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Sun-Earth Day Presents: Eclipse, In a Different Light

Not only the time and date of a solar eclipse were of interest to early forecasters, but eventually the details of the event too.


Aristarchus' Eclipse Drawings

Claudius Ptolemy, who lived in the second century AD., wrote the extremely influential treatise that came to be called the "Almagest" in about 150 AD. This work covered elements of spherical astronomy, solar, lunar, and planetary theory, eclipses, and the fixed stars. It remained the definitive authority on its subject for nearly fifteen hundred years.

Ptolemy represents the epitome of knowledge of Grecian astronomy. Ptolemy knew, for example, the details of the orbit of the Moon including its nodal points. He also knew that the Sun must be within 20 degrees 41' of the node point, and that up to two solar eclipses could occur within seven months in the same part of the world. Lunar eclipses were especially easy to calculate because of the vast area covered by Earth's shadow on the Moon. Solar eclipses however required much greater knowledge. The shadow of the Moon on Earth is less than 100 kilometers wide, and its track across the daytime hemisphere is the result of many complex factors that cannot be anticipated without a nearly complete understanding of the lunar orbit and speed.

Statue of Aristarchus

The image above shows a detail from Book VI, Chapter 7, of a late-1400s copy of George Trebizond's Latin translation (ca. 1451) of this work. The drawing illustrates the computation of the duration of solar and lunar eclipses. This elaborate manuscript of the translation, with the figures drawn in several colors, was dedicated to Pope Sixtus IV by George's son Andreas.

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