Geodetic applications of eclipses and occultations

Abstract

If in imagination we viewed a solar eclipse or the occultation of a star from a point outside the earth, we would see the shadow of the moon advancing across the face of the earth, the earth meanwhile turning on its axis beneath the shadow. When some point on the advancing edge of the shadow overtook a given point on the surface of the earth, an observer at that point would note the beginning of the eclipse or occultation. When the trailing edge of the shadow uncovered that point again, the observer there would note the end of the eclipse or occultation.

The universal time (as distinguished from the local time) of the beginning or ending would depend on the position of the observer with reference to the body of the earth, that is, on his ideal geodetic coordinates. These universal times would not depend in the least on the direction of the observer’s vertical. This fact is the key to the usefulness of eclipses and occultations for geodetic purposes.

Suppose that the prediction for the times of beginning or ending had been made on the basis of the astronomical latitude and longitude of the observer. Since there would be in general deflections of the vertical in latitude and longitude, Δπ and Δλ, these would bring about, even in the absence of any other source of discrepancy, diffe-