Solar eclipse

Total solar eclipse
Annular solar eclipsePartial solar eclipse

A solar eclipse occurs when an observer (on Earth) passes through the shadow cast by the Moon which fully or partially blocks ("occults") the Sun. This can only happen when the Sun, Moon and Earth are nearly aligned on a straight line in three dimensions (syzygy) during a new moon when the Moon is close to the ecliptic plane.[1] 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.

If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every new moon. However, since the Moon's orbit is tilted at more than 5 degrees to the Earth's orbit around the Sun, its shadow usually misses Earth. A solar eclipse can only occur when the moon is close enough to the ecliptic plane during a new moon. Special conditions must occur for the two events to coincide because the Moon's orbit crosses the ecliptic at its orbital nodes twice every draconic month (27.212220 days) while a new moon occurs one every synodic month (29.530587981 days). Solar (and lunar) eclipses therefore happen only during eclipse seasons resulting in at least two, and up to five, solar eclipses each year; no more than two of which can be total eclipses.[2][3]

Total eclipses are rare because the timing of the new moon within the eclipse season needs to be more exact for an alignment between the observer (on Earth) and the centers of the Sun and Moon. In addition, the elliptical orbit of the Moon often takes it far enough away from Earth that its apparent size is not large enough to block the Sun entirely. Total solar eclipses are rare at any particular location because totality exists only along a narrow path on the Earth's surface traced by the Moon's full shadow or umbra.




Partial and annular phases of solar eclipse on May 20, 2012
Partial solar eclipse during annular solar eclipse of May 20, 2012

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 Earth.[6] This narrow track is called the path of totality.[7]






Terminology for central eclipse

Each icon shows the view from the centre of its black spot, representing the Moon (not to scale)

Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse.[13] 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 the Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse.[13] The last (umbral yet) non-central solar eclipse was on April 29, 2014. This was an annular eclipse. The next non-central total solar eclipse will be on April 9, 2043.[14]


The phases observed during a total eclipse are called:[15]




Geometry of a total solar eclipse (not to scale)




  Moon Sun
At perigee
At apogee
At perihelion
At aphelion
Mean radius 1,737.10 km
(1,079.38 mi)
696,000 km
(432,000 mi)
Distance 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)
33' 30"
29' 26"
32' 42"
31' 36"
Apparent size
to scale
Order by
apparent size
1st 4th 2nd 3rd










Besselian elements are used to predict whether an eclipse will be partial, annular, or total (or annular/total), and what the eclipse circumstances will be at any given location.

Calculations with Besselian elements can determine the exact shape of the umbra's shadow on the earth's surface. But at what longitudes on the earth's surface the shadow will fall, is a function of the earth's rotation, and on how much that rotation has slowed down over time. A number called ΔT is used in eclipse prediction to take this slowing into account. As the earth slows, ΔT increases. ΔT for dates in the future can only be roughly estimated because the earth's rotation is slowing irregularly. This means that, although it is possible to predict that there will be a total eclipse on a certain date in the far future, it is not possible to predict in the far future exactly at what longitudes that eclipse will be total.


The following factors determine the duration of a total solar eclipse (in order of decreasing importance):[30][31]

  1. The Moon being almost exactly at perigee (making its angular diameter as large as possible).
  2. The Earth being very near aphelion (furthest away from the Sun in its elliptical orbit, making its angular diameter nearly as small as possible).
  3. The midpoint of the eclipse being very close to the Earth's equator, where the rotational velocity is greatest.
  4. The vector of the eclipse path at the midpoint of the eclipse aligning with the vector of the Earth's rotation (i.e. not diagonal but due east).
  5. The midpoint of the eclipse being near the subsolar point (the part of the Earth closest to the Sun).


Occurrence and cycles


Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average,[33] it is estimated that they recur at any given place only once every 360 to 410 years, on average.[34] The total eclipse lasts for only a maximum of a few minutes at any location, because the Moon's umbra moves eastward at over 1700 km/h.[35] Totality currently can never last more than 7 min 32 s. This value changes over the millennia and is currently decreasing. By the 8th millennium, the longest theoretically possible total eclipse will be less than 7 min 2 s.[30] The last time an eclipse longer than 7 minutes occurred was June 30, 1973 (7 min 3 sec). Observers aboard a Concorde supersonic aircraft were able to stretch totality for this eclipse to about 74 minutes by flying along the path of the Moon's umbra.[36] The next total eclipse exceeding seven minutes in duration will not occur until June 25, 2150. The longest total solar eclipse during the 11,000 year period from 3000 BC to at least 8000 AD will occur on July 16, 2186, when totality will last 7 min 29 s.[30][37] For comparison, the longest total eclipse of the 20th century at 7 min 8 s occurred on June 20, 1955, and there are no total solar eclipses over 7 min in duration in the 21st century.[38]


Frequency per year

Between two and five solar eclipses occur every year, with at least one 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.[40] On average, there are about 240 solar eclipses each century.[41]

The 5 solar eclipses of 1935
January 5 February 3 June 30 July 30 December 25
Saros 111
Saros 149
Saros 116
Saros 154
Saros 121

Final totality


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 ancient calendars may be deduced.[44] A solar eclipse of June 15, 763 BC mentioned in an Assyrian text is important for the chronology of the ancient Near East.[45] There have been other claims to date earlier eclipses. The Book of Joshua 10:13 describes an event that a group of University of Cambridge scholars concluded to be the annular solar eclipse that occurred on 30 October 1207 BC.[46] The Chinese king Zhong Kang supposedly beheaded two astronomers, Hsi and Ho, who failed to predict an eclipse 4,000 years ago.[47] 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.[48]

Eclipses have been interpreted as omens, or portents.[49] The ancient Greek historian Herodotus wrote that Thales of Miletus predicted an eclipse that occurred during a battle between the Medes and the Lydians. Both sides put down their weapons and declared peace as a result of the eclipse.[50] 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 Asia Minor.[51] An eclipse recorded by Herodotus before Xerxes departed for his expedition against Greece,[52] 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.[53] Alternatively, a partial eclipse was visible from Persia on October 2, 480 BC.[54] Herodotus also reports a solar eclipse at Sparta during the Second Persian invasion of Greece.[55] The date of the eclipse (August 1, 477 BC) does not match exactly the conventional dates for the invasion accepted by historians.[56]

Chinese records of eclipses begin at around 720 BC.[57] The 4th century BC astronomer Shi Shen described the prediction of eclipses by using the relative positions of the Moon and Sun.[58] The "radiating influence" theory (i.e., the Moon's light was reflection from the Sun) was existent in Chinese thought from about the sixth century BC (in the Zhi Ran of Zhi Ni Zi),[59] though it was opposed by the 1st century AD philosopher Wang Chong, who made clear in his writing that this theory was nothing new.[58] Ancient Greeks, such as Parmenides and Aristotle, also supported the theory of the Moon shining because of reflected light.[59]

Attempts have been made to establish the exact date of Good Friday by assuming that the darkness described at Jesus's crucifixion was a solar eclipse. This research has not yielded conclusive results,[60][61] and Good Friday is recorded as being at Passover, which is held at the time of a full moon. Further, the darkness lasted from the sixth hour to the ninth, or three hours, which is much, much longer than the eight-minute upper limit for any solar eclipse's totality. In the Western hemisphere, there are few reliable records of eclipses before AD 800, until the advent of Arab and monastic observations in the early medieval period.[57] The first recorded observation of the corona was made in Constantinople in AD 968.[54][57]

The first known telescopic observation of a total solar eclipse was made in France in 1706.[57] Nine years later, English astronomer Edmund Halley accurately predicted and observed the solar eclipse of May 3, 1715.[54][57] 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.[54] Spectroscope observations were made of the solar eclipse of August 18, 1868, which helped to determine the chemical composition of the Sun.[54]







Partial and annular eclipses










Eclipse chasing

A dedicated group of eclipse chasers have pursued the observation of solar eclipses when they occur around the Earth.[76] A person who chases eclipses is known as an umbraphile, meaning shadow lover.[77] Umbraphiles travel for eclipses and use various tools to help view the sun including solar viewing glasses, also known as eclipse glasses, as well as telescopes.[78][79]

Other observations




1919 observations



Gravity anomalies


Observation during the 1997 solar eclipse by Wang et al. suggested a possible gravitational shielding effect,[88] which generated debate. In 2002, Wang and a collaborator published detailed data analysis, which suggested that the phenomenon still remains unexplained.[89]

Eclipses and transits





Artificial satellites



Observations of eclipses from spacecraft or artificial satellites orbiting above the Earth's atmosphere are not subject to weather conditions. The crew of Gemini 12 observed a total solar eclipse from space in 1966.[94] The partial phase of the 1999 total eclipse was visible from Mir.[95]





Recent and forthcoming solar eclipses

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

Eclipses only occur in the eclipse season, when the Sun is close to either the ascending or descending node of the Moon. Each eclipse is separated by one, five or six lunations (synodic months), and the midpoint of each season is separated by 173.3 days, which is the mean time for the Sun to travel from one node to the next. The period is a little less than half a calendar year because the lunar nodes slowly regress. Because 223 synodic months is roughly equal to 239 anomalistic months and 242 draconic months, eclipses with similar geometry recur 223 synodic months (about 6,585.3 days) apart. This period (18 years 11.3 days) is a saros. Because 223 synodic months is not identical to 239 anomalistic months or 242 draconic months, saros cycles do not endlessly repeat. Each cycle begins with the Moon's shadow crossing the Earth near the north or south pole, and subsequent events progress toward the other pole until the Moon's shadow misses the Earth and the series ends.[24] Saros cycles are numbered; currently, cycles 117 to 156 are active. The last solar eclipse occurred on January 6, 2019. It was a partial solar eclipse visible from Asia and Alaska.[100]

Solar eclipses

See also