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<p>Hipparchus based an estimate of the minimum possible value for the rate of precession on measurements of the distance of Spica from the Moon in mid-eclipse—an eclipse observed by himself on April 21, 146 BCE and another observed by Timocharis, on an unspecified date. This paper reaches three conclusions: (1) Hipparchus’s final adopted value for the longitude of Spica was determined by his wish to keep the latitude of Spica constant at 2° South; (2) Hipparchus carefully selected two eclipses for comparison so that the effects of the lunar parallax would nearly cancel out; and (3) Timocharis’s lunar eclipse was one of two possibilities: the eclipse of April 19, 295 BCE or that of April 8, 294 BCE.</p>

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Spica and the ancient astronomers: on Hipparchus’s longitude of Spica and the date of Timocharis’s lunar eclipse

  • 期刊论文

  • 发表日期：

    2026年8月3日

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  • James Evans

摘要

Hipparchus based an estimate of the minimum possible value for the rate of precession on measurements of the distance of Spica from the Moon in mid-eclipse—an eclipse observed by himself on April 21, 146 BCE and another observed by Timocharis, on an unspecified date. This paper reaches three conclusions: (1) Hipparchus’s final adopted value for the longitude of Spica was determined by his wish to keep the latitude of Spica constant at 2° South; (2) Hipparchus carefully selected two eclipses for comparison so that the effects of the lunar parallax would nearly cancel out; and (3) Timocharis’s lunar eclipse was one of two possibilities: the eclipse of April 19, 295 BCE or that of April 8, 294 BCE.