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Eclipse

Lunar Eclipse

A lunar eclipse is an eclipse which occurs whenever the moon passes behind the earth such that the earth blocks the sun's rays from striking the moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon the night of a lunar eclipse. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes.

The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. Within the umbra, there is no direct solar radiation. However, as a result of the Sun's large angular size, solar illumination is only partially blocked in the outer portion of the Earth's shadow, which is given the name penumbra.

A penumbral eclipse occurs when the Moon passes through the Earth's penumbra. The penumbra causes a subtle darkening of the Moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth's penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.

A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth's umbra, one observes a total lunar eclipse. The Moon's speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the Moon's first and last contact with the shadow is much longer, and could last up to 3.8 hours.

The relative distance of the Moon from the Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the Moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease much with distance. Thus, a totally-eclipsed Moon occurring near apogee will lengthen the duration of totality.

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 exist at New Moon, when Sun, Moon and Earth are on a single line with the Moon in the middle.

There are four types of solar eclipses:

A partial solar eclipse occurs when the Sun is only partially overlapped by the Moon.
A total solar eclipse occurs when the Moon completely obscures the Sun. This happens when the Moon is near perigee and its angular diameter as seen from Earth is identical to or slightly larger than that of the Sun. A total solar eclipse is the only opportunity to observe the Sun's corona without specialized equipment.
An annular (ring-formed) eclipse occurs when the Moon's center passes in front of Sun's center while the Moon is near apogee. The Moon's angular diameter is then smaller than that of the Sun so that a ring of the Sun can still be seen around the Moon. This is similar to a penumbral eclipse.
A hybrid eclipse occurs when the curvature of Earth's surface causes a single solar eclipse to be observed as annular from some locations but total from other locations. A total eclipse is seen from places on the Earth's surface that lie along the path of the eclipse and are physically closer to the Moon, and so intersect the Moon's umbra; other locations, further from the Moon, fall in the Moon's antumbra and the eclipse is annular.

The term 'solar eclipse' is a misnomer: the phenomenon is actually an occultation. An 'eclipse' occurs when one celestial object passes into the shadow cast by another (as with an eclipse of the Moon). An 'occultation' occurs when one body passes in front of another. When at its new phase the Moon passes in front of, or occults, the Sun, as seen from Earth, the Moon also casts a small shadow on Earth. An 'occultation' of the Sun is therefore also a partial 'eclipse' of Earth.

Observing a solar eclipse

Deadlands hell on earth core pdf editor. Looking at the Sun is dangerous at any time when any part of the brilliant visible disk of the Sun (its photosphere) is visible; 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's viewfinder- is extremely dangerous.

Safe Solar Viewing

The Sun can be viewed using appropriate filtration to block the harmful part of the Sun's radiation. Note that sunglasses are of little use, since they don't block the harmful and invisible infra-red radiation which causes retinal damage; other improvised methods, such as using a reflection in water, or looking through a compact disk, are equally dangerous. Only properly designed and certified solar filters should ever be used for direct viewing of the Sun; and these must be in perfect condition, as even a small defect could cause damage.

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 visible. The danger here is of being caught out by the end of the total phase, and the return of the 'exposed' Sun; because all parts of the Sun's disk are of similar intensity, even a tiny sliver of the Sun could cause permanent eye damage. For this reason, viewing the total phase of a solar eclipse through binoculars or a telescope should not be recommended.

Eclipse frequency and cause

Diagram of solar eclipse

Total and annular eclipses both occur when the Moon lines up with the Sun exactly, but since the Moon's orbit is not perfectly circular it is sometimes farther away from Earth and doesn't always cover the entire solar disc from an Earthly vantage point.

It is one of the most remarkable coincidences of nature that the Sun lies approximately 400 times as far away from Earth as does the Moon, and the Sun is also approximately 400 times as large in diameter as the Moon. As a result, as seen from Earth, the Sun and the Moon appear to be nearly the same apparent size. The Moon orbits Earth in an elliptical, or elongated orbit, however, and not in a circular orbit.

Thus during about 55-60% of its orbit the Moon is far enough from Earth ('apogee') that it is too small to cover the Sun's surface completely. During the remaining portion of its orbit, it is closer to Earth ('perigee') and large enough in apparent size to cover the Sun completely.

When a solar eclipse occurs near apogee, there is therefore a small ring or annulus of Sun that remains uncovered even at the moment of maximum eclipse. This produces an 'annular' eclipse, during which the brilliant and blinding uncovered ring of the Sun makes the solar corona invisible. When a solar eclipse occurs near perigee, however, the Moon is close enough to Earth and large enough in the sky that it can cover the entire bright surface (the photosphere) of the Sun completely, and the observer sees a total eclipse, at which time the ghostly white solar corona appears.

A solar eclipse can only be seen in a band across Earth as the Moon's shadow moves across its surface, while a total or annular eclipse is actually total or ring-formed in only a small band within this band (the eclipse path), and partial elsewhere (total eclipse takes place where the umbra of the Moon's shadow falls, whereas a partial eclipse is visible

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