Science Fair Project Encyclopedia
- For Solar Eclipse, the alien friend of the rubber doll Betty Spaghetty, see Betty Spaghetty.
A solar eclipse occurs when the Sun, Moon and Earth are on a single line with the Moon 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 eclipsed 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 four 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: A ring of the Sun can still be seen around the Moon (caused by the Moon's umbra failing to reach the Earth's surface, which can happen if the Moon is near apogee). This is similar to a penumbral eclipse.
- A hybrid eclipse: The curvature of Earth's surface causes a single solar eclipse to be observed as annular from some locations but total from other locations. A total eclipse is seen from places on the Earth's surface that lie along the path of the eclipse and are physically closer to the Moon, and so intersect the Moon's umbra; other locations, further from the Moon, fall in the Moon's antumbra and the eclipse is annular.
Observing a solar eclipse
- Never look directly at the sun without using proper safety equipment, even during an eclipse, to do so can cause permanent retinal damage and can seriously affect one's eyesight.
The best and safest way to view a solar eclipse, or any other solar event, is via indirect projection. This can be done by projecting an image of the sun onto a white piece of paper or cardboard using a pair of binoculars (With one of the lenses covered), a telescope, or another piece of cardboard with a very small hole in it (1mm diameter), often called a pinhole camera. The projected image of the sun can then be safely viewed with no worries.
Although not recommended, direct viewing of a solar eclipse can be achieved using proper, certified safety equipment. Special solar filter goggles, made specifically for viewing solar events, can often be purchased at museums, planetariums, and sometimes may even be provided free of charge if an eclipse is upcoming. Another option is to use a piece of welder's glass with a shade rating of 13 or higher (14 being the recommended shade rating). This can be purchased at any welding supply store. Solar filter goggles or welding glass can also be used to protect cameras while photographing an eclipse.
- There are rumors that metallic potato chip packaging and CD-ROM discs can be used to safely view a solar eclipse. This is not true, although these materials may reduce the brightness of sunlight to a tolerable level, they provide absolutely no protection against invisible ultraviolet radiation, which can also cause serious retinal damage.
- Sunglasses do not provide sufficient protection for direct viewing of a solar eclipse, and should not be used as a protective device for viewing an eclipse. This explictly includes crossed polarizing sunglasses, which are not a complete filter, regardless of what some textbooks say.
- If a direct method of viewing an eclipse is chosen (Using proper safety equipment), a good rule of thumb to follow should be to limit the amount of time spent looking at the sun. It is best to not look at the sun for more than 20 seconds at a time, with at least a 30 second break between viewings. This will help reduce the possibility of eye damage that may exist even while using proper safety equipment.
- Perhaps paradoxically, the greatest danger is when totality approaches. When, for example, 95% of the surface of the Sun is covered by the Moon, the total amount of light reaching the eye has decreased enough to make it easier to overcome the normal blink reflex. Furthermore the relative darkness means the pupil is somewhat dilated, allowing more light to enter. Unfortunately, the part of the Sun's surface that still isn't covered is as bright as ever, and is as damaging to eye. The same problem arises at the end of totality. The brightness of the light goes from that of the corona (about 4 times the brightness of the full moon, and perfectly safe to observe with the naked eye) to that of a tiny dot of the uncovered photosphere, which is again damaging to the eye very quickly.
Eclipse frequency and cause
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.
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 in width. 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 width.
Total solar eclipses are rare events. Although they occur somewhere on the Earth approximately every 18 months, it has been estimated that they recur at any given spot only every 300 to 400 years. And after waiting so long, the total solar eclipse only lasts for a few minutes, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 40 s, and is usually a good deal shorter. During each millennium there are typically fewer than 10 total solar eclipses exceeding 7 minutes. The last time this happened was June 30, 1973. Those alive today probably won't live to see it happen again — on June 25, 2150. The longest total solar eclipse during the 8,000-year period from 3000 BC to 5000 AD will occur on July 16, 2186, when totality will last 7 min 29 s. (eclipse predictions by Fred Espenak, NASA/GSFC.)
For astronomers, a total solar eclipse forms a rare opportunity to observe the corona (the outer layer of the Sun's atmosphere). Normally this is not visible because the photosphere is much brighter than the corona.
Calculating the date of a solar eclipse
If you know the date and time of a solar eclipse, you can predict other eclipses using eclipse cycles. Two well-known eclipse cycles are the Saros cycle and the Inex cycle. The Saros cycle is probably the most well known, and one of the best, eclipse cycles. The Inex cycle is itself a poor cycle, but it is very convenient in the classification of eclipse cycles. After a Saros cycle finishes, a new Saros cycle begins 1 Inex later (hence its name: in-ex).
Historical solar eclipses
In the Odyssey, XIV, 151, Homer states that Odysseus will return to his home, and take vengeance on the suitors of Penelope, at the failing of the old moon and the coming of the new. Later 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.
A double (solar and lunar) eclipse took place 23 years after the ascension of king Shulgi of Babylon. This has been identified with eclipses that occurred on 9 May (solar eclipse) and 24 May (lunar eclipse), 2138 BC . This identification is however much less commonly accepted than the eclipse of 763 BC. See also Chronology of Babylonia and Assyria.
Herodotus wrote that Thales of Milete predicted an eclipse which occurred during a war between the Medians and the Lydians. Soldiers on both sides put down their weapons and declared peace as a result of the eclipse. Exactly which eclipse was involved has remained uncertain, although the issue has been studied by hundreds of ancient and modern authorities. One likely candidate took place on May 28, 585 BC, probably near the Halys river in the middle of modern Turkey.
An annular eclipse of the Sun occurred at Sardis on February 17, 478 BC, while Xerxes was departing for his expedition against Greece, as Herodotus, VII, 37 recorded ([Hind and Chambers, 1889: 323] considered this absolute date more than a century ago). Herodotus (book IX, 10, book VIII, 131, and book IX, 1) reports that another solar eclipse was observed in Sparta during the next year, on August 1, 477 BC. 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 of (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 that these two eclipse records have been ignored so far.
The foundation of Rome took place 437 years after the capture of Troy (1182 BC), 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, calling the year of the first consuls "245 ab urbe condita" (a.u.c.). A new study claims that the Varronian date has been superseded. Its correctness has not been proved scientifically but it is used worldwide.
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 BC, 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 of Thoth. The first day of Thoth fell on March 2 in that year (Prof. E. J. Bickerman, 1980: 115). That implies that Rhea Silvia's pregnancy lasted for 281 days. Rome was founded on the ninth day of the month Pharmuthi , which was April 21, 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 and 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 month 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 these data by telling that Romulus was in his 18th year of age when he founded Rome. Therefore, three eclipse records prove that Romulus reigned from 746 BC to 709 BC.
Special observation campaigns
- May 30, 1965: Launch of rockets at Charlestown, USA
- May 20, 1966: Launch of rockets at Karystos, Greece to watch the solar eclipse
- November 12, 1966: Launch of two Titus-rockets fom Las Palmas, Argentinia
- February 26, 1979: Launch of rockets from Red Lake, Kanada
- February 16, 1980: Launch of rockets from San Marco platform
Simultaneous occurrence of solar eclipse and transit of a planet
In principle, the simultaneous occurrence of a Solar eclipse and a transit of a planet is possible. But these events are extremely rare. The next anticipated simultaneous occurrence of a Solar eclipse and a transit of Mercury will be on July 5th, 6757 , and of a Solar eclipse and a transit of Venus is expected on April 5th, 15,232 .
Only 5 hours after the transit of Venus on June 4, 1769 there was a total solar eclipse, which was visible in Northern America, Europe and Northern Asia as partial solar eclipse. This was the lowest time difference between a transit of a planet and a solar eclipse in the historical past.
Solar eclipses by artificial satellites
Artificial satellites can also get in the line between earth and Sun. But these events are difficult to watch, because the zone of visiblity is very small. The satellite passes the Sun in one second. Like a transit of a planet it will not get dark. 
Longest solar eclipses between 0 and 3000
Total solar eclipses
|July 16, 2186||7m29s|
|July 5, 2168||7m26s|
|June 27, 363||7m24s|
|July 27, 2204||7m22s|
|July 8, 381||7m22s|
|June 9, 1062||7m20s|
|June 20, 1080||7m18s|
|June 16, 345||7m17s|
|June 3, 699||7m16s|
|June 13, 717||7m15s|
|June 1, 132||7m14s|
|June 24, 2150||7m14s|
|May 29, 1044||7m12s|
|June 25, 2522||7m12s|
|June 12, 150||7m12s|
|July 19, 399||7m11s|
|July 3, 2885||7m11s|
|June 14, 2504||7m10s|
|June 23, 2867||7m10s|
|May 23, 681||7m09s|
|June 20, 1955||7m08s|
|August 8, 2222||7m06s|
|May 22, 114||7m05s|
|July 1, 1098||7m05s|
|June 8, 1937||7m04s|
|June 30, 1973||7m04s|
|July 5, 2540||7m04s|
|July 16, 2903||7m04s|
|June 6, 327||7m03s|
|June 22, 168||7m02s|
|June 25, 735||7m02s|
|June 12, 2849||7m00s|
Annular solar eclipses
|December 7, 150||12m24s|
|November 25, 132||12m16s|
|December 17, 168||12m15s|
|December 14, 1955||12m09s|
|January 14, 3080||12m09s|
|January 24, 3098||12m05s|
|December 24, 1973||12m03s|
|December 25, 1628||12m02s|
|December 2, 1937||12m00s|
- Lunar eclipse
- Allais effect
- Pharaoh (historical novel by Bolesław Prus, incorporating a culminating solar-eclipse scene).
- Search 5,000 Years of Eclipses At This Site by Hermit Eclipse (loads slowly)
- NASA's Eclipse Home Page
- World Atlas of Solar Eclipse Paths by Fred Espenak
- Wikisource has some detailed information about recently solar eclipses as seen from Beijing, Shanghai and Tianjin
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