Everything Kids' Astronomy Book Read Online Free PDF Page B

years ago, Hipparchus developed a numbering system, one through six, based on the apparent magnitude or brightness of the stars he saw from Earth. A first or number one magnitude star was 100 times brighter than the faint stars in the sixth class. Over time astronomers have found some objects can be brighter than a one, so they started using a minus sign to indicate how much light there seemed to be. Do you think there are numbers higher than six? With modern telescopes, stars can be listed up to a thirty! Hip-parchus might have been surprised to see astronomers still use the information from his catalog today. Newer books have changed the amounts of a few of his numbers and many, many more stars have been added to his list. Suppose you and your friends are walking after dark and you see a light shining in the distance.
    WORDS to KNOW
    MAGNITUDE: The amount of light a star emits or gives off is called its magnitude. Astronomers measure the magnitude of the stars out in the universe by using a number system.
    It could be a streetlight or a flashlight. How bright it is doesn’t tell you much about the light, if you don’t know how far away it is. Modern astronomers have learned how to measure the distance to a star and its apparent magnitude, or how bright it appears to us. They use both of these figures to determine the absolute magnitude or luminosity (true brightness) of a star. Scientists pretend that the sun and the other stars are all the same distance from the earth to figure their absolute magnitude. The sun has an apparent magnitude of –26.7 and absolute magnitude of 4.8. See what a difference a little distance makes!

    Connecting the Dots
    Have you ever seen what looks like animals or people hiding among the stars in the constellations? Ancient stargazers like the Chinese, Egyptians, Greeks, and people living in the Middle East saw some of these same creatures. Some believed they were gods and made up wonderful stories about them that are still retold today. Each of these groups of people had different names for the same clusters of stars or constellations. Sometimes some parts of one constellation were grouped with part of another, and many were never seen because none of the stargazers lived in the Southern Hemisphere. You might not be able see all of them, either. Eventually it was agreed that there were eighty-eight constellations in all, but many of the shapes had been changed. Do you really think Cygnus looks like a swan or Canis Minor and Major look like a pair of dogs? Although the stars that form the figures appear to be close in certain constellations, they may actually be billions of miles apart. And because they are moving, they might be in other constellations eventually. Like the imaginary celestial equator above Earth that helps astronomers locate objects in space, there is also an imaginary circle called an ecliptic. This circle is where the eclipses of the sun and the moon occur. Some of the constellations seem to move along this circle. This circle swings from 23.5 degrees north of the equator to 23.5 degrees south of the equator as the earth orbits around the sun. Would you call the Big Dipper a constellation? To most astronomers it is an asterism, or just part of the constellation known as the Great Bear or Ursa Major!
    WORDS to KNOW
    CONSTELLATION: A constellation is a group of stars that together form an image in the sky. One constellation that is well known is Orion.
    How the Constellations Got Their Names
    Have you read a lot of fairy tales or watched them on TV? Most of these stories were written for children to enjoy, but they were also written to teach children good behavior and keep them safe. Their lessons were things like: don’t talk to strangers, be honest and kind or you might be punished. Ancient storytellers told stories about what they thought they saw in the sky, from mighty hunters like Orion to magical animals like unicorns and winged horses, or even things like a