One of the first things civilizations must do to ensure a coherant society is to establish an accurate calendar to organize planting and harvesting of crops. Most early calendars were lunar calendars, because the monthly duration of the lunar cycle is 29.53 days, 12.37 months during a solar seasonal year. Every year, the lunar "synodic" calendar of 29.53 days slips by 0.38 of a month or 11.2 days relative to the seasonal "planting" year.
At the same time that ancient peoples kept track of how the lunar and solar calenders meshed with each other, they also uncovered some of the factors that lead to lunar and solar eclipses which also require specific timings of the solar and lunar positions across the sky and over the years. In many ways, the ability to predict eclipses was an outgrowth of the pre-existing need to keep track of lunar and solar calender relationships.
Why No Solar Eclipse Predictions?
The diameter of Earth's shadow upon the Moon is over 12,000 kilometers during a LUNAR eclipse. Compare this with the 300 km shadow of the moon on Earth during a SOLAR eclipses. This makes predicting lunar eclipses a forgiving enterprise even when you do not know the precise details of the Moon's orbit. You can be just about anywhere on the side of Earth facing the Moon, and still see a lunar eclipse, but for a solar eclipse you have to be in a very specific geographic location. To forecast a solar eclipse, you would need to know the details of the lunar orbit to a small fraction of a degree of arc to predict where the Moon's shadow will cross Earth's surface. With the exception of the ancient Chinese and Greeks, no written records suggest that the Moon, stars, or planets were routinely measured with this degree of accuracy.
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By 2300 BC, ancient Chinese astrologers, already had sophisticated observatory buildings, and as early as 2650 BC, Li Shu was writing about astronomy. Observing total solar eclipses was a major element of forecasting the future health and successes of the Emperor, and astrologers were left with the onerous task of trying to anticipate when these events might occur. Failure to get the prediction right, in at least one recorded case in 2300 BC resulted in the beheading of two astrologers. Because the pattern of total solar eclipses is erratic in any specific geographic location, many astrologers no doubt lost their heads. By about 20 BC, surviving documents show that Chinese astrologers understood what caused eclipses, and by 8 BC some predictions of total solar eclipse were made using the 135-month recurrence period. By AD 206 Chinese astrologers could predict solar eclipses by analyzing the Moon's motion.
ECLIPSE
QUOTATION
"Here lie the bodies of Ho and Hi, Whose fate, though sad, is risible; Being slain because they could not spy Th' eclipse which was invisible."
Author unknown Refers to the Chinese eclipse Of 2136 BC or 2159 BC.
Babylonian clay tablets that have survived since dawn of civilization in the Mesopotamian region record the earliest total solar eclipse seen in Ugarit on May 3, 1375 BC. Like the Chinese, Babylonian astrologers kept careful records about celestial happenings including the motions of Mercury, Venus, the Sun, and the Moon on tablets dating from 1700 to 1681 BC. Later records identified a total solar eclipse on July 31, 1063 BC, that "turned day into night," and the famous eclipse of June 15, 763 BC, recorded by Assyrian observers in Nineveh. Babylonian astronomers are credited with having discovered the 223-month period for lunar eclipses.
ECLIPSE
QUOTATION"If the sun at its rising is like a crescent and wears a crown like the moon: the king will capture his enemy's land: evil will leave the land, and (the land) will experience good."
Refers to a solar eclipse of 27 May 669 BC. Rasil the Older, Babylonian Scribe to the King
Nearly all we know about ancient Egyptian civilization's knowledge of astronomy comes to us from tomb paintings, various temple inscriptions, and a handful of papyrus documents such as the Rhind Papyrus. Unfortunately, the Great Library in Alexandria was burned during the time of Cleopatra and Julius Caesar. Later burnings in AD 390 and AD 640 destroyed an estimated 400,000 books on Egyptian secular literature, mathematics, medicine, and astronomy. The burnings were classified as one of the greatest intellectual catastrophies in human history. One can only guess what Egyptian knowledge of astronomy was lost. All that survives is fragments that some scholars see as merely the faded ghosts of Egyptian intellectual legacy.
The oldest example of a sundial is Egyptian from about 1500 BC. The fabulous astrological ceiling of Senmut painted around 1460 BC, includes celestial objects such as Orion, Sirius, and the planets Mercury, Venus, Jupiter, and Saturn. The oldest known copies of an almanac date from 1220 BC at the time of Ramses the Great. In 1100 BC Amenhope wrote "Catalog of the Universe" in which he identified the major known constellations. Curiously, the catalog does not mention either Sirius or any of the planets previously known to the Egyptians. At least outwardly, there are no surviving inscriptions or documents to indicate that Eqyptian knowledge of astronomy was more than tomb decoration, and not protected over the ages as a body of knowledge. Numerous temple and pyramid alignments and several papyrus codices suggest a sophisticated knowledge of trigonometry and algebra; no similar astronomy documents survive, or records of astrological observations. The Vienna papyrus which described lunar and solar eclipses and their portent was probably copied by a scribe in the late second century AD, and presents knowledge of astronomy that is regarded as Babylonian in nature.
Thought to be the inspiration for the
"Winged Sun Over Egypt"
"Winged Sun Over Egypt"
Ancient Greece
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.
Ptolemy
Ancient India
Indian astronomy is largely wrapped up in the Vedic religious treatises, but one individual, Aryabhata of Kusumapura, born in AD 476 is noteworthy. He is the first known astronomer on that continent to have used a continuous system of counting solar days. His book, the "Aryabhatiya," published in 498 AD described numerical and geometric rules for eclipse calculations. Indian astronomy at that time was taking much of its lead from cyclic Hindu cosmology in which nature operted in cycles, setting the stage for searching for numerical patterns in the expected time frames for eclipses.
The Mayas
While Chinese, Babylonian, and Greek astronomers dominated the knowledge of old world astronomy half way across the globe, Mayan observers were working on calendars and recording celestial observations. The Dresden Codex records several tables thought to be lunar eclipse tables. As in previous civilizations in other parts of the world, the Mayas used records of historical lunar eclipses to calculate how often they occurred over a 405-month period. There is no mention of recorded total solar eclipses, or discussions in the Codex for how to predict these events. After the Spanish Conquistadores, came the missionaries in the 1600s who intentionally destroyed nearly all native written record. Little survives to tell us whether the Mayas, Incas, or Aztecs achieved a deeper understanding of solar eclipses and their forecasting.
Symbols representing
the months in a Mayan,
365-day year
The Islamic World
Islamic astronomy became the western world's powerhouse of scientific research during the 9th and 10th centuries AD, while the Dark Ages engulfed much of the rest of the western world. The works by Ptolemy, Plato, and Aristotle were translated, amplified upon and spread throughout the Muslim world. Al-Khwarazmi developed the first tables trigonometric functions (ca 825 AD) which remained the standard reference well into the modern era. Al-Khwarazmi was known to the west as "Algorizm" and this is, in fact, the origin of the term 'algorithm'. Al-Khwarazmi's calculations were good to five places, allowing for unprecedented precision in astronomy and other sciences. At Antioch, Muhammad al-Batani (ca 850 AD) began with Ptolemy's works and recalculated the precession of the equinoxes, and produced new, more precise astronomical tables. Following a steady series of advances in Islamic trigonometry, observations by Ibn Yunus of lunar and solar eclipses were recorded in Cairo ca 1000 AD. Ibn Yunus is regarded as one of the greatest observational astronomers of his time. The pace of Islamic science and scholarship eventually slowed down in the 11th and 12th centuries. Many great books and great ideas of the Islamic Age lay fallow for hundreds of years until they were finally translated into Latin and fueled the European revolution in thinking and the birth of science as we know it today.
Courtesy of www.exploratorium.edu
ADDITIONAL LINKS
What Ancients Thought About the Sun- calendar section- Traditions and Cultures