reflect If you look up at the sky on most nights, you ll see the Moon. You may have noticed that sometimes the Moon looks like a circle. Other times it looks like a half circle. On some nights, you can t see it at all. orbit: to travel in a repeating path around an object Earth orbits the Sun as the Moon orbits Earth. Even though it looks different, the Moon does not really change. It looks like it changes because it orbits Earth. The Moon orbits Earth while Earth orbits the Sun. How can you show the orbits of the Moon and Earth? They are so big that it could be difficult to show. What other objects in the Solar System follow orbits? How do the Moon and Earth move through space? Both the Moon and Earth are spheres, or balls, of rock. Unlike the Moon, Earth is a planet. A planet is a large, sphere-shaped object that orbits a star. The star that Earth orbits is called the Sun. The Moon is a satellite, which is a natural object that orbits a planet. The Moon orbits Earth about once every 28 days. That is about one month. Earth s orbit around the Sun is much longer than the Moon s orbit around Earth. How do we know that? Well, it takes about one year for Earth to orbit the Sun. look out! The Moon looks bright in the night sky, but it does not make its own light. It is lit up by the Sun, meaning it reflects the Sun s light back to us. The half of the Moon facing the Sun is always lit up. As the Moon orbits Earth, you can see different parts of the lighted half. The Moon looks like it changes throughout the month as the Moon moves into different positions. But it s really our view of the Moon as it moves that makes it seem to change shape! As the Moon orbits Earth, the shape of the Moon looks different to people on Earth. They can see different parts of the lighted half of the Moon. 77
How can we use models to represent the orbits of the Moon and Earth? model: an idea or a physical structure that describes or represents something else Keep in mind that no model can show Earth, the Sun, or the Moon perfectly. Earth is much bigger than the Moon. About four Moons could fit inside Earth. The Sun is even larger. About one million Earths could fit inside the Sun. It would be very difficult to choose two perfect objects to model Earth and the Sun. The object representing the Sun would have to be one million times larger than the object representing Earth. Your object for Earth would be too tiny to see, or your object for the Sun would be too large to use easily. Studying how Earth, the Sun, and the Moon move can be a little tricky. After all, you can t place them on your desk to look at them. What would happen if you tried to bring the Sun into your classroom? Pretty difficult, right? One useful way to study them is by making a model. You can use everyday objects to represent each space body and show how they move. To make a good model, you need to choose objects that are useful for showing what you want. Suppose you decide to make a model out of balls. You would need to choose the largest ball to represent the Sun. Maybe you would use a beach ball. Then you would need to choose a smaller ball to represent Earth. You might use a baseball. You should use an even smaller ball to represent the Moon. A table tennis ball would be handy. Think about other types of balls that could show these differences in size. Now you need to consider distance. The Moon is much closer to Earth than the Sun is. If you put the object representing the Sun too far away, your model will be too big. You should place the Sun as far away from Earth as you can and place the Moon closer to Earth. You must find a way to show that the Sun is farther away. Even though your distances are not exact, your model will still help show the patterns of how these objects move. Whenever you make a model, you need to know that the model has limitations. It won t be exactly like the real thing, but it can still be very useful. limitation: a factor or problem that prevents full use of something 78
try now Try making your own model. 1. Gather materials that you can use to make a model of the Sun, Earth, and the Moon. Consider the following ideas, or come up with your own. foam balls sports balls clay 2. Arrange the objects to represent the Sun, Earth, and the Moon. 3. Show how the objects move. Describe the model in words. what do you think? This picture shows a model a student made of the Sun, Earth, and the Moon. What are some features of the model that make it useful? What are some limitations of the model? a lamp foods (marshmallows, candies, fruits) balloons What are the eight planets of our solar system, and where are they located? Earth isn t the only planet to orbit the Sun. There are eight planets in our solar system. Each planet orbits at a different distance from the Sun. Mercury is the closest planet to the Sun. It is also the smallest planet. The next planet from the Sun is Venus. This planet is sometimes called Earth s twin because it is about the same size as Earth. Venus is much hotter than Earth, though. solar system: the Sun and any objects, including planets, moons, and asteroids, that orbit it Earth comes next. It is the third planet from the Sun. The distance between Earth and the Sun is just right for living things. If it were closer, Earth might be too hot. If it were farther away, Earth would get too cold. Mars is the planet that comes after Earth. It is known as the Red Planet because its In order from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Notice how much larger the four outer planets are. Yet even the largest planet, Jupiter, is small compared to the Sun. The speck to the right of Neptune is Pluto. Most scientists now consider Pluto a dwarf planet. 79
soil makes it look red. Jupiter is the fifth planet from the Sun. It is the largest planet. Saturn comes after Jupiter. It is known for the rings that go around it. Uranus is located beyond Saturn. It glows green. The farthest planet from the Sun is Neptune. It looks blue from Earth. For a long time, scientists described the Solar System as having nine planets. The ninth planet was Pluto. It is a tiny sphere located very far from the astronomer: a scientist who studies space and the objects in it Sun for most of its orbit, it is farther away than Neptune. In 2006, astronomers met to discuss Pluto. They decided that Pluto really didn t have all the characteristics of a planet. Pluto and two other spheres were reclassified as dwarf planets. Because the scientists were gathering evidence they may not have had in the past, they agreed that the change made sense. The new evidence allowed them to draw new conclusions. Scientists in the Spotlight: Neil de- Grasse Tyson Changing Pluto from a planet to a dwarf planet was not an easy decision for scientists. People were used to thinking that there were nine planets. Many people did not want the change. One of the scientists who played an important role in the decision was Dr. Neil degrasse Tyson. Dr. Tyson has been gazing at the sky since he got his first telescope as a young child. Today, he is a scientist who studies how objects in the universe move and interact. Not only does Dr. Tyson know a lot about the universe, Pluto has one large moon, Charon, and two smaller moons, Hydra and Nix. but he has a talent for sharing that information. Dr. Tyson can describe some of the most difficult ideas about space in ways that are easy for students to understand. What is his secret? Let em play, he said in a 2004 interview. When my daughter was two and she poured her cup of milk on the dining table and watched it drizzle and then drip down to the floor, she was performing experiments. Dr. Tyson helped students realize that just by watching and observing carefully, they were true scientists already. 80
try now Suppose someone asked you to quickly name the fifth planet from the Sun. You might have to think about it for a moment. There are some fun ways to remember the order of the planets. For example, take a look at this sentence: My very educated mother just served us nachos. What Do You Know? The diagram on the next page shows the Sun and the orbits of the planets. Draw the correct planet on each orbit. Write the name of the planet and its position in order from the Sun. Draw smaller pictures to show smaller planets and larger pictures to show larger planets. Include any other details you might know about each planet. The first letter of every word is the first letter of a planet name. So you have Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are lots of different sentences like this that people have made up to remember the order of the planets. Try coming up with one of your own. 81
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connecting with your child A Living Model Work with your child to model the motions of Earth and the Moon. Invite another family member to join you if possible. You won t need to gather any materials in advance because you will be the materials for this activity! 1. Choose an open space in a room inside your home or outside. Clear any objects or furniture from the space. The space does not have to be huge, but it should be large enough so you can walk in a circle without bumping into anything or tripping. 2. Choose who will represent Earth and who will represent the Moon. If you have another family member available, he or she can be the Sun. If not, set up a lamp as the Sun in the center of the area. 3. Whoever is Earth should stand in position in orbit around the Sun. 4. Whoever is the Moon should stand in position in orbit around Earth. 5. When everyone is in position, slowly model the motions of Earth and the Moon. Be careful not to move too quickly. You have probably seen models of the Solar System made from foam balls or even candies. In this activity, you and your child are the model. The goal of this model is to represent the orbits of Earth and the Moon. To create a good representation, you should use something or someone to represent the Sun. The Sun is the star at the center of the Solar System. The planets and everything in the Solar System travel around the Sun, so the Sun will stay in one place in the center of your model. Earth travels around the Sun. To travel around another object is to orbit it, and the path traveled is also called an orbit. The person acting as Earth will have to walk slowly around the Sun. Earth orbits in a counterclockwise direction, so the person should walk that way as well. In real life, it takes Earth one year to complete this motion. The shape of Earth s orbit is actually an ellipse, which is like a circle that has been stretched. For the purpose of the model, you can consider the orbit to be a circle. As Earth orbits the Sun, the Moon orbits Earth. The person acting as the Moon should walk slowly around the person acting as Earth. As with Earth, the Moon moves in a counterclockwise direction. The Moon takes about 28 days to complete one orbit, so the person acting as the Moon should walk a little faster than the person acting as Earth. Everyone should be careful as they walk so as not to trip over one another. Try to make one complete orbit for Earth. (The Moon should make about 12 orbits around Earth in this time.) 83
Here are some questions to discuss with students: How did we model the orbits of Earth and the Moon? How are the orbits of Earth and the Moon alike? How are they different? About how many orbits does the Moon complete during one complete orbit of Earth? In what ways was the model a good way to represent the orbits? What are some things you could do to improve your model? What are some limitations of any kind of model used to represent objects in space? 84