To correct the observations with the sextant and the inaccuracy of the chronometer, etc., are a part of the business of the navigating officer, and he is provided with tables and things to make this work as easy and certain as is possible. =How to Know When You Are at the North Pole.=—If you should ever reach the north pole where overhead is north and every other direction is south how would you know it? Suppose you were standing on the exact top of the world during one of its long polar nights, then the North Star would be directly over your head and the Big Dipper and Cassiopeia would serve to mark the passing of days as well as of nights, that is, if you were at the north pole in the wintertime. [Illustration: FIG. 100.—SHADOWS AT THE NORTH POLE.] But if you were there during a long Arctic day, that is, in the good old summertime, you could see the Sun making a circle, or seeming to, once in 24 hours, parallel to the horizon, and never going higher or getting lower. Explorers use a sextant to find out when they are at the north pole, and they sight the Sun’s height above the horizon at morning, noon and night. If the angle the Sun makes with the horizon is the same every time he is observed, the explorer knows he is standing on the very point round which the Earth turns. Then he plants a stick in the snow and ice on the north pole, hoists the American flag, and hurries home as fast as dogs and ships and trains can carry him to tell about it. If you ever reach the north pole you can know it, too, even though you haven’t a sextant with you. All you need to do, when you think you are standing on the north pole, is to notice the length of your shadow five or six times in 24 hours. If the length of your shadow is exactly the same every time you look at it, as shown in Fig. 100, you are really and truly at the north pole. =Difference Between the True North Pole and the Magnetic North Pole.=—The compass and dipping needle do not point to the _true north pole_, for the _magnetic north pole_ and the true north pole are not located at the same place. The magnetic north pole was found by Captain Ross in 1832. At that time the magnetic north pole was northwest of Hudson’s Bay in about latitude 70 degrees north and longitude 96 degrees and 45 minutes west, that is, west of the zero meridian which runs through Greenwich. CHAPTER VI THE MOON, THE EARTH’S DAUGHTER =How the Moon Was Made.=—There was a time away back in the beginning of the planets when the Earth did not have a Moon. Two ideas among others have been worked out to account for the birth of the Moon and these are somewhat alike, for both agree that just as the Earth was once a part of the Sun and was separated from the Sun and became a planet so the Moon was once a part of the Earth and was thrown off and became her daughter. [Illustration: FIG. 101.—MOON AND EARTH JOINED TOGETHER LIKE A DUMBBELL.]