Problem:Surveying Astronomy: The Equation of Time

## Problem: Surveying Astronomy: The Equation of Time

An hour measured by the apparent motion of the true sun, which is sundial time, varies with the time of year. An hour measured by the imaginary mean sun is constant throughout the year. Which of the following reasons account for this effect?

(A) The orbit of the earth around the sun is elliptical. Therefore, the apparent motion of the true sun is more rapid when the earth is closer to the sun and slower when the earth is farther away.
(B) The apparent motion of the true sun is along the ecliptic and the motion of the imagined mean sun is along the celestial equator. Therefore, the mean sun moves steadily in terms of right ascension, which the true sun does not.
(C) The tidal forces of the moon and other planets of the solar system cause the rate of the rotation of the earth on its axis to vary significantly during the year. This variation causes the apparent motion of the true sun to vary.
(D) Both A and B are true.

This is problem 76(4-43) from the new second edition of1001 Solved Surveying Problemsby Jan Van Sickle. Reprinted with permission from1001 Solved Surveying Problemsby Jan Van Sickle (1997, 728 pp., Professional Publications Inc.). For details on this and other FLS exam-prep books, call 800/426-1178 or visitwww.ppi2pass.com .