Fig. 47.—Brass Semi-Circle with Shadow Wire.
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.
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].