Solar eclipse

This article is about the astronomical phenomenon. For the video game, see Solar Eclipse (video game).

Not to be confused with Lunar eclipse.

Photo of 1999 total eclipse

As seen from the Earth, a solar eclipse occurs when the Sun passes between the Moon and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and Moon are in conjunction as seen from Earth. At least two, and up to five, solar eclipses occur each year; no more than two can be total eclipses. 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 umbra.

Some people, sometimes referred to as "eclipse chasers" or "umbraphiles", will travel to remote locations to observe or witness a predicted central solar eclipse (see Types below). The solar eclipse of August 11, 1999, in Europe helped to increase public awareness of the phenomenon, which apparently led to an unusually large number of journeys made specifically to witness the annular solar eclipse of October 3, 2005, and of March 29, 2006.

The last total solar eclipse was the solar eclipse of July 11, 2010; the next will be the solar eclipse of November 13, 2012. The recent solar eclipse of June 1, 2011 and the Solar eclipse of July 1, 2011, were partial eclipses (see Types below); the next partial eclipse will occur on November 25, 2011.

A total solar 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.

Types

Annular solar eclipse on October 3, 2005

There are four types of solar eclipses:

• A total eclipse occurs when the dark silhouette of the Moon completely obscures the intensely bright light of the Sun, allowing the much fainter solar corona to be visible. During any one eclipse, totality occurs at best only in a narrow track on the surface of the Earth.
• An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon.
• A hybrid eclipse (also called annular/total eclipse) shifts between a total and annular eclipse. At some points on the surface of the Earth it appears as a total eclipse, whereas at others it appears as annular. Hybrid eclipses are comparatively rare.
• A partial eclipse occurs when the Sun and Moon are not exactly in line and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra passes above the Earth's polar regions and never intersects the Earth's surface.

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 transiting the Sun as seen from STEREO-B on February 25, 2007 at 4.4 times the distance between the Earth and the Moon.

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. 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.

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.

Terminology for central eclipse

Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse. 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. 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.

The phases observed during a total eclipse are called:

• First Contact—when the moon's limb first becomes visible on the solar disk. Some also name individual phases between First and Second Contact e.g. Pac-Man phase.
• Second Contact—starting with Baily's Beads {caused by light shining through valleys on the moon's surface} and the Diamond Ring. Almost the entire disk is covered.
• Totality—the limb of the moon obscuring the entire disk of the sun and only the corona visible
• Third Contact—when the first bright light becomes visible and the shadow is moving away from the observer. Again a Diamond Ring may be observed

Predictions

Geometry

Geometry of a Total Solar Eclipse (not to scale)

A Total eclipse in the umbra.
B Annular eclipse in the antumbra.
C Partial eclipse in the penumbra

The diagram to the right shows the alignment of the Sun, Moon and Earth during a solar eclipse. The dark gray region below the Moon 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 only a partial and annular eclipses can be seen.

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.

	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.

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. Since the eclipse limit creates a window of opportunity of up to 36 degrees (24 degrees for central eclipses), it is possible for partial (or rarely a partial and a central) eclipses to occur in consecutive months.

Path

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.

Occurrence and cycles

Total Solar Eclipse Paths: 1001–2000, showing that total solar eclipses occur everywhere on earth. This image was merged from 50 separate images from NASA.

Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, 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. 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.

Frequency per year

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.

The 5 solar eclipses of 1935

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

Final totality

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 many millions of 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 600 million 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.

A complicating factor is that the Sun will increase in size over this timescale. This makes 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 600 million years.

Historical eclipses

Astronomers Studying an Eclipse painted by Antoine Caron in 1571

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 a society's calendar may be deduced. Aryabhata (476–550) concluded the Heliocentric theory in solar eclipse. 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. Perhaps the earliest still-unproven claim is that of archaeologist Bruce Masse asserting on the basis of several ancient flood myths, which mention a total solar eclipse, he links an eclipse that occurred May 10, 2807 BC with a possible meteor impact in the Indian Ocean. 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 (2084 BC) of the regime of Emperor Zhong Kang of Xia dynasty, but these are highly disputed and rely on much supposition.

Herodotus wrote that Thales of Miletus 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 recorded. Hind and Chambers considered this absolute date more than a century ago. Herodotus also 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 day, 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. The modern conventional dates are different by a year or two, and that these two eclipse records have been ignored so far. The Chronicle of Ireland recorded a solar eclipse on June 29, AD 512, and a solar eclipse was reported to have taken place during the Battle of Stiklestad in July, 1030.

In the Indian epic the Mahabharata the incident is related of the thirteenth day when Arjun vows to slay Jayadrath before nightfall, to avenge the death of Abhimanyu at Jayadratha's hands. What may only be described as a solar eclipse brought Jayadrath out to celebrate his surviving the day, only to have the sun reappear and Arjun killed Jayadrath. In the epic astronomers have calculated all possible eclipse pairs matching the above time difference and being visible from Kurukshetra, the battlefield of the Mahabharata war. 3129 BC and 2559 BC appear to be the best candidate for the Mahabharata war.

Attempts have been made to establish the exact date of Good Friday by means of solar eclipses, but this research has not yielded conclusive results. Research has manifested the inability of total solar eclipses to serve as explanations for the recorded Good Friday features of the crucifixion eclipse. (Good Friday is recorded as being at Passover, which is also recorded as being at or near the time of a full moon.)

The ancient Chinese astronomer Shi Shen (fl. fourth century BC) was aware of the relation of the moon in a solar eclipse, as he provided instructions in his writing to predict them by using the relative positions of the moon and sun. The "radiating influence" theory for a solar eclipse (i.e., the moon's light was merely light reflected from the sun) was existent in Chinese thought from about the sixth century BC (in the Zhi Ran of Zhi Ni Zi), and opposed by the Chinese philosopher Wang Chong (AD 27–97), who made clear in his writing that this theory was nothing new. This can be said of Jing Fang's writing in the 1st century BC, which stated:

The moon and the planets are Yin; they have shape but no light. This they receive only when the sun illuminates them. The former masters regarded the sun as round like a crossbow bullet, and they thought the moon had the nature of a mirror. Some of them recognized the moon as a ball too. Those parts of the moon which the sun illuminates look bright, those parts which it does not, remain dark.

The ancient Greeks had known this as well, since it was Parmenides of Elea, around 475 BC, who supported the theory of the moon shining because of reflected light, and was accepted in the time of Aristotle as well. The Chinese astronomer and inventor Zhang Heng (AD 78–139) wrote of both solar and lunar eclipses in the publication of Ling Xian in AD 120, supporting the radiating influence theory that Wang Chong had opposed (Wade-Giles):

The sun is like fire and the moon like water. The fire gives out light and the water reflects it. Thus the moon's brightness is produced from the radiance of the sun, and the moon's darkness (pho) is due to (the light of) the sun being obstructed (pi). The side which faces the sun is fully lit, and the side which is away from it is dark. The planets (as well as the moon) have the nature of water and reflect light. The light pouring forth from the sun (tang jih chih chhung kuang) does not always reach the moon owing to the obstruction (pi) of the earth itself—this is called "an-hsü", a lunar eclipse. When (a similar effect) happens with a planet (we call it) an occultation (hsing wei); when the moon passes across (kuo)(the sun's path) then there is a solar eclipse (shih).

The later Chinese scientist and statesman Shen Kuo (AD 1031–1095) also wrote of eclipses, and his reasoning for why the celestial bodies were round and spherical instead of flat (Wade-Giles spelling):

The Director [of the Astronomical Observatory] asked me about the shapes of the sun and moon; whether they were like balls or (flat) fans. If they were like balls they would surely obstruct (ai) each other when they met. I replied that these celestial bodies were certainly like balls. How do we know this? By the waxing and waning (ying khuei) of the moon. The moon itself gives forth no light, but is like a ball of silver; the light is the light of the sun (reflected). When the brightness is first seen, the sun(-light passes almost) alongside, so the side only is illuminated and looks like a crescent. When the sun gradually gets further away, the light shines slanting, and the moon is full, round like a bullet. If half of a sphere is covered with (white) powder and looked at from the side, the covered part will look like a crescent; if looked at from the front, it will appear round. Thus we know that the celestial bodies are spherical ... Since the sun and moon are in conjunction (ho) and in opposition (tui) once a day, why then do they have eclipses only occasionally?' I answered that the ecliptic and the moon's path are like two rings, lying one over the other (hsiang tieh), but distant by a small amount. (If this obliquity did not exist), the sun would be eclipsed whenever the two bodies were in conjunction, and the moon would be eclipsed whenever they were exactly in position. But (in fact) though they may occupy the same degree, the two paths are not (always) near (each other), and so naturally the bodies do not (intrude) upon one another.

Eclipses have been interpreted as omens, or portents, especially when associated with battles. On 22 January 1879 Zulu warriors successfully defeated a British battalion in the fight against imperialism during the Zulu War in South Africa. At 2:29 PM there was a solar eclipse. The conflict was named the Battle of Isandlwana, the Zulu name for the battle translates as "the day of the dead moon".

Viewing

The Pinhole Projection Method of observing partial Solar Eclipse. At the insert in the upper left corner of this image one can see the partially eclipsed sun that was photographed with a white solar filter. At the bottom of the image one can see the projection of the partially eclipsed sun.

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.

Under normal conditions, the Sun is so bright that it is difficult to stare at it directly, so there is no tendency to look at it in a way that might damage the eye. 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 just 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.

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.

Partial and annular eclipses

Eclipse glasses

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. 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 despite what may have been said in the media.

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. 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.

Totality

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.

Baily's beads

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, but no longer where mountains are present. Totality then begins with the diamond ring effect, the last bright flash of sunlight.

At the end of totality, the same effects will occur in reverse order, and on the opposite side of the moon.

Photography

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.

Other observations

The progression of a solar eclipse on August 1, 2008 in Novosibirsk, Russia. All times UTC (local time was UTC+7). The time span between shots is three minutes.

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. 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.

During a solar eclipse, special (indirect) observations can also be achieved with the unaided eye only. Normally the spots of light which fall through the small openings between the leaves of a tree have a circular shape. These are images of the Sun. During a partial eclipse, the light spots will show the partial shape of the Sun, as seen on the picture.

Another famous phenomenon is 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, and are very difficult to observe. Many professional eclipse chasers have never been able to witness them.

During a partial eclipse, a related effect that can be seen is anisotropy in the shadows of objects. Particularly if the partial eclipse is nearly total, the unobscured part of the sun acts as an approximate line source of light. This means that objects cast shadows which have a very narrow penumbra in one direction, but a broad penumbra in the perpendicular direction.

1919 observations

The original photograph of the 1919 eclipse which was claimed to confirm Einstein's theory of general relativity.

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.

Gravity anomalies

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. 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.

A recent published observation during the 1997 solar eclipse by Wang et al. suggested a possible gravitational shielding effect, though there is some serious debate. Later in 2002, Yang and Wang published detailed data analysis which suggested that the phenomenon still remains unexplained. More studies are being planned by NASA and ESA over the next decade.

Before sunrise, after sunset

The phenomenon of atmospheric refraction makes it possible to observe the Sun (and hence a solar eclipse) even when it is slightly below the horizon. It is, however, possible for a solar eclipse to attain totality (or in the event of a partial eclipse, near-totality) before (visual and actual) sunrise or after sunset from a particular location. When this occurs shortly before the former or after the latter, the sky will appear much darker than it would otherwise be immediately before sunrise or after sunset. On these occasions, an object (especially a planet, often Mercury) may be visible near the sunrise or sunset point of the horizon when it could not have been seen without the eclipse.

Eclipses and transits

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.

Only five 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.

More common, but still infrequent, is a conjunction of any planet (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 during a total solar eclipse. 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

Shadow of the moon above Turkey and Cyprus, seen from the ISS during a 2006 total solar eclipse.

Artificial satellites can also pass in front of, or transit, 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.

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. 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. 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.

Meteorological measurements

A special weather station used for meteorological measurements during solar eclipses.

A marked drop of the intensity of the solar radiation occurs during solar eclipse. It influences the actions in the atmosphere. The variations of the atmospheric actions display in changes of standard meteorological and physical quantities. These may be noticed by a measurement of the air temperature and other meteorological quantities (e.g.: air humidity, soil temperature, colour of the solar radiation).

The progressions of the quantities are usually detected by special weather stations because of a short duration of a total (annular) solar eclipse. The properties of the devices usually are: high speed of measurement, high resolution, and sensitivity. Acquired results show variations in progressions of meteorological and physical quantities (e.g.: colour of the light).

Recent and forthcoming solar eclipses

Main article: List of solar eclipses in the 21st century

Further information: Lists of solar eclipses

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.

Eclipse path for total and hybrid eclipses from 2001-2020.

1997–2000

Further information: Template:Solar eclipse set 1997-2000

2000–2003

Further information: Template:Solar eclipse set 2000-2003

2004–2007

Further information: Template:Solar eclipse set 2004-2007

2008–2011

Further information: Template:Solar eclipse set 2008-2011

2011–2014

Further information: Template:Solar eclipse set 2011-2014

2015–2018

Further information: Template:Solar eclipse set 2015-2018

2018–2021

Further information: Template:Solar eclipse set 2018-2021

2022–2025

Further information: Template:Solar eclipse set 2022-2025

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See also

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Eclipses elsewhere

• Solar eclipses on Jupiter
• Solar eclipses on Pluto
• Transit of Deimos from Mars
• Transit of Phobos from Mars

Eclipse lists

• Articles on individual solar eclipses
• List of solar eclipses

Miscellaneous

• Besselian Elements
• Black Sun (mythology)
• Solar eclipses in fiction

References

• Mobberley, Martin (2007) Total Solar Eclipses and How to Observe Them (Astronomers' Observing Guides). New York: Springer
• Needham, Joseph (1986). Science and Civilization in China: Volume 3. Taipei: Caves Books, Ltd.

External links

• Solar eclipse of January 15, 2010, Fred Espenak, NASA
• Detailed eclipse explanations and predictions, Hermit Eclipse
• Prof. Druckmüller's eclipse photography site
• World Atlas of Solar Eclipse Paths, Fred Espenak (archived from the original on 2010-05027)
• Solar eclipse time sequence
• NASA's Eclipse Home Page, Fred Espenak (archived from the original on 2010-05-27)
• Animated maps of past and future solar eclipses
• Search among the 11,898 solar eclipses over five millennium and display interactive maps
• Looking Back at an Eclipsed Earth 1999 August 11 from Mir EO-27 - Astronomy Picture of the Day 10 June 2007
• Animated explanation of the mechanics of a solar eclipse, University of Glamorgan
• Eclipse Image Gallery at The World at Night

Eye safety

• Eye Safety During Solar Eclipses, F. Espenak (NASA Goddard Space Flight Center)
• How to Watch a Partial Solar Eclipse Safely, A. M. MacRobert (Sky & Telescope magazine)
• UK hospitals assess eye damage after solar eclipse, British Medical Journal, August 21, 1999, p. 319–469