The Solar System Teacher Background Information

Transcription

1 The Solar System Teacher Background Information What is a solar system? Our solar system consists of the sun and all the objects that revolve around it: nine planets and their moons, thousands of asteroids and comets and countless meteoroids (small bits of rock and metal). What makes day and night? (Why does the Sun appear to travel across the sky each day?) Each of the nine planets rotates around an axis an imaginary line through its center while also traveling in a counterclockwise direction around the sun. Whether it is day or night depends on the way the planet is turned in its rotation. The part of the planet that is turned toward the sun experiences daytime. The part that is turned away from the sun experiences nighttime. The time it takes a planet to completely spin around on its axis is that planet s day. Earth completes a spin on its axis every 24 hours. Other planets spin faster or slower than Earth, so their days are longer or shorter than the days on Earth. What makes a year? The time it takes to make a complete orbit around the sun is its year. The farther a planet is from the sun, the longer its year. A planet s distance from the sun also affects its climate and atmosphere. The exact distance between the planets and the sun, as well as between the planets themselves, varies because the planets move in elliptical (oval) rather than circular orbits. What causes our seasons? The Earth travels (revolves) in an elliptical orbit around the Sun once each year. Because the Earth is always tilted on its axis and tilted in the same direction, sometimes the top half of the Earth (northern hemisphere) is tilted away from the Sun, so the season is winter; and the bottom half of the Earth (southern hemisphere) is then tilted towards the Sun, so it is summer. In spring and summer, both hemispheres get equal amounts of sunlight. For a graphic of the seasons and for other great information on this topic visit Windows to the Universe. The Seasonal Merry-Go-Round. Boulder, CO: University Corporation of Atmospheric Research (UCAR), , The Regents of the University of Michigan, December 6, Online. Available: July 18, S4E2a, b, c, & d 26 Background

2 Why are there both rocky and gaseous planets? Astronomers believe that both kinds of planets formed while the sun was forming about five billion years ago. The sun s heat and solar winds (streams of gases from the sun) drove off the gases in the cloud of dust and gas that comprised the early solar system, leaving metal and rock behind. From these materials came the inner planets. The inner planets those closest to the Sun (Mercury, Venus, Earth, Mars)--have many characteristics in common. Because they are similar to Earth, they are sometimes called the terrestrial planets. All four terrestrial planets have hard crusts, composed of lightweight silicate rocks. Rocky mantles of heavier rock lie beneath the crusts. Heavy iron cores form their centers. Four of the outer planets (Jupiter, Saturn, Uranus, and Neptune) are made entirely of gases and are often referred to as the gas giants. Because these planets are so far from the Sun, they did not lose great amounts of lightweight gases such as hydrogen, the way the inner planets did. They became much more massive than the terrestrial planets. The atmospheres of the giant gas planets are composed mostly of hydrogen. It is, of course, very cold on these planets. Leftover gas, dust, and ice continued orbiting around these planets forming the rings and multiple moons found around them. Pluto, the farthest planet from the sun, is also the least known. In the mid 1990 s, the Hubble Space Telescope did provide some information. Pluto is nothing like the other outer planets. It is a rocky, icy planet and is very small. It is the smallest planet in the solar system. Some astronomers suspect that Pluto is actually a large moon of Neptune that was pulled out of its regular orbit. In addition to Earth, only Mercury, Venus, Mars, Jupiter, and Saturn were known to ancient people. The others weren t discovered until astronomers began using telescopes for their observations. Uranus was discovered in 1781; Neptune, in 1846; and Pluto, in What keeps the planets in orbit around the Sun? All the planets are ruled by the Sun s gravity. The Sun s gravitational pull keeps the planets in a path around itself, just like Earth s gravitational pull keeps our feet on the ground, as well as the moon in our orbit. How is Planet Earth unique? Within the solar system, Earth is unique in a number of ways. First 70 percent of the Earth s surface is covered by water-more water than on any other planet. Second, Earth as a unique atmosphere made up of nitrogen (about 80%) and oxygen (about 20 %) with traces of argon, carbon dioxide, water vapor, and other gases. This atmosphere helps keep the Earth warm and absorbs some of the damaging forms of energy from the sun (like ultraviolet radiation). And most important to us, Earth is the only planet in the solar system (as far as we know) that sustains life. The Sun s light and heat make life possible on Earth. Plants are dependent on the light from the Sun to make food through the process of photosynthesis. And all other life is dependent on plants or plant eaters. S4E2a, b, c, & d 27 Background

3 How long will the Sun last? The Sun s hydrogen fuel is only half used up, so it will be a while before it burns out. Some scientists think Earth will be uninhabitable because of a dying Sun in about 5 billion years. How does the solar system fit into this universe? Many years ago, as people saw the Sun going by every day, they thought that the Sun was going around the Earth. They believed that the Earth and human beings were the center of the universe (geocentric theory of the universe). They felt smaller and less significant when it was discovered that the Earth is a planet traveling around the Sun. Later, it was established that our Sun is one of the billions of stars that forms a huge cluster of stars---a galaxy called the Milky Way. The Milky Way is a one of billions of galaxies that make up our universe. How much of the solar system has been explored? Of all the bodies in the solar system, human beings have set foot on two: Earth and its Moon. But robot probes built and guided by scientists have traveled to the edge of the solar system, beaming spectacular photos and other information back to Earth. These probes have landed on Mars and Venus and have flown by all of the planets except Pluto. Most of what we know about the planets comes from these robot probes. Before the first probe, all of what we knew about the solar system came from careful observation and calculations, made with the help of powerful telescopes. Are there other planets in the universe? Although most scientists are extremely skeptical about reports of UFOs, ETs, or visitors from space, they generally accept the view that planets probably do exist elsewhere in the universe beyond our solar system. In fact, using a huge antenna, called a radio telescope, astronomers have found evidence of several planets (one was the size of the moon) orbiting a star other than our sun. But no one has yet viewed these planets. Considering that the stars we now see appear to be little more than points of light, it will be a long while before we have telescopes powerful enough to see these and any other planets in other solar systems. Those that argue for the existence of other planets make two main points. First, that of the billions upon billions of stars in space, some if not all must have formed the way our Sun formed with leftover material later developing into planets. Among the vast numbers of stars that took shape this way, by sheer chance, some must have also produced orbiting planets like those of our solar system. Second, almost nothing in the natural world is unique. Therefore, if the Sun has planets around it, there is a good likelihood that other stars do as well. For several years, a number of astronomers have been trying to make contact with living things on planets outside our solar system. They have sent up rockets and radio signals in the hope that they will be received by intelligent beings who would respond. Meanwhile S4E2a, b, c, & d 28 Background

4 scientists use radio telescopes to listen for organized radio signals. Perhaps some day, a tour of the planets will include the planets of another solar system. How does the moon move? The moon spins. It spins more slowly than the Earth. It takes about 28 days for the moon to turn around once. It takes the Earth just 24 hours to turn around once. A day on the moon is about two weeks long. How would you like to have a night two weeks long? The moon moves in another way, too. The Earth s pull helps keep the moon traveling in a path around the Earth. The moon travels around the Earth once in 28 days. Because the moon spins once in 28 days and takes 28 days to travel around the Earth, we always see the same side of the moon. Why does our Moon change shapes and sometimes seem to disappear? The moon is the brightest light visible in our night sky, yet it produces no light of its own. It only reflects light given off by the Sun. The sun is a star; and like other stars is composed of burning gases which emit light. This light reflects off the moon and other planets in our solar system, allowing us to see them at night. The phases of the moon are a result of the moon s position relative to the Sun as it orbits the Earth. The moon is said to be a new moon when its orbit places it between the Sun and the Earth. The next several nights, the slice of the moon that is visible is called a crescent moon. It is called a waxing crescent until the moon has completed one-quarter of its 28 day orbit. Even though at this point one-half of the moon is visible to us, the phase is called the first quarter moon. As the moon continues its orbit between the first quarter and full moon stages, it is called a waxing gibbous moon. The moon is considered full when it has traveled half way around the Earth. At this point, the Earth is between the moon and the Sun, so we see a full circle illuminated in the night sky. The moon is visible at this stage because of the relative positions of the Sun, moon, and Earth. Over the next several days, the moon is in the gibbous stage and is said to be waning. When the moon has traveled three-quarters of the way around the Earth, it is called the last quarter moon. The last few days of the moon s orbit are called the waning crescent moon. When the moon has completed its orbit, it becomes a new moon once more and the cycle continues. (If you were on the moon, you would see the Earth go through phases!) What have scientists learned about the moon? The moon, Earth s only natural satellite, is about one-quarter the size of Earth. Its orbit is elliptical, just like those of the other bodies in our solar system. The moon is our closest neighbor in space about 240,000 miles (384,000 kilometers) away. Long ago, people had some strange ideas about the moon. Science fiction stories were written about travel to the moon. Eventually, unmanned space craft were sent; we learned a lot from the pictures they sent back. Then on July 20, 1969, science fiction became a reality. As part of the Apollo 11 mission, astronauts Neil Armstrong and Edwin Aldrin, Jr. landed on the moon! Third crew member Michael Collins remained in the command module. S4E2a, b, c, & d 29 Background

5 Armstrong, the first human to set foot on the moon, described its surface as fine and powdery. He and Aldrin set up machines to send back information to Earth. They also gathered rocks and moon dust for scientific study. From Apollo 11 and other missions, we ve learned a lot about the moon. It s a gray and lifeless place---a place of high, sharp mountains and broad, flat plains. Some of these plains are called maria, or seas, although there is no water on them. It s also a place of long, narrow, trenchlike valleys, called rilles. Many craters dot the surface. Some of the craters may be the result of volcanoes whose tops were blown off or collapsed. Others are probably the result of asteroids, meteors, or comets which hit the moon s surface. From the rock samples brought back by astronauts we know that moon rocks have a slightly different composition from those on Earth. Earth s crust and the moon s crust were learned to have similar amounts of oxygen, silicon, and aluminum. The moon s crust, however was shown to have more iron, and titanium, but less alkali metals, carbon, and nitrogen. No water was found on the moon. In fact, the only hydrogen found was carried in by the solar wind. The atmosphere of the moon is very thin; therefore, the sky is always black. There is no oxygen in the atmosphere. That s one of the reasons astronauts have to wear space suits while on the moon. Another reason is to protect themselves from the extremely hot daytime temperatures and the extremely cold nighttime temperatures. They also have to carry devices to communicate with each other, for there is no air to carry sound. S4E2a, b, c, & d 30 Background