CHAPTER 22 THE SUN and ITS SOLAR SYSTEM II. Observing the Solar System

Topic 7: The Solar System (a) The sun’s family is called the solar system. It includes 9 planets, at least 61 natural satellites (moons), millions of asteroids, millions of meteroids, and many comets. All travel around the sun at very high speeds in paths called orbits.

Topic 7: The Solar System (b) Mercury, Venus, Mars, Jupiter and Saturn are planets that can be seen from Earth with the unaided eye. Meteors can also be regularly observed with the unaided eye.

Topic 8: Planets and Stars (a) Over the lifetime of an observer the positions of stars relative to each other (and therefore the shapes of constellations) do not noticeably change. The position of the planets among the constellations, however, change constantly. The planets appear to move through the stars because they are much closer to us.

Topic 8: Planets and Stars (b) Most of the time the planets move eastward in front of the background of the constellations, but they periodically make westward loops called retrograde motion. These loops occur because each planet orbits the sun at a different speed. Whenever Earth overtakes and passes another planet, that planet appears to move backward, or westward, among the stars.

Topic 9: Solar System Models (a) A universe with Earth at its centre is called a geocentric, or earth-centered system. (b) The Greek astronomer Ptolemy (AD 140) developed an earth-centered model that he used to predict the locations of the planets. He imagined the planets in small orbits, called epicycles. The center of each small orbit moved around Earth in a larger orbit called a deferent. Retrograd motion occurred when the planet made a trip around the epicycle.

Topic 9: Solar System Models (c) Copernicus is credited with proposing a heliocentric, or sun- centered, solar system. The planets revolve around the sun in this model and epicycles are no longer needed to explain retrograd motion. Retrograd motion will occur whenever Earth passes another planet.

Topic 9: Solar System Models (d) The heliocentric system provides a much simpler explanation of the observed motions in the sky. This system marked the beginning of our modern understanding of the structure of the universe.