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Solar eclipse

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Photo taken during the French 1999 eclipse.
29 March 2006 solar eclipse.

A solar eclipse occurs when the Moon passes in front of the Sun and obscures it totally or partially. This configuration can only occur at New Moon, when the Sun and Moon are in conjunction, as seen from Earth. A total solar eclipse is considered by many to be the most spectacular natural phenomenon that one can observe.

Types of solar eclipses

There are four types of solar eclipses:

  • A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark outline of the Moon, and the much fainter corona is visible (see image above). During any one eclipse, totality is visible only from at most 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 surrounding the outline of the Moon.
  • A hybrid eclipse is intermediate between a total and annular eclipse. At some points on the Earth it is visible as a total eclipse; whereas at others it is annular. The generic term for a total, annular or hybrid eclipse is a central eclipse.
  • 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 a central eclipse. However, some eclipses can only be seen as a partial eclipse, because the central line never intersects the Earth's surface.

The reason some solar eclipses are total and others are annular has to do with the elliptical nature of the Moon's orbit around Earth. One of the most remarkable coincidences in nature is that (i) the Sun lies about 400 times as far from Earth as does the Moon, and (ii) the Sun is also about 400 times the diameter of the Moon. As seen from Earth, therefore, the Sun and the Moon appear to be about the same size in the sky - about 1/2 of a degree in angular measure. Because the Moon's orbit around Earth is an ellipse rather than a circle, however, at some times during the month the Moon is further away, and at other times it is closer to Earth, than average.

When a solar eclipse occurs while the Moon is at its closest (near its perigee), it appears large enough to cover the bright disk, or photosphere, of the Sun completely, and a total eclipse occurs. When it is at its farthest, however (near apogee), it appears smaller, and it cannot cover the Sun completely. In that case, at the time of greatest eclipse there remains a thin annulus (or ring) of brilliant Sun left uncovered. Hence the term annular eclipse. Slightly more annular eclipses than total eclipses occur, because on average the Moon lies too far away from Earth to cover the Sun completely.

Terminology

The term eclipse is actually a misnomer: The phenomenon of the Moon passing in front of the Sun is actually an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. When the Moon disappears at Full Moon by passing into Earth's shadow, the event is properly called an eclipse, but when the Moon passes in front of the Sun, we see an occultation of the Sun by the Moon.

Photo taken by Wikipedia editor Luc Viatour (Lviatour) during the French 1999 eclipse.
Photo taken during the Spanish 2005 annular eclipse.
Photo taken in Valladolid (Spain) during the October 3 2005 annular eclipse.
People observing a Solar eclipse in Iceland 2002.

Observing a solar eclipse

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's 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 even more hazardous, although just viewing it with the naked eye can easily cause damage.

Viewing 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.

The Sun's disk can be viewed using appropriate filtration to block the harmful part of the Sun's radiation. Sunglasses are not safe, since they do not block the harmful and invisible infrared radiation which causes retinal damage. Only properly designed and certified solar filters should ever be used for direct viewing of the Sun's disk.

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. However, care must be taken 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.

These precautions apply to viewing the Sun at any time except during the totality phase of a total solar eclipse (see below).

Viewing totality during total eclipses

Contrary to popular belief, it is safe to observe the total phase of a total solar eclipse directly with the unaided eye, binoculars or a telescope, when the Sun's photosphere is completely covered by the Moon; indeed, this is a very spectacular and beautiful sight, and it is too dim to be seen through filters. The Sun's faint corona will be visible, and even the chromosphere, solar prominences, and possibly even a solar flare may be seen. However, it is important to stop directly viewing the Sun promptly at the end of totality. The exact time and duration of totality for the location from which the eclipse is being observed should be determined from a reliable source (local astronomers, etc.). Note that it is never safe to look at an annular or partial eclipse directly, because the Sun's disk is never completely covered during this type of eclipse.

Additional information

For more information on safe eclipse viewing, see:

Eclipse Predictions

Geometry of an Eclipse

Diagram of solar eclipse (not to scale).

The diagram to the right shows the alignment of the Sun, Moon and Earth at a solar eclipse. The dark gray region to the right of 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 will be seen. The larger light gray area is the penumbra, in which a partial eclipse will be seen.

Motion of the Moon and Earth

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 – hence the name.

The Moon's orbit is also elliptical, which means that the distance of the Moon from the Earth can vary by about 6% from its average value. This means that the apparent size of the Moon is sometimes larger or smaller than average, 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.

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 moved on in its orbit around the Sun. This means that the average time between one New Moon and the next is longer, and is approximately 29.6 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. 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 18.5 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 draconitic 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.

Frequency of Solar Eclipses

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 either above or below the earth, and others are central only in remote regions of the arctic or antarctic.

Path of an Eclipse

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).

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 of Eclipses at a given place

Total Solar Eclipse Paths: 1001-2000. This image was merged from 50 separated images from http://sunearth.gsfc.nasa.gov/eclipse/

Total solar eclipses are rare events. Although they occur somewhere on Earth approximately every 18 months, it has been estimated that they recur at any given place only once every 370 years, on average (Stephenson, p.54). Then, after waiting so long, the total eclipse only lasts for a few minutes, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 40 s, and is usually 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. 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 of comparable 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. (eclipse predictions by Fred Espenak, NASA/GSFC.)

For astronomers, a total solar eclipse forms a rare opportunity to observe the corona (the outer layer of the Sun's atmosphere). Normally this is not visible because the photosphere is much brighter than the corona.

Eclipse Cycles

If the date and time of a solar eclipse is known, it is possible to predict other eclipses using eclipse cycles. Two such cycles are the Saros and the Inex. The Saros cycle is probably the most well known, and one of the best, eclipse cycles. The Inex cycle is itself a poor cycle, but it is very convenient in the classification of eclipse cycles. After a Saros cycle finishes, a new Saros cycle begins 1 Inex later (hence its name: in-ex).

Historical solar eclipses

A solar eclipse of 15 June, 763 BC mentioned in an Assyrian text is important for the Chronology of the Ancient Orient. This is the earliest solar eclipse mentioned in historical sources that has been identified beyond reasonable doubt. There have been other claims to date earlier eclipses, notably that of Mursili II (likely 1312 BC), in Babylonia, and also in China, but these are highly disputed and rely on much supposition. For a discussion, see Stephenson (1997).

Herodotus wrote that Thales of Milete predicted an eclipse which occurred during a war between the Medians and the Lydians. Soldiers on both sides put down their weapons and declared peace as a result of the eclipse. Exactly which eclipse was involved has remained uncertain, although the issue has been studied by hundreds of ancient and modern authorities. One likely candidate took place on May 28, 585 BC, probably near the Halys river in the middle of modern Turkey.

An annular eclipse of the Sun occurred at Sardis on February 17, 478 BC, while Xerxes was departing for his expedition against Greece, as Herodotus, VII, 37 recorded ([Hind and Chambers, 1889: 323] considered this absolute date more than a century ago). Herodotus (book IX, 10, book VIII, 131, and book IX, 1) reports that another solar eclipse was observed in Sparta during the next year, on August 1, 477 BC. The sky suddenly darkened in the middle of the sky, well after the battles of Thermopylae and Salamis, after the departure of Mardonius to Thessaly at the beginning of the spring of (477 BC) and his second attack on Athens, after the return of Cleombrotus to Sparta. Note that the modern conventional dates are different by a year or two, and that these two eclipse records have been ignored so far.

Other Observations

During a solar eclipse special observations can be done with the unaided eye. 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.

Images of the sun during a partial eclipse through the leaves of a tree.

Special observation campaigns

Solar eclipse before sunrise or after sunset

It is possible for a solar eclipse to attain totality (or in the event of a partial eclipse, near totality) before 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.

Simultaneous occurrence of solar eclipse and transit of a planet

In principle, the simultaneous occurrence of a Solar eclipse and a transit of a planet is possible. But these events are extremely rare 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 of a Solar eclipse and a transit of Venus is expected on April 5, 15232.

Only 5 hours after the transit of Venus on June 4, 1769 there was a total solar eclipse, which was visible in Northern America, Europe and Northern Asia as partial solar eclipse. This was the lowest time difference between a transit of a planet and a solar eclipse in the historical past.

More common — but still quite rare — is a conjunction of any planet (not confined exclusively to Mercury or Venus) at the time 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 — including Albert Einstein — hypothesized that there may have been a planet 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. When no such planet was found during such an eclipse, the possibility of its existence was ruled out.

Solar eclipses by artificial satellites

Artificial satellites can also get in the line between Earth and Sun. But none are 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 across to blot the Sun out entirely. This means the best you can get is a satellite transit, but these events 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. Like a transit of a planet it will not get dark. [1]

Past and future eclipses

Although there is a total eclipse visible somewhere on Earth most years, some are more conveniently observed than others. Eclipses where the path of totality crosses major population centres generate the most interest in the general public.

Selected past and upcoming eclipses are:

Selected Solar Eclipses
Date of
eclipse 		Time (UTC) Type Max Duration Eclipse Path Notes
Start Mid End
May 29, 1919 - - - total   West Africa Photographed by Arthur Eddington to verify general relativity
August 11, 1999 - - - total - Europe, Asia
June 21, 2001 - - - total 04:57 min South America, Africa
December 14, 2001 - - - annular 03:53 min North and Middle America
June 10, 2002 - - - annular 00:23 min Asia, Australia, North America
December 4, 2002 - - - total 02:04 min South Africa, Antarctica, Indonesia, Australia
May 31, 2003 - - - annular 03:37 min Europe, Asia, North America
November 23, 2003 - - - total 01:57 min Australia, New Zealand, Antarctica, South America
April 19, 2004 - - - partial - Antarctica, South Africa
October 14, 2004 - - - partial - Asia, Hawaii, Alaska
April 8, 2005 - - - hybrid 00:42 min Pacific, Middle America
October 3, 2005 08:41 10:31 12:22 annular 04:32 min Northern Africa, Europe, Western Asia, Middle East and India [2]
March 29, 2006 - - - total 04:07 min Brazil, Northern Africa, Central Asia, Mongolia [3]
September 22, 2006 - - - annular 07:09 min South America, West Africa, Antarctica
March 19, 2007 - - - partial - Asia, Alaska
September 11, 2007 - - - partial - South America, Antarctica
February 7, 2008 - - - annular 02:12 min Antarctica, Australia, New Zealand
August 1, 2008 - - - total 02:27 min North America, Europe, Asia
January 26, 2009 - - - annular 07:54 min Southern Africa, Antarctica, South East Asia, Australia
July 22, 2009 - - - total 06:39 min India, China, Pacific Ocean, best view in Shanghai, Hangzhou or Wuhan. Longest duration of totality in the 21st century
January 15, 2010 - - - annular 11:08 min Africa, Asia
July 11, 2010 - - - total 05:20 min Southern South America, Tahiti
January 4, 2011 - - - partial - Europe, Africa, Central Asia
June 1, 2011 - - - partial - Iceland, northern North America, East Asia
July 1, 2011 - - - partial - Southern Indian Ocean
November 25, 2011 - - - partial - Southern Africa, Antarctica, Tasmania, New Zealand
May 20, 2012 - - - annular 05:46 min Pacific, Asia, North America
November 13, 2012 - - - total 04:02 min Australia, New Zealand, southern South America, southern Pacific
May 10, 2013 - - - annular 06:03 min Australia, New Zealand, Central Pacific
November 3, 2013 - - - hybrid 01:40 min Eastern America, South Europe, Africa
April 29, 2014 - - - annular 00:00 min South India, Australia, Antarctica
October 23, 2014 - - - partial - Northern Pacific, North America
March 20, 2015 - - - total 02:47 min Atlantic before England, Norway, North Pole (!)
September 13, 2015 - - - partial - South Africa, South India, Antarctica
March 9 2016 - - - total 04m09s South Asia, Pacific
September 1 2016 - - - annular 03m06s Africa
February 26 2017 - - - annular 00m44s Southern Africa, southern South America
August 21 2017 - - - total 02m40s North America
February 15 2018 - - - partial - Antarctic, southern South America
July 13 2018 - - - partial - South Australia
August 11 2018 - - - partial - Northern Europe, north Asia
January 6 2019 - - - partial - Eastern Asia
July 2 2019 - - - total 04m33s South America
December 26 2019 - - - annular 03m39s South Asia
June 21 2020 - - - annular 00m38s South Asia
December 14 2020 - - - total 02m10s South America

(*) Duration of central eclipse.

See also

Wikimedia Commons has media related to Solar eclipse.

Further reading

F.R. Stephenson, Historical Eclipses and Earth's Rotation (Cambridge University Press, 1997).

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