HOW COME? Seeing pictures in the night sky

Why do stars form pictures, like a Big Dipper? asks a readerThink of the night sky as a giant connect-the-dots drawing, with each star a dot. Except that there's no right or wrong way to connect them. The pictures the dots form can be yours alone -- just like those you see in puffy clouds on sunny days.

Of course, there's a difference between stars and clouds.

Clouds are collections of water vapor, constantly shifting and re-forming. That pug you see one minute is a Beluga whale the next.

Stars, on the other hand, are faraway suns. Their enormous distance makes their positions appear fixed in the sky. So as the Earth turns, star patterns remain the same, for as long as they're visible during the night.

While you can dream up your own constellations, chances are you'll be renaming groups of stars people have been describing for thousands of years. Throughout history, when people saw pictures in the night sky, they saw what interested them. For example, European sailors saw stars patterned in the shape of a compass. (In fact, astronomers say that one of the most important uses for star pictures was to help people navigate the featureless sea -- for example, looking for the northerly Big Dipper.)

And the pictures could be wildly creative. In a big group of stars surrounding the Dipper, ancient Egyptians saw a bull, a reclining man, and a hippopotamus walking on two legs -- carrying a crocodile on his back.

Modern astronomy divides the sky into 88 separate areas, each represented by a named constellation. These include Orion's starry belt, Leo the lion, a giraffe, a chameleon, a dolphin, a telescope, a painter's easel, and a winged horse.

Each half of the Earth sees a different night sky. The Big Dipper is the most familiar constellation in the Northern Hemisphere, easy to recognize and visible in the northern sky all night in most of the United States. In the Southern Hemisphere, the best-known is the kite-shaped Southern Cross.

But you don't have to cross the equator to see the other hemisphere's most familiar constellations. In the U.S., in the Florida Keys and Hawaii, look for the Southern Cross just above the southern horizon. In northernmost spots in the Southern Hemisphere, you'll find the Big Dipper resting on the northern horizon -- upside down.

Stars are always being born or are dying, and constantly moving through space. So over millennia, constellations change. A million years ago, the Big Dipper looked like a long spear.