Sun, Earth, and Moon

Transcription

1 CHAPTER 19 1 SECTION The Solar System Sun,, and Moon KEY IDEAS As you read this section, keep these questions in mind: Why does the sky look the way it does from? How is the solar system organized? How does the moon affect? What Does the Sky Look Like from? If you have ever watched a sunrise or a sunset, it may have seemed that the sun moved up or down. However, does not stay still as the sun moves through the sky. The sun appears to rise and set each day because rotates, or spins, on its axis. As rotates, different parts of s surface face toward or away from the sun. When an area is facing the sun, it experiences day. When it is facing away from the sun, it experiences night. In addition to rotating, also orbits the sun. As s position around the sun changes, the stars in the sky seem to revolve around. Therefore, the night sky looks different at different times of the year. The stars seem to move across the sky over the course of a year. THE CLOSEST STAR We often think of stars as things we see only at night. In fact, the sun is a star. We cannot see other stars during the day because the sun s light makes the sky too bright. Our sun may look brighter than all other stars, but it is actually a very ordinary star. It is not particularly hot or cool, and it is of average size. The sun looks so bright to us because it is much closer to us than other stars. READING TOOLBOX Summarize As you read this section, underline the main ideas. When you finish reading, write a short summary of the section using the underlined ideas. 1. Explain What is the main reason that the stars seem to move in the sky over the course of a year? If it were closer to, the sun would look much larger. Sun 2. Explain Why does the sun appear much brighter than other stars? Interactive Reader 415 The Solar System

2 3. Describe Name two reasons the sun s energy is important to life on. 4. Apply Concepts Saturn is farther from the sun than Mars. Which planet takes the longest to orbit the sun? THE SUN S IMPORTANCE The sun is the major source of heat and light for. Most organisms depend on the sun s energy for food, either directly or indirectly. Energy from the sun is also a main cause of weather patterns and ocean currents on. As rotates, we see the sun rise and set once every 24 hours. Many of our activities, such as waking up, eating meals at certain times, and falling asleep, follow the sun s cycle. Many other organisms also have patterns of activity and inactivity that follow the sun s daily cycle. What Objects Are Visible in the Night Sky? The planets and many stars are visible in the night sky. A planet is an object that orbits the sun, is round because of its own gravity, and has cleared other objects from its path. Eight planets orbit our sun. Planets closer to the sun orbit the sun more quickly than planets that are farther away. takes days, or one year, to orbit the sun. Mercury takes only 88 days. Neptune, the most distant planet, takes 60,225 days, or 165 years, to orbit the sun. CONSTELLATIONS From, some groups of stars appear to form the outlines of shapes. These groups are called constellations. Ancient Greeks gave names to many of the constellations. By studying the sky over many years, the ancient Greeks calculated that the stars are more distant than the planets. 5. Define What is a constellation? October 2007 This group of stars make up the constellation Leo. The ancient Greeks thought that these stars looked like the outline of a lion. Interactive Reader 416 The Solar System

3 What Is the Structure of Our Solar System? The solar system consists of the sun and all the objects that orbit it. The sun is the most important part of our solar system. It makes up almost 99% of the mass of the solar system. The planets and their moons make up most of the other 1%. The solar system also contains many smaller objects, such as asteroids, comets, meteoroids, dust, and gas. GRAVITY Remember that gravity pulls objects together. Every object in the solar system pulls on every other object. The force of gravity between two objects depends upon their masses and how far apart they are. The greater the masses of the two objects, the larger the gravitational force between them. The closer two objects are, the larger the gravitational force between them is. Because the sun s mass is so great, it pulls on all other objects in the solar system with the greatest force. The pull of the sun keeps and the other planets in their orbits. Gravity also keeps moons in orbit around planets. 6. Identify What object makes up most of the mass in our solar system? 7. Describe How do the masses of two objects affect the gravitational force between them? Without the pull of the sun, would fly off into space in a straight line. The pull of the sun s gravity curves s path and keeps in its orbit. Pull of gravity Forward motion 8. Explain Why does move in a curved path around the sun, instead of a straight line? Sun Resulting path (orbit) Imagine swinging a ball on a string in a circle. If you let go of the string, the ball will fly off in a straight line. Gravity is like an invisible string holding the planets in orbit around the sun. If there was no gravity, the planets would fly out into space. The figure at the top of the next page shows the orbits of the planets in the solar system. Interactive Reader 417 The Solar System

4 Sun Saturn Neptune 9. Identify Which planet is farthest from the sun? Jupiter Uranus The sun s gravitational force holds the planets in almost circular orbits. Planets far from the sun take a longer time to orbit the sun than planets that are closer. THE SIZES OF THE PLANETS Compare the sizes of the planets in the figure below. The diameters of some planets, such as Jupiter and Saturn, are much larger than the diameter of. However, these planets are still only a fraction of the size of the sun. In fact, the sun s diameter is almost 10 times greater than Jupiter s. Sun 10. Identify Name the smallest and largest planets in our solar system. Mercury Mars Venus Jupiter Saturn Uranus Neptune SATELLITES A satellite is an object in space that orbits a body that has a larger mass. For example, the moon is a satellite of. The gravitational force between a satellite and the larger object keeps the satellite in its orbit. Six of the eight planets in the solar system have at least one satellite. Only Mercury and Venus do not have any known satellites. Interactive Reader 418 The Solar System

5 What Are the Features of s Moon? As you can see in the photograph below, the moon s surface has many light and dark patches. Many of the light patches are craters. These craters formed when rocks from space collided with the moon. The large dark patches on the moon are called maria. Maria formed when lava flowed out of the moon s interior, filled large craters, and cooled to form solid rock. The dark parts of the moon are maria. The lighter parts are highlands and craters. 11. Describe How did the maria on the moon s surface form? 12. Identify Label some of the maria that are visible in this photograph of the moon. Why Does the Moon Have Phases? The moon appears to have different shapes throughout the month. The different appearances of the moon are called phases. The relative positions of, the moon, and the sun determine the phases of the moon. You can see this in the figure at the top of the next page. At any given time the sun lights up, or illuminates, half the moon s surface. As the moon revolves around, we see different amounts of the illuminated part of the moon. Sometimes, we see the entire illuminated half of the moon. Other times, we see part or none of the illuminated half. When the moon is full, the half that is lit is facing you. When the moon is new, the side of the moon that is dark is facing you. The moon revolves around in 29.5 days, or about one month. Therefore, the time it takes for the moon to pass through all of its phases is about 29.5 days. 13. Describe What causes the phases of the moon? 14. Infer About how long is it between two full moons? Interactive Reader 419 The Solar System

6 Waxing gibbous First quarter Waxing crescent 15. Describe What fraction of the illuminated side of the moon do we see during a first-quarter moon? Full moon New moon Waning gibbous Waning crescent Last quarter The positions of the moon, sun, and determine which phase the moon is in. The photographs show how the moon looks from at each phase. 16. Define What is an eclipse? 17. Explain Why can t everyone on observe a solar eclipse? What Is an Eclipse? As the moon revolves around, revolves around the sun. Sometimes, the sun, and the moon line up in a straight line. When this happens, observers on see an event called an eclipse. An eclipse happens when one object in space moves into the shadow of another object. During a new moon, the moon may come between and the sun and cast a shadow onto parts of. People in these parts of see the sky go dark as the moon blocks their view of the sun. This is called a solar eclipse. Penumbra Umbra Sun s rays When the moon is full, it may pass into the shadow of. blocks the light of the sun from reaching the moon. To observers on, the moon is blocked from view. This event is called a lunar eclipse. Interactive Reader 420 The Solar System

7 Umbra Sun s rays 18. Describe What happens during a lunar eclipse? Penumbra UMBRA AND PENUMBRA Look at the regions labeled umbra and penumbra in the previous two figures. During an eclipse, observers in the umbra will see a total eclipse. Observers in the penumbra will see a partial eclipse. No one outside the penumbra will see an eclipse. What Causes Tides? The gravitational pull between and the moon is the main cause of tides on. The pull of the moon s gravity is strongest on the side of nearest the moon. On this side of, water in the oceans is pulled toward the moon. This creates a bulge of water. The ocean water on the opposite side of bulges away from the moon. Therefore, has two tidal bulges on opposite sides of its surface. Because rotates, the water level at coastal areas changes over the course of a day. This occurs as the water at these areas bulges because of the moon s gravity. Times when the water level is highest in an area are called high tides. Times when the water level is lowest are called low tides. Because the moon orbits, the times of high and low tides in an area change throughout the month. 19. Compare How is the umbra different from the penumbra? Orbit of moon Moon 20. Define What is a high tide? The change in the water level between high tide and low tide can be very large. Low tide Pull of moon High tide Average sea level Interactive Reader 421 The Solar System

8 Section 1 Review Section Vocabulary eclipse an event in which the shadow of one celestial body falls on another phase in astronomy, the change in the illuminated area of one celestial body as seen from another celestial body; phases of the moon are caused by the changing positions of, the sun, and the moon planet a celestial body that orbits the sun, is round because of its own gravity, and has cleared the neighborhood around its orbital path satellite a natural or artificial body that revolves around a celestial body that is greater in mass solar system the sun and all of the planets and other bodies that travel around it 1. Explain Why do positions of objects in the sky change over time? 2. Infer Jupiter has a much greater mass than. Why do we not notice the pull of its gravity? 3. Describe What is the basic structure of the solar system? 4. Describe Identify two features of the moon and describe how each formed. 5. Identify What causes a solar eclipse? 6. Identify What is the role of the moon in forming tides? Interactive Reader 422 The Solar System