Solar eclipse

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File:Solar eclips 1999 4.jpg
Photo taken during the French 1999 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 the Earth.

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 right). During any one eclipse, a total eclipse is visible only from a fairly 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.
File:Film eclipse soleil 1999.jpg
Photo taken by Wikipedia editor Luc Viatour (Lviatour) during the French 1999 eclipse
File:Eclipse anular 2005.png
Photo taken during the Spanish 2005 annular eclipse

Contents

Observing a solar eclipse

Looking at the Sun is dangerous at any time – to do so can cause permanent eye damage. This is true at any time, including during solar eclipses. Since an eclipse offers an unusually high temptation to look at the Sun, there is a high incidence of eye damage caused during solar eclipses. Viewing the Sun through any kind of optical aid (binoculars, a telescope, or even a camera viewfinder) is extremely dangerous.

The Sun 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 infra-red radiation which causes retinal damage. Only properly designed and certified solar filters should ever be used for direct viewing of the Sun.

The safest way to view the Sun is by indirect projection. This can be done by projecting an image of the sun 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, as this will cause severe eye damage; particular care should be taken if children are present.

It is safe to directly observe the total phase of a total solar eclipse, when the Sun's photosphere is completely covered by the Moon; indeed, this is a very beautiful sight. 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 ensure that one stops directly viewing the sun several seconds before the end of totality.

For more information on safe eclipse viewing, see:

Eclipse Predictions

Geometry of an Eclipse

File:Solar eclipse.png
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 the 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 which 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

File:Total Solar Eclipse Paths- 1001-2000.gif
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 every 300 to 400 years. Then, after waiting so long, the total solar 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

In the Odyssey, XIV, 151, Homer states that Odysseus will return to his home, and take vengeance on the suitors of Penelope, at the failing of the old moon and the coming of the new. Later in the Odyssey (XX, 356-357 and 390), Homer adds that the Sun vanished out of heaven and an evil gloom covered all things about the hour of the midday meal, during the celebration of the new moon. A total eclipse of the Sun was visible from the Greek island of Ithaca on April 16, 1178 BC. This would be six years after the end of the Trojan War, as traditionally dated (1184 BC), though within the Odyssey narrative it is ten years after the war.

File:Solar Eclipse Path 16 Jun 763 BC.png
Path of the total eclipse of 763 BC described in an Assyrian text

A solar eclipse of 15 June, 763 BC mentioned in an Assyrian text is important for the Chronology of the Ancient Orient.

A double (solar and lunar) eclipse took place 23 years after the ascension of king Shulgi of Babylon. This has been identified with eclipses that occurred on 9 May (solar eclipse) and 24 May (lunar eclipse), 2138 BC . This identification is however much less commonly accepted than the eclipse of 763 BC. See also Chronology of Babylonia and Assyria.

On June 4, 780 BC a solar eclipse was recorded in China.

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.

The foundation of Rome took place 437 years after the capture of Troy (1182 BC), according to Velleius Paterculus (VIII, 5). It took place shortly before an eclipse of the Sun that was observed at Rome on June 25, 745 BC and had a magnitude of 50.3%. Its beginning occurred at 16:38, its middle at 17:28, and its end at 18:16. Varro may have used the consular list with its mistakes, calling the year of the first consuls "245 ab urbe condita" (a.u.c.). A new study claims that the Varronian date has been superseded. Its correctness has not been proved scientifically but it is used worldwide.

According to Lucius Tarrutius of Firmum, Romulus was conceived in the womb on the 23rd day of the Egyptian month Choiac, at the time of a total eclipse of the Sun. This eclipse occurred on June 15, 763 BC, with a magnitude of 62.5% at Rome. Its beginning took place at 6:49, its middle at 7:47 and its end at 8:51. He was born on the 21st day of the month of Thoth. The first day of Thoth fell on 2 March in that year (Prof. E. J. Bickerman, 1980: 115). That implies that Rhea Silvia's pregnancy lasted for 281 days. Rome was founded on the ninth day of the month Pharmuthi, which was April 21, as universally agreed. The Romans add that, about the time Romulus started to build the city, an eclipse of the Sun was observed by Antimachus, the Teian poet, on the 30th day of the lunar month. This eclipse (see above) had a magnitude of 54.6% at Teos, Asia Minor. It started at 17:49 and was still eclipsed at sunset, at 19:20. Romulus vanished in the 54th year of his life, on the Nones of Quintilis (July), on a day when the Sun was darkened. The day turned into night, which sudden darkness was believed to be an eclipse of the Sun. It occurred on July 17, 709 BC, with a magnitude of 93.7%, beginning at 5:04 and ending at 6:57. All these eclipse data have been calculated by Prof. Aurél Ponori-Thewrewk, retired director of the Planetarium of Budapest. Plutarch placed it in the 37th year from the foundation of Rome, on the fifth of our month July, then called Quintilis, on "Caprotine Nones". Livy (I, 21) also states that Romulus ruled for 37 years. He was slain by the Senate or disappeared in the 38th year of his reign. Most of these have been recorded by Plutarch (Lives of Romulus, Numa Pompilius and Camillus), Florus (Book I, I), Cicero (The Republic VI, 22: Scipio's Dream), Dio (Dion) Cassius and Dionysius of Halicarnassus (L. 2). Dio in his Roman History (Book I) confirms these data by telling that Romulus was in his 18th year of age when he founded Rome. Therefore, three eclipse records prove that Romulus reigned from 746 BC to 709 BC.

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.

File:IMG 1650 zonsverduistering Malta.JPG
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

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External links

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