   #copyright

Solar eclipse

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Photo taken during the 1999 eclipse.
   Enlarge
   Photo taken during the 1999 eclipse.

   A solar eclipse occurs when the Moon passes between Earth and the Sun,
   thereby totally or partially obscuring Earth's view of the Sun. This
   configuration can only occur during a New Moon, when the Sun and Moon
   are in conjunction as seen from the Earth. In ancient times, and in
   some cultures today, solar eclipses are attributed to mythical
   properties. Total solar eclipses can be frightening events for people
   unaware of their astronomical nature, as the Sun suddenly disappears in
   the middle of the day and the sky darkens in a matter of minutes.
   However, the spiritual attribution of solar eclipses is now largely
   disregarded.

   Total solar eclipses are very rare events for any given place on Earth
   because totality is only seen where the Moon's umbra touches the
   Earth's surface. A total solar eclipse is a spectacular natural
   phenomenon and many people consider travel to remote locations in order
   to observe one. The 1999 total eclipse in Europe, said by some to be
   the most-watched eclipse in human history, helped to increase public
   awareness of the phenomenon. This was illustrated by the number of
   people willing to make the trip to witness the 2005 annular eclipse and
   the 2006 total eclipse. The next solar eclipse takes place on March 19,
   2007, while the next total solar eclipse will occur on August 1, 2008.

Types of solar eclipses

   An annular eclipse.
   Enlarge
   An annular eclipse.

   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, or annulus,
       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 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 Earth's distance from the Sun is about 400 times the Moon's
   distance from the Earth. The Sun's diameter is about 400 times the
   diameter of the Moon. Because these ratios are approximately the same,
   the sizes of the Sun and the Moon as seen from Earth appear to be
   approximately the same: about 0.5 degree of arc in angular measure.

   Because 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
   vary. 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
   when the Moon is near its closest distance from 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
   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 is very close to 1: the eclipse will appear to be total at some
   locations on Earth and annular at other locations.

   The Earth's orbit around the Sun is also elliptical, so the Earth's
   distance from the Sun varies throughout the year. This also affects the
   apparent sizes of the Sun and Moon, but not so much as the Moon's
   varying distance from the Earth. When the Earth approaches its farthest
   distance from the Sun (the aphelion) in July, this tends to favour a
   total eclipse. As the Earth approaches its closest distance from the
   Sun (the perihelion) in January, this tends to favour an annular
   eclipse.

Terminology

   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.

   The term solar eclipse itself is technically a misnomer. The phenomenon
   of the Moon passing in front of the Sun is not an eclipse, but an
   occultation. Properly speaking, an eclipse occurs when one object
   passes into the shadow cast by another object. For example, when the
   Moon disappears at Full Moon by passing into Earth's shadow, the event
   is properly called a lunar eclipse. Therefore, the proper, but rarely
   used, term for what is commonly called a solar eclipse is eclipse of
   the Earth.

Eclipse predictions

Geometry of an eclipse

   Diagram of solar eclipse (not to scale).
   Enlarge
   Diagram of solar eclipse (not to scale).

   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 eclipse 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.

   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 29.6 days. This is known
   as the synodic month, and corresponds to what is commonly called the
   lunar month.
   A Total eclipse. B Annular eclipse. C Partial eclipse
   A Total eclipse. B Annular eclipse. C Partial eclipse

   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 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.

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 Eclipse Paths: 1001–2000. This image was merged from 50
   separate images from NASA.
   Enlarge
   Total Solar Eclipse Paths: 1001–2000. This image was merged from 50
   separate images from NASA.

   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.

   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 cycle 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 cycle finishes, a new Saros cycle begins one Inex
   later, hence its name: in-ex. A Saros cycle 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 lasts 1226 to 1550 years and 69 to 87 eclipses, with about 40 to
   60 central.

Final totality

   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 disk. 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. Perhaps the earliest still-unproven
   claim is that of archaeologist Bruce Masse; on the basis of several
   ancient flood myths that mention a total solar eclipse, he links an
   eclipse that occurred 10 May 2807 BC with a possible meteor impact in
   the Indian Ocean. 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.

   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.

   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. Hind and Chambers considered this absolute date
   more than a century ago. Herodotus also 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 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. The Chronicle of
   Ireland recorded a solar eclipse on June 29, 512 AD.

   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.

Observing a solar eclipse

   Photo taken in Valladolid (Spain) during the October 2005 annular
   eclipse.
   Enlarge
   Photo taken in Valladolid (Spain) during the October 2005 annular
   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 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 even more hazardous.

   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.

Viewing partial and annular eclipses

   Eclipse glasses.
   Enlarge
   Eclipse glasses.

   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.

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.
   Baily's beads.
   Enlarge
   Baily's beads.

   Also very beautiful are the effects just before (and just after)
   totality. When the shrinking visible part of the photosphere becomes
   very small, Baily's beads will occur (see picture). 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. Note that it is
   not entirely safe to view Baily beads or the diamond ring without
   proper eye protection (because in both cases the photosphere is still
   visible).

Other observations

   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.

   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 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 seen them.

   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.

1919 observation campaign

   The original photograph of the 1919 eclipse which was claimed to
   confirm Einstein's theory of general relativity.
   Enlarge
   The original photograph of the 1919 eclipse which was claimed to
   confirm Einstein's theory of general relativity.

   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 between them, the
   theoretical predictions about gravitational lenses were confirmed
   (though the data were ambiguous at the time). Of course the observation
   with the Sun between was only possible during totality, since the stars
   are visible then.

Solar eclipse before sunrise or 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.

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 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 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. However, it is now known that
   no such planet exists. While there does remain some possibility for
   small Vulcanoid asteroids to exist, none have ever been found.

Solar eclipses by and from artificial satellites

   The shadow of the moon as seen from the ISS in 2006.
   Enlarge
   The shadow of the moon as seen from the ISS in 2006.

   Artificial satellites can also get in the line between the Earth and
   the 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. As with a
   transit of a planet, it will not get dark.

   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. Observations of eclipses from satellites orbiting
   above the Earth's atmosphere are of course not subject to weather
   conditions.

   The direct observation of a total solar eclipse from space is rather
   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.
   Retrieved from " http://en.wikipedia.org/wiki/Solar_eclipse"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
