Solar Eclipses: Ancient Greece

March 28, 2009, 10:21 pm
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Introduction

By 450 BC, the Greek civilization was in its ascendancy. The historian Herodotus (ca 460 BC) mentions that Thales was able to predict the year when a total solar eclipse would occur. Details of how this prediction was made do not survive. The eclipse occurred in either 610 BC or 585 BC. Apparently the method used worked only once because what is known of Greek scientific history does not suggest that the method was ever reliably used again.

Thales is said to have visited Egypt, and from the empirical rules in use there for land surveying, brought back to Greece the ideas of deductive geometry later codified by Euclid. Before 450 BC, Meton realized that a single period of 235 lunar months (19 years) would cause the popular lunar calendar to return to synchrony with the solar, seasonal calendar. At this time, the same lunar phase would be recorded at the same time of the solar calendar year. This period also gives a rough guide to when a lunar eclipse will recur at the same geographic location.

Ptolemy (ca 150 AD) represents the epitome of knowledge of Grecian astronomy. Records such as the "Almagest" show he had a sophisticated scheme for predicting both lunar and solar eclipses. 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.

Aristarchus "On the Distances and Sizes of the Sun and Moon."

Aristarchus "On the Distances and Sizes of the Sun and Moon."

The image above is from a tenth century Greek copy of a noteworthy work by Aristarchus of Samos written in the second century B.C.E. titled "On the Distances and Sizes of the Sun and Moon," in which he calculates the ratio of the distance between Earth and the Sun to that between Earth and the Moon. (His estimate was more than an order of magnitude too small, but the fault was in his lack of accurate instruments rather than in his correct method of reasoning.) This detail compares the line subtending the arc dividing the light and dark portions of the Moon in a lunar eclipse with the relative diameters of the Moon and the Sun. Aristarchus also found an improved value for the length of the solar year.

In the Odyssey (XX, 356-357 and 390), Homer adds that the Sun vanished out of heaven and an evil gloom covered all things about the hour of the midday meal, during the celebration of the new moon. A total eclipse of the Sun was visible from the Greek island of Ithaca on April 16, 1178 BC. This would be six years after the end of the Trojan War, as traditionally dated (1184 BC), though within the Odyssey narrative it's supposed to be ten years after it.

References

  • Aristarchus Sun-Moon drawing - Library of Congress Vatican Exhibit, Greek Mathematics and Its Modern Heirs, Vat. gr. 204 fol. 116 recto math06 NS.02

Preview Image

"Detail 1 from Ptolemy's Almagest" - Claudius Ptolemy, who lived in the second century C.E., wrote the extremely influential treatise that came to be called the "Almagest" in about 150 C.E. 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. This image 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.  (Source: Library of Congress Vatican Exhibit, Greek Astronomy, Vat. lat. 2055 fol. 101 verso math17 NS.08.)

 

Citation

Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Solar Eclipses: Ancient Greece." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 26, 2007].
<http://www.cosmosportal.org/articles/view/137332/>

 

Glossary

Citation

(2009). Solar Eclipses: Ancient Greece. Retrieved from http://www.trunity.net/cosmos/view/article/51cbeab47896bb431f685660

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