Page:Popular Science Monthly Volume 75.djvu/422

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the declination of the zenith—declination being defined in a way exactly similar to latitude, i. e., the angular distance of a point on the sky north or south of the celestial equator. Latitude is usually designated by the Greek letter Φ, and it may be seen from the figure that a third definition of latitude is the angular distance of the celestial pole above the horizon—the altitude of the celestial pole.

Many methods of determining latitude have been devised, some of them coming down to us from the ancient Chaldean and Egyptian astronomers. The simplest method is to measure the altitude of the sun at noon on the day it passes through the equinox. On that day, the sun will cross the meridian at the point E', and its altitude will then be 90° − Φ, as may be readily seen from the figure. A rough value of

PSM V75 D422 Shifting of the earth axis.png

Fig. 1.

this angle may be obtained by measuring the shortest shadow of a vertical stick on a level piece of ground on the day of the equinox. The height of the stick divided by the length of the shortest shadow is the tangent of the complement of the latitude.

If the earth be considered a rigid body and the axis upon which it rotates be fixed within the body of the earth, the latitudes of all places upon its surface will remain always the same. If, however, the axis should shift its position within the earth, then the equatorial plane, which must be always perpendicular to the axis, must shift and consequently the latitudes of all places on the earth's surface must change accordingly.

It is well established that, at least during historic times, no changes of any considerable magnitude have occurred in the latitudes of places on the earth. It has long been suspected by astronomers, however, that