As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses only part of the Sun is obscured. At least two, and up to five, solar eclipses occur each year; no more than two can be total eclipses.[1][2] Total solar eclipses are nevertheless rare at any particular location because totality exists only along a narrow path on the Earth's surface traced by the Moon's shadow or umbra.
An eclipse is a natural phenomenon. Nevertheless, in ancient times, and in some cultures today, solar eclipses have been attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of their astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.
As it is dangerous to look directly at the Sun, observers should use special eye protection or indirect viewing techniques. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses.[3][4]
The last total solar eclipse was on July 11, 2010; the next will be on November 13, 2012. The solar eclipses of 2011 were all partial eclipses; the last one occurred on November 25, 2011. The next solar eclipse will be an annular eclipse on May 20, 2012.
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There are four types of solar eclipses:
The Sun's distance from the Earth is about 400 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size: about 0.5 degree of arc in angular measure.
The Moon's orbit around the Earth is an ellipse, as is the Earth's orbit around the Sun; the apparent sizes of the Sun and Moon therefore vary.[6] The magnitude of an eclipse is the ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse. An eclipse that occurs when the Moon is near its closest distance to the Earth (i.e., near its perigee) can be a total eclipse because the Moon will appear to be large enough to cover completely the Sun's bright disk, or photosphere; a total eclipse has a magnitude greater than 1. Conversely, an eclipse that occurs when the Moon is near its farthest distance from the Earth (i.e., near its apogee) can only be an annular eclipse because the Moon will appear to be slightly smaller than the Sun; the magnitude of an annular eclipse is less than 1. Slightly more solar eclipses are annular than total because, on average, the Moon lies too far from Earth to cover the Sun completely. A hybrid eclipse occurs when the magnitude of an eclipse changes during the event from smaller than one to larger than one—or vice versa—so the eclipse appears to be total at some locations on Earth and annular at other locations.[7]
Because the Earth's orbit around the Sun is also elliptical, the Earth's distance from the Sun similarly varies throughout the year. This affects the apparent sizes of the Sun and Moon in the same way, but not so much as the Moon's varying distance from the Earth. When the Earth approaches its farthest distance from the Sun in July, a total eclipse is somewhat more likely, whereas conditions favour an annular eclipse when the Earth approaches its closest distance to the Sun in January.
Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse.[8] This is, however, not completely correct: the definition of a central eclipse is an eclipse during which the central line of the umbra touches the Earth's surface. It is possible, though extremely rare, that part of the umbra intersects with Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse.[8] The next non-central solar eclipse will be on April 29, 2014. This will be an annular eclipse. The next non-central total solar eclipse will be on April 9, 2043.[9]
The phases observed during a total eclipse are called:
The diagrams to the right show the alignment of the Sun, Moon and Earth during a solar eclipse. The dark gray region between the Moon and the Earth is the umbra, where the Sun is completely obscured by the Moon. The small area where the umbra touches the Earth's surface is where a total eclipse can be seen. The larger light gray area is the penumbra, in which a partial eclipse can be seen. An observer in the antumbra, the area of shadow beyond the umbra, will see an annular eclipse.
The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ecliptic). Because of this, at the time of a new moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the new moon occurs close to one of the points (known as nodes) where the Moon's orbit crosses the ecliptic.
As noted above, the Moon's orbit is also elliptical. The Moon's distance from the Earth can vary by about 6% from its average value. Therefore, the Moon's apparent size varies with its distance from the Earth, and it is this effect that leads to the difference between total and annular eclipses. The distance of the Earth from the Sun also varies during the year, but this is a smaller effect. On average, the Moon appears to be slightly smaller than the Sun, so the majority (about 60%) of central eclipses are annular. It is only when the Moon is closer to the Earth than average (near its perigee) that a total eclipse occurs.[10][11]
| Moon | Sun | |||
|---|---|---|---|---|
| At perigee (nearest) |
At apogee (farthest) |
At perihelion (nearest) |
At aphelion (farthest) |
|
| Mean radius, r | 1,737.10 kilometres (1,079.38 miles) |
696,000 kilometres (432,000 miles) |
||
| Distance, d | 363,104 km (225,622 mi) |
405,696 km (252,088 mi) |
147,098,070 km (91,402,500 mi) |
152,097,700 km (94,509,100 mi) |
| Angular diameter, 2 × arctan(r / d) |
32' 54" (0.5482°) |
29' 26" (0.4907°) |
32' 32" (0.5422°) |
31' 28" (0.5244°) |
| Apparent size to scale |
||||
| Rank in descending order |
1st | 4th | 2nd | 3rd |
The Moon orbits the Earth in approximately 27.3 days, relative to a fixed frame of reference. This is known as the sidereal month. However, during one sidereal month, the Earth has revolved part way around the Sun, making the average time between one new moon and the next longer than the sidereal month: it is approximately 29.5 days. This is known as the synodic month, and corresponds to what is commonly called the lunar month.
The Moon crosses from south to north of the ecliptic at its ascending node, and vice versa at its descending node. However, the nodes of the Moon's orbit are gradually moving in a retrograde motion, due to the action of the Sun's gravity on the Moon's motion, and they make a complete circuit every 18.6 years. This means that the time between each passage of the Moon through the ascending node is slightly shorter than the sidereal month. This period is called the draconic month.
Finally, the Moon's perigee is moving forwards in its orbit, and makes a complete circuit in about 9 years. The time between one perigee and the next is known as the anomalistic month.
The Moon's orbit intersects with the ecliptic at the two nodes that are 180 degrees apart. Therefore, the new moon occurs close to the nodes at two periods of the year approximately six months apart, and there will always be at least one solar eclipse during these periods. Sometimes the new moon occurs close enough to a node during two consecutive months. This means that in any given year, there will always be at least two solar eclipses, and there can be as many as five. However, some are visible only as partial eclipses, because the umbra passes above Earth's north or south pole, and others are central only in remote regions of the Arctic or Antarctic.[12][13]
Eclipses can only occur when the sun is within about 15 to 18 degrees of a node, (10 to 12 degrees for central eclipses). This is referred to as an eclipse limit. In the time it takes for the moon to return to a node (draconic month), the apparent position of the sun has moved about 29 degrees, relative to the nodes.[1] Since the eclipse limit creates a window of opportunity of up to 36 degrees (24 degrees for central eclipses), it is possible for partial eclipses (or rarely a partial and a central eclipse) to occur in consecutive months.[14][15]
During a central eclipse, the Moon's umbra (or antumbra, in the case of an annular eclipse) moves rapidly from west to east across the Earth. The Earth is also rotating from west to east, but the umbra always moves faster than any given point on the Earth's surface, so it almost always appears to move in a roughly west-east direction across a map of the Earth (there are some rare exceptions to this which can occur during an eclipse of the midnight sun in Arctic or Antarctic regions, for example on June 10 and December 4, 2021).
The width of the track of a central eclipse varies according to the relative apparent diameters of the Sun and Moon. In the most favourable circumstances, when a total eclipse occurs very close to perigee, the track can be over 250 km wide and the duration of totality may be over 7 minutes. Outside of the central track, a partial eclipse can usually be seen over a much larger area of the Earth.
Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average,[17] it has been estimated that they recur at any given place only once every 370 years, on average. The total eclipse only lasts for a few minutes at that location, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 31 s, and is usually much 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 (7 min 3 sec). Observers aboard a Concorde aircraft were able to stretch totality to about 74 minutes by flying along the path of the Moon's umbra. The next eclipse exceeding seven minutes in duration will not occur until 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.[18] For comparison, the longest eclipse of the 20th century occurred on June 20, 1955 and lasted 7 min 8 sec.
If the date and time of any solar eclipse are known, it is possible to predict other eclipses using eclipse cycles. Two such cycles are the saros and the inex. The saros is probably the best known and one of the most accurate eclipse cycles. The inex cycle is itself a poor cycle, but it is very convenient in the classification of eclipse cycles. After a saros finishes, a new saros series begins one inex later, hence its name: in-ex. A saros lasts 6,585.3 days (a little over 18 years), which means that after this period a practically identical eclipse will occur. The most notable difference will be a shift of 120° in longitude (due to the 0.3 days) and a little in latitude. A saros series always starts with a partial eclipse near one of Earth's polar regions, then shifts over the globe through a series of annular or total eclipses, and ends at the opposite polar region. A saros series lasts 1226 to 1550 years and 69 to 87 eclipses, with about 40 to 60 central.[19]
Solar eclipses can occur 2 to 5 times per year, at least once per eclipse season. Since the Gregorian calendar was instituted in 1582, years that have had five solar eclipses were 1693, 1758, 1805, 1823, 1870, and 1935. The next occurrence will be 2206.[20]
| January 5 | February 3 | June 30 | July 30 | December 25 |
|---|---|---|---|---|
| Partial (south) |
Partial (north) |
Partial (north) |
Partial (south) |
Annular (south) |
Saros 111 |
Saros 149 |
Saros 116 |
Saros 154 |
Saros 121 |
Solar eclipses are seen on Earth because of a fortuitous combination of circumstances. Even on Earth, eclipses of the type familiar to people today are a temporary (on a geological time scale) phenomenon. Hundreds of millions of years in the past, the Moon was too close to the Earth to precisely occlude the Sun as it does during eclipses today; and over a billion years in the future, it will be too far away to do so.
Due to tidal acceleration, the orbit of the Moon around the Earth becomes approximately 3.8 cm more distant each year. It is estimated that in 1.4 billion years, the distance from the Earth to the Moon will have increased by 23,500 km, meaning that it will no longer be able to completely cover the Sun's disk. This will be true even when the Moon is at perigee, and the Earth at aphelion.[21]
In addition, the Sun will increase in size over this timescale, making it even more unlikely that the Moon will be able to cause a total eclipse. Therefore, the last total solar eclipse on Earth will occur in slightly less than 1.4 billion years.[21]
Historical eclipses are a very valuable resource for historians, in that they allow a few historical events to be dated precisely, from which other dates and ancient calendars may be deduced. A solar eclipse of June 15, 763 BC mentioned in an Assyrian text is important for the Chronology of the Ancient Orient. Also known as the eclipse of Bur Sagale, it is the earliest solar eclipse mentioned in historical sources that has been identified successfully. There have been other claims to date earlier eclipses, notably that of Mursili II (likely 1312 BC), in Babylonia, and also in China, during the Fifth Year (possibly 1876 BC) of the reign of Emperor Zhong Kang, but these are highly disputed and rely on much supposition.[22] Perhaps the earliest still-unproven claim is that of archaeologist Bruce Masse, who putatively links an eclipse that occurred on May 10, 2807 BC with a possible meteor impact in the Indian Ocean on the basis of several ancient flood myths that mention a total solar eclipse.[23]
Eclipses have been interpreted as omens, or portents. The ancient Greek historian Herodotus wrote that Thales of Miletus predicted an eclipse that 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. The exact eclipse involved remains 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.[24] An eclipse recorded by Herodotus before Xerxes departed for his expedition against Greece,[25] which is traditionally dated to 480 BC, was matched by John Russell Hind to an annular eclipse of the Sun at Sardis on February 17, 478 BC.[26] Alternatively, a partial eclipse was visible from Persia on October 2, 480 BC.[27] Herodotus also reports a solar eclipse at Sparta during the Second Persian invasion of Greece.[28] The date of the eclipse (August 1, 477 BC) does not match exactly the conventional dates for the invasion accepted by historians.[29]
Chinese records of eclipses begin at around 720 BC.[30] The 4th century BC astronomer Shi Shen described the prediction of eclipses by using the relative positions of the moon and sun.[31] The "radiating influence" theory (i.e., the moon's light was light reflected from the sun) was existent in Chinese thought from about the sixth century BC (in the Zhi Ran of Zhi Ni Zi),[32] though it was opposed by the 1st century AD philosopher Wang Chong, who made clear in his writing that this theory was nothing new.[33] Ancient Greeks, such as Parmenides and Aristotle, also supported the theory of the moon shining because of reflected light.[32]
Attempts have been made to establish the exact date of Good Friday by assuming the darkness described at Christ's crucifixion was a solar eclipse. This research has not yielded conclusive results,[34][35] and Good Friday is recorded as being at Passover, which is held at the time of a full moon. In the Western hemisphere, there are few reliable records of eclipses before 800 AD, until the advent of Arab and monastic observations in the early medieval period.[30] The first recorded observation of the corona was made in Constantinople in 968 AD.[27][30]
The first known telescopic observation of a total solar eclipse was made in France in 1706.[30] Nine years later, English astronomer Edmund Halley observed the solar eclipse of May 3, 1715.[27][30] By the mid-19th century, scientific understanding of the Sun was improving through observations of the Sun's corona during solar eclipses. The corona was identified as part of the Sun's atmosphere in 1842, and the first photograph (or daguerreotype) of a total eclipse was taken of the solar eclipse of July 28, 1851.[27] Spectroscope observations were made of the solar eclipse of August 18, 1868, which helped to determine the chemical composition of the Sun.[27]
Looking directly at the photosphere of the Sun (the bright disk of the Sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in permanent impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring.[36]
Under normal conditions, the Sun is so bright that it is difficult to stare at it directly. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare at it. Unfortunately, looking at the Sun during an eclipse is as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun's disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse). Viewing the Sun's disk through any kind of optical aid (binoculars, a telescope, or even an optical camera viewfinder) is extremely hazardous and can cause irreversible eye damage in a fraction of a second.[37][38]
Glancing at the Sun with all or most of its disk visible is unlikely to result in permanent harm, as the pupil will close down and reduce the brightness of the whole scene. If the eclipse is near total, the low average amount of light causes the pupil to open. Unfortunately the remaining parts of the Sun are still just as bright, so they are now brighter on the retina than when looking at a full Sun. As the eye has a small fovea, for detailed viewing, the tendency will be to track the image on to this best part of the retina, causing damage.
Viewing the Sun during partial and annular eclipses (and during total eclipses outside the brief period of totality) requires special eye protection, or indirect viewing methods, if eye damage is to be avoided. The Sun's disk can be viewed using appropriate filtration to block the harmful part of the Sun's radiation. Sunglasses do not make viewing the sun safe. Only properly designed and certified solar filters should be used for direct viewing of the Sun's disk.[39] Especially, self-made filters using common objects such as a floppy disk removed from its case, a Compact Disc, a black colour slide film, etc. must be avoided.[40]
The safest way to view the Sun's disk is by indirect projection. This can be done by projecting an image of the disk onto a white piece of paper or card using a pair of binoculars (with one of the lenses covered), a telescope, or another piece of cardboard with a small hole in it (about 1 mm diameter), often called a pinhole camera. The projected image of the Sun can then be safely viewed; this technique can be used to observe sunspots, as well as eclipses. Care must be taken, however, to ensure that no one looks through the projector (telescope, pinhole, etc.) directly. Viewing the Sun's disk on a video display screen (provided by a video camera or digital camera) is safe, although the camera itself may be damaged by direct exposure to the Sun. The optical viewfinders provided with some video and digital cameras are not safe. Securely mounting #14 welder's glass in front of the lens and viewfinder protects the equipment and makes viewing possible.[41] Professional workmanship is essential because of the dire consequences any gaps or detaching mountings will have. In the partial eclipse path one will not be able to see the corona or nearly complete darkening of the sky, yet, depending on how much of the sun's disk is obscured, some darkening may be noticeable. If two-thirds or more of the sun is obscured, then an effect can be observed by which the daylight appears to be dim, as if the sky were overcast, yet objects still cast sharp shadows.
When the shrinking visible part of the photosphere becomes very small, Baily's beads will occur. These are caused by the sunlight still being able to reach Earth through lunar valleys. Totality then begins with the diamond ring effect, the last bright flash of sunlight.[42]
It is safe to observe the total phase of a solar eclipse directly with the unaided eye, binoculars or a telescope, only when the Sun's photosphere is completely covered by the Moon. During this period the sun is too dim to be seen through filters. The Sun's faint corona will be visible, and the chromosphere, solar prominences, and possibly even a solar flare may be seen. However, viewing the Sun after totality is dangerous.
At the end of totality, the same effects will occur in reverse order, and on the opposite side of the moon.
Photographing an eclipse is possible with fairly common camera equipment. In order for the disk of the Sun/Moon to be easily visible, a fairly high magnification long focus lens is needed (70–200 mm for a 35 mm camera), and for the disk to fill most of the frame, a longer lens is needed (over 500 mm). As with viewing the Sun directly, looking at it through the viewfinder of a camera can produce damage to the retina, so care is advised.[43]
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. According to the point reached in the solar cycle, the corona may appear small and symmetric, or large and fuzzy. It is very hard to predict this prior to totality.[44]
Phenomena associated with eclipses include shadow bands (also known as flying shadows), which are similar to shadows on the bottom of a swimming pool. They only occur just prior to and after totality, when a narrow solar crescent acts as an anisotropic light source.[45]
The observation of a total solar eclipse of May 29, 1919 helped to confirm Einstein's theory of general relativity. By comparing the apparent distance between two stars, with and without the Sun between them, Arthur Eddington stated that the theoretical predictions about gravitational lenses were confirmed, though it now appears the data was ambiguous at the time. The observation with the Sun between the stars was only possible during totality, since the stars are then visible.[46]
There is a long history of observations of gravity-related phenomena during solar eclipses, especially around totality. In 1954 and again in 1959, Maurice Allais reported observations of strange and unexplained movement during solar eclipses.[47] This phenomenon is now called the Allais Effect. Similarly, Saxl and Allen in 1970 observed sudden change in motion of a torsion pendulum, and this phenomenon is called the Saxl effect.[48]
A recent published observation during the 1997 solar eclipse by Wang et al. suggested a possible gravitational shielding effect,[49] which generated debate. Later in 2002, Yang and Wang published detailed data analysis which suggested that the phenomenon still remains unexplained.[50] More studies are being planned by NASA and ESA over the next decade.
In principle, the simultaneous occurrence of a Solar eclipse and a transit of a planet is possible. But these events are extremely rare because of their short durations. The next anticipated simultaneous occurrence of a Solar eclipse and a transit of Mercury will be on July 5, 6757, and a Solar eclipse and a transit of Venus is expected on April 5, 15232.[51]
Only five hours after the transit of Venus on June 4, 1769, there was a total solar eclipse, which was visible in the Northern hemisphere as a partial eclipse. This was the lowest time difference between a transit of a planet and a solar eclipse in the historical past.
More common, but still infrequent, is a conjunction of a planet (especially but not only Mercury or Venus) at the time of a total solar eclipse, in which event the planet will be visible very near the eclipsed Sun, when without the eclipse it would have been lost in the Sun's glare. At one time, some scientists hypothesized that there may be a planet (often given the name Vulcan) even closer to the Sun than Mercury; the only way to confirm its existence would have been to observe it in transit or during a total solar eclipse.[52] It now is known that no such planet exists. While there does remain some possibility for small Vulcanoid asteroids to exist, none has ever been found.
Artificial satellites can also pass in front of the Sun as seen from Earth, but none is large enough to cause an eclipse. At the altitude of the International Space Station, for example, an object would need to be about 3.35 km (2.08 mi) across to blot the Sun out entirely. These transits are difficult to watch, because the zone of visibility is very small. The satellite passes over the face of the Sun in about a second, typically. As with a transit of a planet, it will not get dark.[53]
Artificial satellites do play an important role in documenting solar eclipses. Images of the umbra on the Earth's surface taken from Mir and the International Space Station are among the most spectacular of all eclipse images.[54] Observations of eclipses from satellites orbiting above the Earth's atmosphere are not subject to weather conditions.
The direct observation of a total solar eclipse from space is rare. The only documented case is Gemini 12 in 1966.[55] The partial phase of the 2006 total eclipse was visible from the International Space Station. At first, it looked as though an orbit correction in the middle of March would bring the ISS in the path of totality, but this correction was postponed.
Short term eclipse cycles repeat every six lunations (every 177 days), each set lasting three–four years. They occur at either the ascending or descending node of the moon's orbit. Each set has the moon's shadow crossing the earth near the north or south pole, and subsequent events progress toward the other pole until it misses the earth and the series ends. The Saros cycle increments by 5 within each set, and 5 different sets repeat every 18 years, the Saros period.
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| Book: Solar Eclipses | |
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