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 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.
[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
umbra.
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
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.
[5]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][7] 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.
[8]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.
[edit]Terminology for central eclipse
Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse.
[9] 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.
[10] 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.
[11] 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
[edit]Predictions
[edit]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.
[12][13]
| 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.
[14][15]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 (or rarely a partial and a central) eclipses to occur in consecutive months.
[16][17]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.
[18][edit]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.
[19]Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average,
[20] 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.
[21] 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.
[22][edit]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.
[23][edit]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.
[24]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.
[edit]Historical eclipses
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.
[25] 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.
[26][27]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.
[28] 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.
[29] Hind and Chambers considered this absolute date more than a century ago.
[30] Herodotus also reports that another solar eclipse was observed in
Sparta during the next year, on August 1, 477 BC.
[31][32][33] 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.
[34] 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.
[35]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.
[36] Research has manifested the inability of total solar eclipses to serve as explanations for the recorded
Good Friday features of the
crucifixion eclipse.
[37] (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.
[38] 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),
[39] and opposed by the
Chinese philosopher Wang Chong (AD 27–97), who made clear in his writing that this theory was nothing new.
[40] 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.
[39]
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.
[39] 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).
[41]
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.
[42]
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.
[43] The conflict was named the
Battle of Isandlwana, the Zulu name for the battle translates as "the day of the dead moon".
[44][edit]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.
[45]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.
[46][47]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.
[edit]Partial and annular eclipses
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.
[48] 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.
[49]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.
[50] 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.
[edit]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.
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.
[51]At the end of totality, the same effects will occur in reverse order, and on the opposite side of the moon.
[edit]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.
[52][edit]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.
[53]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.
[54]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.
[edit]1919 observations
[edit]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.
[56] 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.
[57]A recent published observation during the 1997 solar eclipse by Wang
et al. suggested a possible gravitational shielding effect,
[58] though there is some serious debate. Later in 2002, Yang and Wang published detailed data analysis which suggested that the phenomenon still remains unexplained.
[59] More studies are being planned by NASA and ESA over the next decade.
[edit]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.
[60][edit]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.
[61]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.
[edit]Artificial satellites
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.
[62]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.
[63] 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.
[64][edit]Meteorological measurements
A special weather station used for meteorological measurements during solar eclipses.
[65]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).
[65]
