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Solar eclipse

A solar eclipse is an eclipse occurring when the Sun, Moon and Earth are on a single line, the Moon being in the middle. Seen from the Earth, the Moon is in front of the Sun and thus part or all of the light of the Sun is hidden by the Moon. Thus it may seem that a piece has been taken out of the Sun, or that it has suddenly disappeared. There are three types of solar eclipses:

  • A partial solar eclipse: Only part of the Sun and the Moon overlap
  • A total solar eclipse: All of the Sun is hidden by the Moon
  • A ring-formed or annular eclipse: Sometimes the Moon's umbra does not reach the Earth's surface. This can happen if the Moon is near apogee. This is similar to a penumbral eclipse. During an annular eclipse, a ring of the Sun can still be seen around the Moon. The Moon's shadow umbra often fails to reach the Earth.

Solar eclipses can only happen during the new moon phase.

Warning: Never look at the Sun directly, even during an eclipse. To do so can cause retinal damage and affect one's eyesight Always use protection, such as welder's glasses, that are recommended for the extreme brightness of the sun, or view the image of the Sun indirectly on, say, a piece of paper, using a simple pinhole camera. Sunglasses are not sufficient!

Total and annular eclipses both occur when the Moon lines up with the Sun exactly, but since the Moon's orbit is not perfectly circular it is sometimes farther away from Earth and doesn't always cover the entire solar disc from an Earthly vantage point.

(Photo taken by John Walker during the Zambia 2001 eclipse)

A solar eclipse can only be seen in a band across the Earth as the Moon's shadow moves across its surface, while a total or annular eclipse is actually total or ring-formed in only a small band within this band (the eclipse path), and partial elsewhere (total eclipse takes place where the umbra of the Moon's shadow falls, whereas a partial eclipse is visible where the penumbra falls). The full band is generally around 100 km across. The eclipse path will be widest if the Moon happens to be at perigee, in which case the eclipse path alone can reach 270 km in diameter.

Total solar eclipses are rare. Total solar eclipse can usually be seen from an area, for only a few moments. The Moon's umbra moves eastward at over 1700 km per hour. Totality can never last more than 7 1/2 minutes. Typically, totality lasts for a much shorter period of time.

For astronomers, a total solar eclipse forms a rare opportunity to watch the corona, the outer layer of the Sun. Normally this is not visible because the light of the Sun overshines the corona.

Calculating the Date of a Solar Eclipse

In order to predict a solar eclipse, one would need to know how many synodic months equal some whole number of eclipse years.

  • A solar eclipse can only occur when the Sun lines up behind the moon, during a new moon. It takes 29.53 days to go from one new moon to the next.
  • The eclipse year is 346.6 days.
  • 223 x 29.53 = 19 x 346.6 = 6585 days
    • 223 lunar months have the same length of time as 19 eclipse years.

This calculation is accurate to within a few hours. A more accurate calculation (see: saros cycle) gives 6595.3 days. Because of the extra one-third day, the Earth will rotate an extra 120º. One must thus wait three full saros intervals (54 years) before the eclipse path will return to nearly the same location, a cycle sometimes called Exeligmos.

Another notable eclipse cycle called Inex takes 358 lunar months (28.9 years), after which an eclipse returns to nearly the same geographical longitude but opposite longitude.

History of Solar Eclipses

In 585 BC, Thales of Miletus predicted an eclipse which occurred during a war between Medians and Lydians. Soldiers on both sides put down their weapons and declared to peace as a result of the eclipse. The date has been confirmed by hundreds of ancient and modern authorities, it took place on May 28, 585 BC, probably near the Halys river, in the middle of the modern Turkey.

April 16, 1178 BC[?], a real total eclipse of the sun took place at Ithaca Greek island, about four years after the end of the Trojan War. Homer XIV, 151 states that at the failing of the old moon and the coming of the new shall Odysseus return to his home, and take vengeance on the suitors of Penelope. Homer addas (XX, 356-357 and 390) that the sun vanished out of heaven and an evil gloom had covered all things about the hour of the midday meal, during the celebration of new moon.

February 17, 478 BC: a real eclipse of the sun occurred at Sardis, when Xerxes was departing for his expedition against Greece, as Herodotus, VII, 37 recorded. (Hind and Chambers, 1889: 323 have considered this absolute date more than a century ago.) During the next year, another solar eclipse was observed in Sparta. It took place on August 1, 477 BC, says Herodotus IX, 10 and VIII, 131, and IX, 1. The sky suddenly darkened in the middle of the sky, well after the battles of Thermopylae and Salamis, after the departure of Mardonius to Thessaly at the beginning of the spring (477 BC) and his second attack on Athens, after the return of Cleombrotus to Sparta. Note that the modern conventional dates are different by a year or two, and these two eclipse records have been ignored so far.

A new study claims that the Varronian date is superseded. Its correctness have not been proved scientifically but it is used worldwide. The foundation of Rome took place 437 years after the capture of Troy (1182 BCE), according to Velleius Paterculus (VIII, 5). It took place shortly before an eclipse of the Sun that was observed at Rome on June 25, 745 BC and had a magnitude of 50.3%. Its beginning occurred at 16:38, its middle at 17:28, and its end at 18:16. Varro may have used the consular list with its mistakes, and called the year of the first consuls "245 ab urbe condita" (a.u.c.).

According to Lucius Tarrutius of Firmum, Romulus was conceived in the womb on the 23rd day of the Egyptian month Choiac, at the time of a total eclipse of the Sun. (This eclipse occurred on June 15, 763 BCE, with a magnitude of 62.5% at Rome. Its beginning took place at 6:49, its middle at 7:47 and its end at 8:51.) He was born on the 21st day of the month Thoth. The first day of Thoth fell on March 2 in that year (Prof. E.J. Bickerman, 1980: 115). It means that Rhea Silvia's pregnancy lasted for 281 days. Rome was founded on the ninth day of the month Pharmuthi, which was the 21st of April, as universally agreed. The Romans add that about the time Romulus started to build the city, an eclipse of the Sun was observed by Antimachus, the Teian poet, on the 30th day of the lunar month. This eclipse (see above) had a magnitude of 54.6% at Teos, Asia Minor. It started at 17:49 it was still eclipsed at sunset, at 19:20. Romulus vanished in the 54th year of his life, on the Nones of Quintilis (July), on a day when the Sun was darkened. The day turned into night, which sudden darkness was believed to be an eclipse of the Sun. It occurred on July 17, 709 BC[?], with a magnitude of 93.7%, beginning at 5:04 and ending at 6:57. (All these eclipse data have been calculated by Prof. Aurél Ponori-Thewrewk, retired director of the Planetarium of Budapest.) Plutarch placed it in the 37th year from the foundation of Rome, on the fifth of our July, then called Quintilis, on "Caprotine Nones," Livy (I, 21) also states that Romulus ruled for 37 years. He was slain by the senate or disappeared in the 38th year of his reign. Most of these have been recorded by Plutarch (Lives of Romulus, Numa Pompilius and Camillus), Florus[?] (Book I, I), Cicero (The Republic VI, 22: Scipio's Dream), Dio (Dion) Cassius and Dionysius of Halicarnassus (L. 2). Dio in his Roman History (Book I) confirms this data by telling that Romulus was in his 18th year of age whan he had founded Rome. Therefore, three eclipse records prove that Romulus reigned from 746 to 709 BCE.

See also: eclipse, lunar eclipse

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