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A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring Earth's view of the Sun totally or partially. This configuration can only occur at the New Moon phase, when the Sun and Moon are in conjunction, as seen from the Earth.
In ancient times, and in some countries today, solar eclipses are arrtibuted mythical properties. Total solar eclipses are no doubt very frightening events for people unaware of their astronomical nature: the Sun, source of all life, suddenly disappears in the middle of the day. The sky darkens in a matter of minutes, and some animals start panicking or go to sleep. Fortunately however, the spiritual attribution to solar eclipses is now generally limited to an illustration of the humbleness of mankind.
Total solar eclipses are very rare events for a given place on Earth. This is because totality is only visible where the umbra of the Moon touches the Earth's surface. Some people travel to the most remote places imaginable to observe eclipses. A total solar eclipse is considered by them to be the most spectacular natural phenomenon that one can observe.
The 1999 total eclipse in Europe, which was without doubt the most watched eclipse in human history, helped to increase public awareness of the phenomenon. This was illustrated by many people willing to make the displacement to witness the 2005 annular eclipse and the 2006 total eclipse. The next total solar eclipse will be that of August 1, 2008.
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 surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are therefore rather 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 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 the Earth. One of the most remarkable coincidences in nature is that the Sun lies about 400 times as far from Earth as does the Moon, and the Sun is also about 400 times larger in diameter than the Moon. As seen from Earth, therefore, the Sun and the Moon appear to be about the same size in the sky - about half a degree in angular measure. Because the Moon's orbit around the Earth is an ellipse rather than a circle, 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. The ratio between the apparent sizes of the Moon and that of the Sun is called the magnitude of the eclipse. Hence, a total eclipse has a magnitude larger than 1, while an annular or partial eclipse has a magnitude between 0 and 1. Hybrid eclipses have a magnitude so close to 1 that in some places on Earth is it larger, and in others smaller. <ref>Solar Eclipses for Beginners, O. Staiger</ref>
Central eclipse is often used as a generic term for a total, annular or hybrid eclipse. This is however not completely correct: the definition of a central eclipse is an eclipse during which the central line of the umbra touches the Earth's surface. It is possible, though extremely rare, that part of the umbra intersects with Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse.<ref>Central Solar Eclipses, F. Espenak</ref>
The term eclipse itself 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 Lunar eclipse, but when the Moon passes in front of the Sun, we see an occultation of the Sun by the Moon. Therefore, 'eclipse of the Earth' would actually be a better, though uncommon, term.
Observing a solar eclipse
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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. <ref>Eye Safety During Solar Eclipses, F. Espenak</ref>
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.<ref>How to Watch a Partial Solar Eclipse Safely, A. M. MacRobert (Sky & Telescope magazine)</ref>
Glancing at the Sun with all or most of its disc 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 are now brighter on the retina than when looking at a full Sun. As the eye has a small sweet spot, or fovea, for detailed viewing, the tendency will be to track the image on to this best part of the retina, causing 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. <ref>Observing Eclipses Safely, O. Staiger</ref>
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.
Viewing totality during total eclipses
Contrary to popular belief, it is safe to observe the total phase of a 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.
Also very beautiful are the effects just before (and just after) totality. When the shrinking visible part of the photosphere becomes very small, Baily beads will occur. These are caused by the sunlight still being able to reach Earth through lunar valleys, but no longer were mountains are present. Totality then begins with the diamond ring effect, the last bright flash of sunlight.
Geometry of an eclipse
The diagram to the right shows the alignment of the Sun, Moon and Earth during 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.
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.
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.<ref>F. Espenak, Fifty Year Canon of Solar Eclipses: 1986-2035 (NASA RP-1178, Greenbelt, MD, 1987)</ref><ref>J. Meeus, C. Grosjean, W. and Vanderleen, Canon of Solar Eclipses (Pergamon Press, New York, 1966)</ref>
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 and eclipse cycles
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. 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.<ref>F.R. Stephenson, Historical Eclipses and Earth's Rotation (Cambridge University Press, 1997, p.54)</ref>
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 one Inex later, hence its name: in-ex. A Saros cycle lasts 6,585.3 days (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 longtitude (due to the 0.3 days) and a little in latitude. An Saros series always starts with a partial eclipse at a pole, then shifts over the globe through a series of annular or total eclipses, and ends on the other pole a couple of milennia later.<ref>Eclipses and the Saros, F. Espenak</ref>
Due to tidal acceleration, the orbit of the Moon around the Earth is unstable, and 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 23500 km, meaning that it will no longer be able to completely cover the Sun's disc. This will be true even when the Moon is at perigee, and the Earth at aphelion.
A complicating factor is that the Sun will increase in size over this timescale. This makes it even more unlikely that the Moon will be able to cause a total eclipse. We can therefore say that the last total solar eclipse on Earth will occur in slightly less than 600 million years.
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.<ref>Solar Eclipses of Historical Interest, F. Espenak</ref><ref>F.R. Stephenson, Historical Eclipses and Earth's Rotation (Cambridge University Press, 1997)</ref>
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.<ref>Eclipse Quotations, D. Le Conte</ref>
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.<ref>Herodotus book VII, 37</ref> Hind and Chambers considered this absolute date more than a century ago.<ref>Hind and Chambers, 1889: 323</ref> Herodotus also reports that another solar eclipse was observed in Sparta during the next year, on August 1, 477 BC.<ref>Herodotus book IX, 10, book VIII, 131, and book IX, 1</ref> 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. Note that the modern conventional dates are different by a year or two, and that these two eclipse records have been ignored so far.<ref>B. E. Schaefer, Solar Eclipses That Changed the World (Sky and Telescope, May 1994, p.36-39)</ref>
It has also been attempted to establish the exact date of Good Friday by means of solar eclipses, but this research has not yielded conclusive results.<ref>C. J. Humphreys and W. G. Waddington, Dating the Crucifixion (Nature, Vol. 306, No. 5945, p.743-746, 22 December 1983)</ref>
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 can appear rather small and symmetric, or large and fuzzy. It is very hard to predict this prior to totality.<ref>The science of eclipses, ESA</ref>
During a solar eclipse special (indirect) observations can also be done 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 observation are the so called flying shadows, which are similar to those 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 seen them.<ref>Flying Shadows, D. Dravins (Lund Observatory)</ref>
Special observation campaigns
In 1919, the observation of a total solar eclipse helped to confirm Einstein's theory of general relativity. By comparing the apparent distance between two stars, with and without the Sun in between them, the predictions about gravitational lenses were confirmed. Of course the observation with the Sun in between was only possible during totality, since the stars are visible then.<ref>Relativity and the 1919 eclipse, ESA</ref>
Over the years, some less important special observations took place:
- May 30, 1965: Launch of rockets at Charlestown, USA
- May 20, 1966: Launch of rockets at Karystos, Greece to watch the solar eclipse
- November 12, 1966: Launch of two Titus-rockets fom Las Palmas, Argentina
- February 26, 1979: Launch of rockets from Red Lake, Canada
- February 16, 1980: Launch of rockets from San Marco platform
Solar eclipse before sunrise or after sunset
The phenomenon of atmospheric diffraction makes it possible to observe the Sun (and hence a solar eclipse) even when it it 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.
Simultaneous occurrence of 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 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 hypothesized that there may be 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 and from 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.<ref>ISS-Venustransit (German)</ref>
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 eclipse images in history.<ref>Looking Back on an Eclipsed Earth, Astronomy Picture of the Day</ref>
The direct observation of a total solar eclipse from space is rather rare. The only documented case is Gemini 12. 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.<ref>JSC Digital Image Collection</ref>
Eclipses on other planets:
- Solar eclipses on Jupiter
- Solar eclipses on Pluto
- Transit of Phobos from Mars
- Transit of Deimos from Mars
- List of solar eclipses
- List of solar eclipses seen from China
- List of solar eclipses visible from the United Kingdom 1000 AD - 2006 AD
- NASA's Eclipse Home Page, Fred Espenak
- Solar Eclipse Resources
- Detailed eclipse explanations and predictions, Hermit Eclipse
- Prof. Druckmüller's eclipse photography site
- World Atlas of Solar Eclipse Paths, F. Espenak
- Animated maps of present and past eclipses all over the world (ShadowandSubstance.com)
- Eye Safety During Solar Eclipses, F. Espenak (NASA Goddard Space Flight Center)
- How to Watch a Partial Solar Eclipse Safely, A. M. MacRobert (Sky & Telescope magazine)
- UK hospitals assess eye damage after solar eclipse, British Medical Journal, 21 August 1999, p. 319-469
Dedicated eclipse pages:
- Pictures of the 2005-04-08 eclipse
- March 29th 2006: Solar Eclipse Visualizations, P. Hemmingsson (Sciss AB)
- Solar Eclipse March 29 2006: All you need to know!
- Antalya: Preparations made in Antalya, Turkey, for expected visits by 29 March 2006 solar eclipse observers
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