Next get a strip of iron or brass ¼ inch wide, ¹/₁₆ inch thick and 13 inches long; drill a ¹/₁₆ inch hole through each end and a ⅛ inch hole 4 inches from one end; bend this strip into an exact semi-circle, as shown in Fig. 47 and solder the tin ring, at the point where numbers 1 and 24 meet, to the middle of the brass semi-circle. Through the holes in the ends of the brass semi-circle fasten a wire, and this wire must run exactly through the center of the tin ring. Now screw the brass semi-circle to a baseboard (see Fig. 48), so that the angle made by the wire and the surface of the board will be equal to the latitude of the place where it is to be used; in other words, when the board is level the wire should point directly to the North Star and when this is done it will be adjusted to the proper angle. The board should be about 12 inches square and 1 inch thick. [Illustration: FIG. 47.—BRASS SEMI-CIRCLE WITH SHADOW WIRE.] [Illustration: FIG. 48.—SUNDIAL COMPLETE.] Since the shadow of the wire made by the sunlight will fall on XII at noon, it will be plain that the shadow of the wire falling on the numbers on one side or the other of the ring will mark the Sun time. To change Sun time to _mean solar time_, or ordinary time, see _Equation of Time_, Appendix M. [Illustration: FIG. 49.—TO FIND THE NORTH BY A WATCH.] =To Find the North by a Watch=.—To use a watch as a compass, that is to find the north by means of a watch, is easy if the Sun is shining. The watch should be held face upward; then turn the watch around until the hour hand points in the direction of the Sun. Draw an imaginary line from the hour XII to the center of your watch. If, now, in the middle between this line and the hour hand you draw another line from the center of the watch and produce, or extend it, the middle line will point just about north, all of which is clearly shown in Fig. 49. CHAPTER IV THE PLANETS, THE SUN’S KIDDIES In the last chapter we said that all the stars in the sky, including our Sun, are fixed in their positions; by this we mean that if we were to look at the Big Dipper every night for a hundred years we could see no change in the positions of any of the stars forming this constellation. But if we look at the sky along the line of the _ecliptic_—that is the path of the Sun—one night after another we are likely to see a bright point of light which looks exactly like a star and yet it is certain that this point of light really does move among the other stars. What kind of a heavenly object then is this?