Exploring the Solar System: A History with 22 Activities

Exploring the Solar System: A History with 22 Activities — page 8 — bright dots But you would have miscounted by a few That's because some of the very brightest dots aren't actually stars The ones that shine without twinkling are really planets Depending on when you look and how much city light is around you can see the planets Mercury Venus Mars Jupiter and Sat urn with just your eyes WONDERING ABOUT WANDER ERS In ancient times the Chinese Babylonians Greeks and Egyp tians recorded their observations of stars They noticed that five stars were different from the thousands of others they didn't twinkle They also noted that these brightly shining stars seemed to move differently too On most nights these five wandering stars travel from east to west But they show up in different places on the starscape from one night to the next And their speed and direction change too Sometimes they move quickly but other times slowly or even stop then go backward The odd movements of the wandering stars seemed purpose ful or intelligent to some ancient cultures Many believed that the wanderers were gods moving back and forth as they went about their heavenly business The five wandering stars are of course not stars at all They're the planets Mercury Venus Mars Jupiter and Saturn They seem to wander across the night sky because unlike stars planets really do move Planets don't twinkle like stars because planets are so much closer to us The strong steady light of the nearby Sun reflects off a planet s entire lit side caus ing it to shine a beam of light toward Earth By comparison from Earth faraway stars look like single points of light Those tiny points of weak starlight get bounced and blurred coming through Earth's atmosphere That s what causes stars to twin kle The nearest planet Venus is 67 million miles 108 million km from Earth That seems far but not compared to the nearest star Alpha Centauri It's 25 trillion miles 40 trillion km away That's the difference between walking a single step and hiking across the state of Indiana These five planets are not a part of the unchanging starscape background They're part of our solar system Everything in the solar system planets and their moons

Exploring the Solar System: A History with 22 Activities — page 9 — dwarf planets asteroids and comets travels around the Sun But each planet revolves or orbits around the Sun at its own uneven pace all while the Earth doing the Looking is same at moving planets from a world that is also on the go makes for some odd tricks of perspective It s like watching a truck as you're passing it on the highway The truck can look like it s standing still or even slipping backward but it isn't really Your car is just moving faster and passing it by see Why They Wan ler page 2 Why They Wander west The planets can appear to slow down stop and change direction against the background of the unchanging starscape This is because the closer the plan ets are to the Sun the speedier their orbit This diagram shows how Mars looks like it's moving backward each time the faster moving Earth passes it up The looping path on the right shows how Mars's travels look from Earth Sometime around the sixth century B C ancient Greek scholars decided that the five wandering stars were not really gods who were out for heavenly strolls The scholars began to carefully chart the paths of the planets create tables of measure ments and work on ideas that would explain the planets move ments They were some of the world's first astronomers FINDING THE COSMOS S CENTER By the second century ancient scholars had come up with an explanation of how the planets moved that didn't involve gods It was hammered out by a Greek astronomer

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