Jori Ann Mae Reed, age 10, of St. Catherines, Ontario, Canada, for her question:
Can explorers tell when they cross the Arctic Circle?
A trained explorer carries a special kit to locate his exact position on land or sea. He can use these gadgets to figure out precisely when he crosses the Arctic Circle, even in mid ocean with no landmarks in sight. But let's suppose he had a mishap and lost all his equipment. If he remembered his homework, he could figure out more or less where he was and which way to go. For example, after sunset the North Star can help him locate the Arctic Circle.
Jori Ann wonders whether an ocean explorer can tell if he has gone past the line of the Arctic Circle. If he uses his ship's clock, charts and sextant he knows exactly when he crosses the line and also how many miles east and west he is from the nearest land. But let's make him solve the problem in his head, without the help of these instruments. His figuring might be a few miles off, but he would know more or less when he crossed the line.
Chances are, he would wait until he could see Polaris, the North Star. Then he would figure how high it was in the sky. Being a trained pilot, he sees the sky as a huge dome. Its highest point is the zenith, directly overhead. Its rim rests around the horizon, where the sky meets the sea on every side. He measures the big dome in degrees. It is handy to know that the width of the full moon equals about half a degree of sky distance. If you hold a pencil at arm's length and squint, it covers the face of the full moon. Two of these widths equal about one degree of sky distance.
Our explorer looks up and pictures circles and parts of circles. These lines divided the sky into sections. For example, around the horizon, neatly spaced lines reach up and meet overhead at the zenith. Each one is a quarter circle that measures 90 degrees of sky distance. This clever plan helps him to measure the height of a star. Its distances above the horizon and below the zenith must total 90 degrees. When Polaris is 23 1/2 degrees below the zenith it must be 66 1/Z degrees above the horizon. This is when our explorer knows he is crossing the Arctic Circle.
A pilot matches the sky degrees to the latitude and longitude lines on the globe. The North Pole is a pinpoint at Latitude 90 degrees North of the equator. It is directly below the North Pole on the Big Dome. The Arctic Circle is about 1,630 miles south of the pole, along Latitude 66 1/2 degrees North of the equator. In the sky, it is 23 degrees from the Celestial North Pole, which is just about where Polaris hangs above the earth.
As our explorer sails onward to the North Pole, Polaris appears higher in the sky. When he reaches his destination, the zenith matches the North Pole fn the sky ¬and Polaris is directly overhead. On the return trip, Polaris moves down in the sky. If he visits Saint Catherines, or any other place near Latitude 45 degrees, the faithful North Star appears 45 degrees above the horizon.