The History and Practice of Ancient Astronomy by Evans James;

Author:Evans, James;
Language: eng
Format: epub
Publisher: Oxford University Press, Incorporated
Published: 1998-07-14T16:00:00+00:00

The first term on the right Hipparchus computed from his solar theory. The use of a lunar eclipse helped him get around the difficulty of measuring the second term, the longitudinal arc from the Sun to Spica.

During a lunar eclipse, the middle of the Earth’s shadow marks a point in the sky that is directly opposite the Sun. Midway through the eclipse, the Moon’s longitude is almost exactly 180° different from the Sun’s. The longitudinal arc from the Moon to Spica at this moment can be measured. The longitudinal arc from the Sun to Spica must be 180° greater. In this method of measuring star longitudes, the Moon provides a screen on which the Earth’s shadow may fall, permitting us to “see” the Sun indirectly. Once a small correction is made for the Moon’s parallax, the longitude of the star can be determined:

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