Physical Science 8th grade (1).pdf

This observation proved that not all objects revolve

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orbiting Jupiter. This observation proved that not all objects revolve around Earth. planet - a massive collection of matter that revolves around a star. 15.1 The Solar System Ancient observers noticed that five bright objects seemed to wander among the stars at night. They called these objects planets , from the Greek word meaning “wandering star,” and named them Mercury, Venus, Mars, Jupiter, and Saturn. In A.D. 140, the Greek astronomer Ptolemy “explained” that planets and the moon orbited Earth. For the next 1,400 years, people believed those ideas, until science proved Ptolemy wrong. How the solar system was discovered Planets shine by reflecting sunlight Today we know that planets are not stars. Stars give off their own light. We see the planets because they reflect light from the sun . For example, Venus appears as a crescent like the moon, becoming dark at times. This is because Venus does not give off its own light. When Earth is on the same side of the sun as Venus, we see Venus’s shadowed side (Figure 15.1 top). The phases of Venus were discovered by Galileo in the 1600s and were part of the evidence that eventually overturned Ptolemy’s model of the solar system. Changing ideas about the solar system Almost 100 years before Galileo, Polish astronomer Nicolaus Copernicus had proposed that the planets orbited the sun, but few believed him. Then came Galileo, using a telescope he built himself to make two discoveries that strongly supported Copernicus’s ideas. First, he argued that the phases of Venus could not be explained if Earth were at the center of the planets (Figure 15.1). Second, he saw that there were four moons orbiting Jupiter. This showed that not everything in the sky revolved around Earth. Discovery of the outer planets The three most distant planets (Uranus, Neptune, and Pluto) are far from the sun and don’t reflect much light back to Earth. These planets were not discovered until telescopes became large enough to see very faint objects. Pluto is so far away that even today we have only a blurry image of it. Astronomers believe that thousands of objects, some of them bigger than Pluto, orbit the sun farther out, in the Kuiper Belt . These objects reflect so little sunlight that the two largest ones have only just recently been discovered.
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313 15.1 T HE S OLAR S YSTEM C HAPTER 15: T HE S OLAR S YSTEM Organization of the solar system The sun, planets, and other objects Today, we define the solar system as the sun and all objects that are gravitationally bound to the sun. The gravitational force of the sun keeps the solar system together just as gravity keeps the moon in orbit around Earth. The traditional solar system includes nine planets and their moons (also called planetary satellites ), and a large number of smaller objects (asteroids, comets, and meteors).
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