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
