sent to the Moon. | text | null |
L_0055 | the sun | T_0537 | The Sun is made almost entirely of the elements hydrogen and helium. The Sun has no solid material. Most atoms in the Sun exist as plasma. Plasma is superheated gas with an electrical charge. Because the Sun is made of gases, it does not have a defined outer boundary. Like Earth, the Sun has an internal structure. The inner three layers make up what we would actually call the Sun. | text | null |
L_0055 | the sun | T_0538 | The core is the Suns innermost layer. The core is plasma. It has a temperature of around 15 million degrees Celsius (C). Nuclear fusion reactions create the immense temperature. In these reactions, hydrogen atoms fuse to form helium. This releases vast amounts of energy. The energy moves towards the outer layers of the Sun. Energy from the Suns core powers most of the solar system. | text | null |
L_0055 | the sun | T_0539 | The radiative zone is the next layer out. It has a temperature of about 4 million degrees C. Energy from the core travels through the radiative zone. The rate the energy travels is extremely slow. Light particles, called photons, can only travel a few millimeters before they hit another particle. The particles are absorbed and then released again. It may take 50 million years for a photon to travel all the way through the radiative zone. | text | null |
L_0055 | the sun | T_0540 | The convection zone surrounds the radiative zone. In the convection zone, hot material from near the Suns center rises. This material cools at the surface, and then plunges back downward. The material then receives more heat from the radiative zone. | text | null |
L_0055 | the sun | T_0541 | The three outer layers of the Sun are its atmosphere. | text | null |
L_0055 | the sun | T_0542 | The photosphere is the visible surface of the Sun (Figure 24.17). Its the part that we see shining. Surprisingly, the photosphere is also one of the coolest layers of the Sun. It is only about 6000 degrees C. | text | null |
L_0055 | the sun | T_0543 | The chromosphere lies above the photosphere. It is about 2,000 km thick. The thin chromosphere is heated by energy from the photosphere. Temperatures range from about 4000 degrees C to about 10,000 degrees C. The chromosphere is not as hot as other parts of the Sun, and it glows red. Jets of gas sometimes fly up through the chromosphere. With speeds up to 72,000 km per hour, the jets can fly as high as 10,000 kilometers. | text | null |
L_0055 | the sun | T_0544 | The corona is the outermost part of the Suns atmosphere. It is the Suns halo, or crown. With a temperature of 1 to 3 million K, the corona is much hotter than the photosphere. The corona extends millions of kilometers into space. Sometime you should try to see a total solar eclipse. If you do you will see the Suns corona shining out into space. | text | null |
L_0055 | the sun | T_0545 | The Sun has many incredible surface features. Dont try to look at them though! Looking directly at the Sun can cause blindness. Find the appropriate filters for a pair of binoculars or a telescope and enjoy! | text | null |
L_0055 | the sun | T_0546 | The most noticeable magnetic activity of the Sun is the appearance of sunspots. Sunspots are cooler, darker areas on the Suns surface (Figure 24.18). Sunspots occur in an 11 year cycle. The number of sunspots begins at a minimum. The number gradually increases to the maximum. Then the number returns to a minimum again. Sunspots form because loops of the Suns magnetic field break through the surface. Sunspots usually occur in pairs. The loop breaks through the surface where it comes out of the Sun. It breaks through again where it goes back into the Sun. Sunspots disrupt the transfer of heat from the Suns lower layers. | text | null |
L_0055 | the sun | T_0547 | A loop of the Suns magnetic field may break. This creates solar flares. Solar flares are violent explosions that release huge amounts of energy (Figure 24.19). The streams of high energy particles they emit make up the solar wind. Solar wind is dangerous to spacecraft and astronauts. Solar flares can even cause damage on Earth. They have knocked out entire power grids and can disturb radio, satellite, and cell phone communications. | text | null |
L_0055 | the sun | T_0548 | Another amazing feature on the Sun is solar prominences. Plasma flows along the loop that connects sunspots. This plasma forms a glowing arch. The arch is a solar prominence. Solar prominences can reach thousands of kilometers into the Suns atmosphere. Prominences can last for a day to several months. Prominences can be seen during a total solar eclipse. NASAs Solar Dynamics Observatory (SDO) was launched on February 11, 2010. SDO is studying the Suns magnetic field. This includes how the Sun affects Earths atmosphere and climate. SDO provides extremely high resolution images. The craft gathers data faster than anything that ever studied the Sun. To learn more about the SDO mission, visit: http://sdo.gsfc.nasa.gov To find these videos for download, check out: There are other ways to connect with NASA. Subscribe to NASAs Goddard Shorts HD podcast (http://svs.gsfc.nasa | text | null |
L_0056 | the sun and the earthmoon system | T_0549 | When a new moon passes directly between the Earth and the Sun, it causes a solar eclipse (Figure 24.20). The Moon casts a shadow on the Earth and blocks our view of the Sun. This happens only all three are lined up and in the same plane. This plane is called the ecliptic. The ecliptic is the plane of Earths orbit around the Sun. The Moons shadow has two distinct parts. The umbra is the inner, cone-shaped part of the shadow. It is the part in which all of the light has been blocked. The penumbra is the outer part of Moons shadow. It is where the light is only partially blocked. When the Moons shadow completely blocks the Sun, it is a total solar eclipse (Figure 24.21). If only part of the Sun is out of view, it is a partial solar eclipse. Solar eclipses are rare events. They usually only last a few minutes. That is because the Moons shadow only covers a very small area on Earth and Earth is turning very rapidly. Solar eclipses are amazing to experience. It appears like night only strange. Birds may sing as they do at dusk. Stars become visible in the sky and it gets colder outside. Unlike at night, the Sun is out. So during a solar eclipse, its easy to see the Suns corona and solar prominences. This NASA page will inform you on when solar eclipses are expected: http://eclipse.gsfc.nasa.gov/solar.html | text | null |
L_0056 | the sun and the earthmoon system | T_0549 | When a new moon passes directly between the Earth and the Sun, it causes a solar eclipse (Figure 24.20). The Moon casts a shadow on the Earth and blocks our view of the Sun. This happens only all three are lined up and in the same plane. This plane is called the ecliptic. The ecliptic is the plane of Earths orbit around the Sun. The Moons shadow has two distinct parts. The umbra is the inner, cone-shaped part of the shadow. It is the part in which all of the light has been blocked. The penumbra is the outer part of Moons shadow. It is where the light is only partially blocked. When the Moons shadow completely blocks the Sun, it is a total solar eclipse (Figure 24.21). If only part of the Sun is out of view, it is a partial solar eclipse. Solar eclipses are rare events. They usually only last a few minutes. That is because the Moons shadow only covers a very small area on Earth and Earth is turning very rapidly. Solar eclipses are amazing to experience. It appears like night only strange. Birds may sing as they do at dusk. Stars become visible in the sky and it gets colder outside. Unlike at night, the Sun is out. So during a solar eclipse, its easy to see the Suns corona and solar prominences. This NASA page will inform you on when solar eclipses are expected: http://eclipse.gsfc.nasa.gov/solar.html | text | null |
L_0056 | the sun and the earthmoon system | T_0550 | Sometimes a full moon moves through Earths shadow. This is a lunar eclipse (Figure 24.22). During a total lunar eclipse, the Moon travels completely in Earths umbra. During a partial lunar eclipse, only a portion of the Moon enters Earths umbra. When the Moon passes through Earths penumbra, it is a penumbral eclipse. Since Earths shadow is large, a lunar eclipse lasts for hours. Anyone with a view of the Moon can see a lunar eclipse. Partial lunar eclipses occur at least twice a year, but total lunar eclipses are less common. The Moon glows with a dull red coloring during a total lunar eclipse. | text | null |
L_0056 | the sun and the earthmoon system | T_0551 | The Moon does not produce any light of its own. It only reflects light from the Sun. As the Moon moves around the Earth, we see different parts of the Moon lit up by the Sun. This causes the phases of the Moon. As the Moon revolves around Earth, it changes from fully lit to completely dark and back again. A full moon occurs when the whole side facing Earth is lit. This happens when Earth is between the Moon and the Sun. About one week later, the Moon enters the quarter-moon phase. Only half of the Moons lit surface is visible from Earth, so it appears as a half circle. When the Moon moves between Earth and the Sun, the side facing Earth is completely dark. This is called the new moon phase. Sometimes you can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the Moon. Before and after the quarter-moon phases are the gibbous and crescent phases. During the crescent moon phase, the Moon is less than half lit. It is seen as only a sliver or crescent shape. During the gibbous moon phase, the Moon is more than half lit. It is not full. The Moon undergoes a complete cycle of phases about every 29.5 days. | text | null |
L_0056 | the sun and the earthmoon system | T_0552 | null | text | null |
L_0057 | introduction to the solar system | T_0553 | The Sun and all the objects that are held by the Suns gravity are known as the solar system. These objects all revolve around the Sun. The ancient Greeks recognized five planets. These lights in the night sky changed their position against the background of stars. They appeared to wander. In fact, the word planet comes from a Greek word meaning wanderer. These objects were thought to be important, so they named them after gods from their mythology. The names for the planets Mercury, Venus, Mars, Jupiter, and Saturn came from the names of gods and a goddess. | text | null |
L_0057 | introduction to the solar system | T_0554 | The ancient Greeks thought that Earth was at the center of the universe, as shown in Figure 25.1. The sky had a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres. The object moved around Earth as that sphere rotated. These spheres contained the Moon, the Sun, and the five planets they recognized: Mercury, Venus, Mars, Jupiter, and Saturn. An outer sphere contained all the stars. The planets appear to move much faster than the stars, so the Greeks placed them closer to Earth. Ptolemy published this model of the solar system around 150 AD. | text | null |
L_0057 | introduction to the solar system | T_0555 | About 1,500 years after Ptolemy, Copernicus proposed a startling idea. He suggested that the Sun is at the center of the universe. Copernicus developed his model because it better explained the motions of the planets. Figure 25.2 shows both the Earth-centered and Sun-centered models. Copernicus did not publish his new model until his death. He knew that it was heresy to say that Earth was not the center of the universe. It wasnt until Galileo developed his telescope that people would take the Copernican | text | null |
L_0057 | introduction to the solar system | T_0556 | Today we know that we have eight planets, five dwarf planets, over 165 moons, and many, many asteroids and other small objects in our solar system. We also know that the Sun is not the center of the universe. But it is the center of the solar system. Figure 25.3 shows our solar system. The planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Table 25.1 gives some data on the mass and diameter of the Sun and planets relative to Earth. Object Mass (Relative to Earth) Sun Mercury Venus Earth Mars Jupiter Saturn Uranus 333,000 Earths mass 0.06 Earths mass 0.82 Earths mass 1.00 Earths mass 0.11 Earths mass 317.8 Earths mass 95.2 Earths mass 14.6 Earths mass Diameter of Planet (Relative to Earth) 109.2 Earths diameter 0.39 Earths diameter 0.95 Earths diameter 1.00 Earths diameter 0.53 Earths diameter 11.21 Earths diameter 9.41 Earths diameter 3.98 Earths diameter Neptune 17.2 Earths mass | text | null |
L_0057 | introduction to the solar system | T_0557 | Youve probably heard about Pluto. When it was discovered in 1930, Pluto was called the ninth planet. Astronomers later found out that Pluto was not like other planets. For one thing, what they were calling Pluto was not a single object. They were actually seeing Pluto and its moon, Charon. In older telescopes, they looked like one object. This one object looked big enough to be a planet. Alone, Pluto was not very big. Astronomers also discovered many objects like Pluto. They were rocky and icy and there were a whole lot of them. Astronomers were faced with a problem. They needed to call these other objects planets. Or they needed to decide that Pluto was something else. In 2006, these scientists decided what a planet is. According to the new definition, a planet must: Orbit a star. Be big enough that its own gravity causes it to be round. Be small enough that it isnt a star itself. Have cleared the area of its orbit of smaller objects. If the first three are true but not the fourth, then that object is a dwarf planet. We now call Pluto a dwarf planet. There are other dwarf planets in the solar system. They are Eris, Ceres, Makemake and Haumea. There are many other reasons why Pluto does not fit with the other planets in our solar system. | text | null |
L_0057 | introduction to the solar system | T_0558 | Figure 25.4 shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. In Figure 25.5, you can see that the orbits of the planets are nearly circular. Plutos orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune. Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table 25.2 shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 (In | text | null |
L_0057 | introduction to the solar system | T_0558 | Figure 25.4 shows the Sun and planets with the correct sizes. The distances between them are way too small. In general, the farther away from the Sun, the greater the distance from one planets orbit to the next. In Figure 25.5, you can see that the orbits of the planets are nearly circular. Plutos orbit is a much longer ellipse. Some astronomers think Pluto was dragged into its orbit by Neptune. Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles). Table 25.2 shows the distance from the Sun to each planet in AU. The table shows how long it takes each planet to spin on its axis. It also shows how long it takes each planet to complete an orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Average Distance from Sun (AU) 0.39 AU 0.72 1.00 1.52 5.20 9.54 19.22 30.06 Length of Day (In Earth Days) 56.84 days 243.02 1.00 1.03 0.41 0.43 0.72 0.67 Length of Year Earth Years) 0.24 years 0.62 1.00 1.88 11.86 29.46 84.01 164.8 (In | text | null |
L_0057 | introduction to the solar system | T_0559 | Planets are held in their orbits by the force of gravity. What would happen without gravity? Imagine that you are swinging a ball on a string in a circular motion. Now let go of the string. The ball will fly away from you in a straight line. It was the string pulling on the ball that kept the ball moving in a circle. The motion of a planet is very similar to the ball on a string. The force pulling the planet is the pull of gravity between the planet and the Sun. Every object is attracted to every other object by gravity. The force of gravity between two objects depends on the mass of the objects. It also depends on how far apart the objects are. When you are sitting next to your dog, there is a gravitational force between the two of you. That force is far too weak for you to notice. You can feel the force of gravity between you and Earth because Earth has a lot of mass. The force of gravity between the Sun and planets is also very large. This is because the Sun and the planets are very large objects. Gravity is great enough to hold the planets to the Sun even though the distances between them are enormous. Gravity also holds moons in orbit around planets. | text | null |
L_0057 | introduction to the solar system | T_0560 | Since the early 1990s, astronomers have discovered other solar systems. A solar system has one or more planets orbiting one or more stars. We call these planets extrasolar planets, or exoplanets. They are called exoplanets because they orbit a star other than the Sun. As of June 2013, 891 exoplanets have been found. More exoplanets are found all the time. You can check out how many we have found at http://planetquest.jpl.nasa.gov/. We have been able to take pictures of only a few exoplanets. Most are discovered because of some tell-tale signs. One sign is a very slight motion of a star that must be caused by the pull of a planet. Another sign is the partial dimming of a stars light as the planet passes in front of it. | text | null |
L_0057 | introduction to the solar system | T_0561 | To figure out how the solar system formed, we need to put together what we have learned. There are two other important features to consider. First, all the planets orbit in nearly the same flat, disk-like region. Second, all the planets orbit in the same direction around the Sun. These two features are clues to how the solar system formed. | text | null |
L_0057 | introduction to the solar system | T_0562 | Scientists think the solar system formed from a big cloud of gas and dust, called a nebula. This is the solar nebula hypothesis. The nebula was made mostly of hydrogen and helium. There were heavier elements too. Gravity caused the nebula to contract (Figure 25.6). As the nebula contracted, it started to spin. As it got smaller and smaller, it spun faster and faster. This is what happens when an ice skater pulls her arms to her sides during a spin move. She spins faster. The spinning caused the nebula to form into a disk shape. This model explains why all the planets are found in the flat, disk-shaped region. It also explains why all the planets revolve in the same direction. The solar system formed from the nebula about 4.6 billion years ago | text | null |
L_0057 | introduction to the solar system | T_0563 | The Sun was the first object to form in the solar system. Gravity pulled matter together to the center of the disk. Density and pressure increased tremendously. Nuclear fusion reactions begin. In these reactions, the nuclei of atoms come together to form new, heavier chemical elements. Fusion reactions release huge amounts of nuclear energy. From these reactions a star was born, the Sun. Meanwhile, the outer parts of the disk were cooling off. Small pieces of dust started clumping together. These clumps collided and combined with other clumps. Larger clumps attracted smaller clumps with their gravity. Eventually, all these pieces grew into the planets and moons that we find in our solar system today. The outer planets Jupiter, Saturn, Uranus, and Neptune condensed from lighter materials. Hydrogen, helium, water, ammonia, and methane were among them. Its so cold by Jupiter and beyond that these materials can form solid particles. Closer to the Sun, they are gases. Since the gases can escape, the inner planets Mercury, Venus, Earth, and Mars formed from denser elements. These elements are solid even when close to the Sun. | text | null |
L_0058 | inner planets | T_0564 | Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. | text | null |
L_0058 | inner planets | T_0564 | Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. | text | null |
L_0058 | inner planets | T_0565 | Mercury is named for the Roman messenger god. Mercury was a messenger because he could run extremely fast. The Greeks gave the planet this name because Mercury moves very quickly in its orbit around the Sun. Mercury orbits the Sun in just 88 Earth days. Mercury has a very short year, but it also has very long days. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 58 Earth days long. | text | null |
L_0058 | inner planets | T_0566 | Mercury is very close to the Sun, so it can get very hot. Mercury also has virtually no atmosphere. As the planet rotates very slowly, the temperature varies tremendously. In direct sunlight, the surface can be as hot as 427C (801F). On the dark side, the surface can be as cold as 183C (297F)! The coldest temperatures may be on the insides of craters. Most of Mercury is extremely dry. Scientists think that there may be a small amount of water, in the form of ice, at the planets poles. The poles never receive direct sunlight. | text | null |
L_0058 | inner planets | T_0567 | Figure 25.10 shows a diagram of Mercurys interior. Mercury is one of the densest planets. Scientists think that the interior contains a large core made mostly of melted iron. Mercurys core takes up about 42% of the planets volume. Mercurys highly cratered surface is evidence that Mercury is not geologically active. | text | null |
L_0058 | inner planets | T_0568 | Named after the Roman goddess of love, Venus is the only planet named after a female. Venus is sometimes called Earths sister planet. But just how similar is Venus to Earth? Venus is our nearest neighbor. Venus is most like Earth in size. | text | null |
L_0058 | inner planets | T_0569 | Viewed through a telescope, Venus looks smooth and featureless. The planet is covered by a thick layer of clouds. You can see the clouds in pictures of Venus, such as Figure 25.11. We make maps of the surface using radar, because the thick clouds wont allow us to take photographs of the surface of Venus. Figure 25.12 shows the topographical features of Venus. The image was produced by the Magellan probe on a flyby. Radar waves sent by the spacecraft reveal mountains, valleys, vast lava plains, and canyons. Like Mercury, Venus does not have a moon. Clouds on Earth are made of water vapor. Venuss clouds are a lot less pleasant. They are made of carbon dioxide, sulfur dioxide and large amounts of corrosive sulfuric acid! The atmosphere of Venus is so thick that the pressure on the surface of Venus is very high. In fact, it is 90 times greater than the pressure at Earths surface! The thick atmosphere causes a strong greenhouse effect. As a result, Venus is the hottest planet. Even though it is farther from the Sun, Venus is much hotter even than Mercury. Temperatures at the surface reach 465C (860F). Thats hot enough to melt lead! | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0570 | Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means that Venus doesnt have tectonic plates. Plate tectonics on Earth erases features over time. Figure 25.13 is an image made using radar data. The volcano is Maat Mons. Lava beds are in the foreground. Scientists think the color of sunlight on Venus is | text | null |
L_0058 | inner planets | T_0571 | Venus is the only planet that rotates clockwise as viewed from its North Pole. All of the other planets rotate counterclockwise. Venus turns slowly, making only one turn every 243 days. This is longer than a year on Venus! It takes Venus only 225 days to orbit the Sun. Because the orbit of Venus is inside Earths orbit, Venus always appears close to the Sun. You can see Venus rising early in the morning, just before the Sun rises. For this reason, Venus is sometimes called the morning star. When it sets in the evening, just after the Sun sets, it may be called the evening star. Since planets only reflect the Suns light, Venus should not be called a star at all! Venus is very bright because its clouds reflect sunlight very well. Venus is the brightest object in the sky besides the Sun and the Moon. | text | null |
L_0058 | inner planets | T_0572 | Earth is the third planet out from the Sun, shown in Figure 25.14. Because it is our planet, we know a lot more about Earth than we do about any other planet. What are main features of Earth? | text | null |
L_0058 | inner planets | T_0573 | Earth is a very diverse planet, seen in Figure 25.14. Water appears as vast oceans of liquid. Water is also seen as ice at the poles or as clouds of vapor. Earth also has large masses of land. Earths average surface temperature is 14C (57F). At this temperature, water is a liquid. The oceans and the atmosphere help keep Earths surface temperatures fairly steady. Earth is the only planet known to have life. Conditions on Earth are ideal for life! The atmosphere filters out harmful radiation. Water is abundant. Carbon dioxide was available for early life forms. The evolution of plants introduced more oxygen for animals. | text | null |
L_0058 | inner planets | T_0574 | The Earth is divided into many plates. These plates move around on the surface. The plates collide or slide past each other. One may even plunge beneath another. Plate motions cause most geological activity. This activity includes earthquakes, volcanoes, and the buildup of mountains. The reason for plate movement is convection in the mantle. Earth is the only planet that we know has plate tectonics. | text | null |
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