Did ancient peoples really predict solar eclipses?

Ancient observations of solar eclipses have a long history among many different cultures and civilizations which stretches back to at least 2500 BC in the writings that have survived from ancient China and Babylon.

Ancient Chinese astrologers, by 2300 BC, 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 instance in 2300 BC resulted in the beheading of two astrologers. Since the pattern of total solar eclipses is a very erratic one in time at a 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 reoccurrence period. By 206 AD they could predict solar eclipses by analyzing the motion of the moon itself.

In the western world, meanwhile, Babylonian clay tablets that have survived since the time of this civilization in the Mesopotamian region, record the first total solar eclipse seen by observers in Ugarit on May 3, 1375 BC. Like the Chinese observers, Babylonian astrologers kept careful records about celestial goings-on including the motions of Mercury, Venus the Sun and the Moon which survive from 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

In ancient Egypt, nearly all of what we know about this civilizations astronomical knowledge comes to us from tomb paintings, a variety of temple inscriptions, and literally a handful of papyrus documents such as the Rhind Papyrus. Sadly, the Great Library in Alexandria was burned at the time of Cleopatra and Julius Caesar, and later burnings in 390 AD and 640 AD destroyed by one estimate over 400,000 books on Egyptian secular literature, mathematics, medicine and astronomy. It is classified as the greatest intellectual catastrophe in human history. One can only guess what Egyptian astronomical knowledge may have been lost. All that now survives are fragments which some scholars see as merely the faded ghosts of what may have been the true Egyptian legacy.

The oldest known example of a sundial dates from Egypt ca 1500 BC. The fabulous astronomical ceiling of Senmut was painted around 1460 BC which include 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, and later in 1100 BC Amenhope wrote the 'Catalog of the Universe' in which he identifies the major constellations known by that time. Curiously, this 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 that Egyptian astronomical knowledge was more than tomb decoration, and not very carefully husbanded over the ages as a body of knowledge. Despite a number of temple and pyramid alignments, and several papyra codices suggesting a sophisticated knowledge of trigonometry and even algebra, no similar astronomical documents have survived, or records of astronomical observations. The Vienna papyrus which described lunar and solar eclipses and their portends, was probably copied by a scribe in the late second century AD, and presents astronomical knowledge that is regarded as essentially Babylonian in nature.

By 700 BC, the Greek civilization was in its ascendancy. The historian Herodotus ( ca 600 BC) mentions that Thales was able to predict the year when a total solar eclipse would occur, but the details upon what this prediction was based does not survive. The eclipses in question occurred in either 610 or 585 BC. Apparently the method used worked only once since 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. Prior to 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 reoccur at the same geographic location. Ptolemy ( ca 150 BC) represents the epitome of Greecian astronomy, and surviving records show that 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, and that the Sun must be within 20d 41' of the Node point, and that up to two solar eclipse could occur within seven months in the same part of the world. Lunar eclipse were especially easy to calculate because of the vast area covered by the Earth's shadow on the Moon. Solar eclipses, however, required much greater finesse and knowledge. The shadow of the Moon on the 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.

Arabic astronomy became the western world's power house 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 and Aristotle were translated and amplified upon and spread throughout the Muslim world. At Antioch, Muhammad al-Batani (ca 850AD) began with Ptolemy's works and recalculated the precession of the equinoxes, and produced new astronomical tables. Following a steady series of advances in trigonometry, observations by Ibn Junis of lunar and solar eclipses were recorded in Cairo ca 1000 AD, and he is regarded as one of the greatest Muslim astronomers of his time. The decline of Arabic and Muslim learning, however, had begun by the 11th century.

While Chinese, Babylonian and Greek astrologers dominated the astronomical knowledge of the 'Old World', half way across the globe, Maya observers were also working on calendars, and recording celestial observations to their own ends. The Dresden Codex records several tables which are widely thought to be lunar eclipse tables. As many civilizations had before them in other parts of the world, the Mayas used records of historical lunar eclipses to identify how often they occur 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 conquest by the Spanish Conquistadors and the intentional destruction of nearly all native written records by the Missionaries by the 1600s, little survives today to tell us whether the Mayas, Incas or Aztecs had achieved a deeper understanding of solar eclipses and their forecasting.

Why the interest in eclipses?

One of the first things that civilizations must do to insure a coherent society, and the harvesting and planting of crops, is to establish an accurate calendar. Most of the early calendars were lunar 'monthly' calendars, but since the time between like lunar phases is 29.5 days, this only leads to 12.38 months during a solar ( seasonal) year, so that every year, the lunar calendar slips by 11 days relative to the seasonal 'planting' year. While establishing an accurate luni-solar calendar, ancient peoples observed the moon quite regularly, and over time would discover evenings when the moon was eclipsed by the Earth's shadow. Because the Earth's shadow is so vast, lunar eclipses were the first major celestial events that ancient astrologers would learn how to predict by using local historical observational records.

Why no solar eclipse predictions?

The diameter of the Earth's shadow at the distance of the Moon is over 12,000 kilometers across. This makes predicting lunar eclipses a very forgiving enterprise even when you do not know the precise details of the orbit of the Moon. For total solar eclipses, however, the shadow of the Moon upon the Earth's surface is only about 300 km across. At the distance of the Moon's orbit, this subtends an angle of less than 1/20 of a degree of arc. To forecast a solar eclipse, you would need to know the details of the lunar orbit to at least this degree of accuracy. With the exception of the ancient Chinese and Greeks, there are no written records that suggest that the Moon, stars or planets were routinely measured with this degree of accuracy. Some have proposed that many ancient civilizations kept track of when total solar eclipses occurred, and that from these local historical records, numerical patterns allowed ancient astrologers to make total solar eclipse forecasts. This also seems not to be possible.

From the 'Canon of Eclipses' from 1207 BC to 1600 AD, you can follow a series of total solar eclipses that were visible over Egypt between 1157 and 115 BC spanning over 1000 years of potential observations. There is no simple, or discernible, pattern. Sometimes 8 months separates two total solar eclipses, sometimes as many as 400 months separate the two! This means that, knowing that an eclipse happened during a particular month and year, there is no simple additive period that lets you anticipate when the next total solar eclipse will happen, even in the same geographical location. This doesn't even allow for the possibility of bad weather which would further reduce the number of observable eclipses. Lunar eclipses can be found to follow after periods of 135 and 223 months. Plotting the distribution of months in the above exercise does show two peaks near 120 months and 270 months, but only 1/2 of the total solar eclipses cluster near these two intervals. An astrologer using the 135 month or 223 month lunar eclipse periods to forecast total solar eclipses would be wrong at least 1/2 the time; similar to simply tossing a coin that this month one would occur.

Only after the lunar orbit was observationally determined, and the changing speed of the Moon and the Earth in their orbits, were established, could total solar eclipses be reliably forecast to within the nearest month or less. This level of astronomical sophistication was apparently reached by ancient Greek astronomers around the first century BC, and several centuries later by Chinese astronomers based on surviving records. We can only speculate what may have been lost in the fires of the Great Library at Alexandria, or burned by Spanish Conquistadors and Missionaries in ancient Mexico, Peru and Chile.

For further readings:

"A History of Science", William Dampier, 1948, Cambridge University Press, Cambridge.

"A Chronicle of Pre-Telescopic Astronomy", Barry Hetherington, 1996,

"Ancient Astronomical Observations and the Accelerations of the Earth and Moon", Robert R. Newton, 1970, The Johns Hopkins Press (Baltimore, Maryland)

"Canon of Eclipses", Theodor von Oppolzer and Owen Gingerich, Dover Publications, New York.

"A Vienna Demotic Papyrus on Eclipse and Lunar Omina", Richard Parker, 1959, Brown University Press, Providence RI

"Early Astronomy", Hugh Thurston, Springer-Verlag Publishing

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