Exploring the Solar System: A History with 22 Activities
Prehistory 1900 Spying on the Heavens he next time you're outside on a clear night look up You won't be the first person to marvel at the Moon and stars Studying the lights in the night sky is some thing that humans have always done People have used recognizable star patterns called constellations to mark the passing of time for thousands of years Ancient peoples used star calendars to help time crop plantings and to move to new hunt ing grounds as the seasons changed The night sky’s pattern of stars or starscape is like a back ground of lights out in space Our view of the starscape shifts during the year as the Earth travels around the Sun The Big Dipper for example appears handle up in the sky during the summer and handle down during the winter But the Big Dipper always keeps its ladle shape because it's not the stars but Earth that is moving This changing view allows us to use the constel lations as a kind of calendar The bright evening star near the Moon isn't a star at all It's the planet Venus If you counted all the stars you could see while looking up at the night sky you'd get to about 3 000 before running out of
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
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
and geographer working in the great Egyptian city of Alexandria His name was Ptolemy TALL uh me activity Spy the Evening Star There are five planets visible to the naked eye But Venus is by far the easiest to see Often called the Evening Star Venus is the third brightest object in Earth's sky after the Sun and the Moon Look for Venus around sunrise or sunset not in the mid dle of the night It will appear close to the horizon near the Sun Remember never look directly at the Sun When and where Venus appears in the sky depends on where it is in its orbit around the Sun Check a night sky calendar in a magazine about astronomy or telescopes in the weather section of many news papers or on a sky calendar Web site see page 165 If you have a pair of seven power 7 or stronger binocu lars you can see Venus change shape over time You can even track the shapes Venus goes through called phases and prove that Venus orbits the Sun just like Galileo did Just sketch Venus's shape night after night and see how it changes phases Hope for clear
