Next Saturday marks the anniversary of the day when two U.S. explorers became the first humans to reach the North Pole. Robert E. Peary and Matthew Henson (along with four Eskimos they had hired) traveled by dog sled over hundreds of miles of snow and ice, and braved temperatures of 50 degrees below zero and winds of hurricane force for seven grueling months, but on April 6, 1909, they arrived at the "top of the world."

Did you ever wonder how they knew that they had reached their destination and could stop looking? I mean, there is no physical "pole" or marking of any kind; in fact, the North Pole isn't even on land, just a floating pack of ice. So how did they know exactly where it was? First, they had to know what it was, and so should we.

The "pole" that Peary and Henson were looking for was the North Geographic Pole, which is the northern end of the Earth's axis, and is the point at which all the Earth's lines of longitude meet. This north pole is invisible, but it is not imaginary, for there is a point in the arctic (and one in the antarctic) on which you could stand and have the entire planet Earth turn below your feet. (Having a globe of the Earth on hand is a great help in explaining and understanding concepts like poles and lines of longitude.)

So, why didn't Peary and Henson just follow a compass needle right to the pole? Because magnetic compasses point to the North Magnetic Pole, which has nothing to do with the North Geographic Pole or the Earth's axis. In fact, the North Magnetic Pole is about a thousand miles away from the Geographic Pole, and its position is constantly changing, moving many miles in just a few years.

So they must have determined their position by the stars, right? Well, in a way, yes. But remember that the North Pole experiences six months of daylight and then six months of darkness each year. If you happen to arrive there after the spring equinox (March 21), there will be no nighttime and, hence, no stars to guide you. Except one - the one we call the sun.

During the daylight months, the sun will appear to rise and sink in the sky, but it will never drop below the horizon. By measuring the angle of the sun over the horizon, and then measuring it again 12 hours later, and by knowing the correct date, you can figure out exactly where you are and whether you are at the North Pole.

Have your children try to imagine how the six months of night sky would appear from the top of the world. The star Polaris is called the North Star because it sits almost directly over the North Pole. This star, then, would be straight overhead - every day of the year - to someone standing at the pole. All the other stars would pivot around Polaris and swing around the sky in circles that are parallel to the horizon. No stars would rise or set as they do in the sky we see; instead, they would stay at a fixed height above the horizon all year 'round.

Can your children locate Polaris in the night sky? The easiest way is to look for the seven bright stars that make up the Big Dipper (which is part of the constellation called Ursa Major, the Great Bear). Draw a line between the two stars on the outside of the bowl, extend that line about five times its length (up from the bowl), and there you'll find Polaris, which will always tell you where north is. Now notice that Polaris is at the end of the handle of the Little Dipper (or Ursa Minor, the Little Bear).

The farther north you go on Earth, the higher Ursa Major and Ursa Minor will appear in the sky. The ancient Greeks named the most northern lands after their word for "bear," because the bear ("arktos") was overhead: the arctic.- Dr. William F. Russell's books for parents and children include "Classic Myths to Read Aloud." Send your questions and comments about Family Learning to him at P.O. Box 1279, Menlo Park, CA 94026.