Eclipses

27 01 2009

An eclipse (from the Greek ekleipô, “to vanish”) is an astronomical event that occurs when one celestial object moves into the shadow of another. The term is most often used to describe either a solar eclipse, when the Moon’s shadow crosses Earth’s surface, or a lunar eclipse, when the Moon moves into the shadow of Earth. However, it can also refer to such events beyond the Earth-Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its parent planet, or a moon passing into the shadow of another moon. An eclipse is a type of syzygy (a line up of astronomical bodies), as are transits and occultations. Eclipses are impossible on Mercury and Venus, which have no moons.

Earth-Moon system
An eclipse involving the Sun, Earth and Moon can occur only when they are nearly in a straight line. Because the orbital plane of the Moon is tilted with respect to the orbital plane of the Earth (the ecliptic), eclipses can occur only when the Moon is close to the intersection of these two planes (the nodes). The Sun, Earth and nodes are aligned twice a year, and eclipses can occur during a period of about two month around these times. There can be from four to seven eclipses in a calendar year, which repeat according to various eclipse cycles, such as the Saros cycle (an eclipse cycle with a period of about 18 years 11 days 8 hours that can be used to predict eclipses of the Sun and Moon).

1999 Total Solar Eclipse (Luc Viatour)

1999 Total Solar Eclipse (Luc Viatour)

The Earth happens to be in the midst of a cosmic coincidence. During a solar eclipse, the Moon can sometimes perfectly cover the Sun because its apparent size is nearly the same as the Sun when viewed from the Earth. This was not the case 100 million years ago (when the Moon was closer to the Earth), and it will cease to be the case in the distant future. A solar eclipse is actually a misnomer; the phenomenon is more correctly described as an occultation.

The general eclipse begins when the Moon’s penumbra starts to sweep across the Earth’s surface. The beginning of a total or annular eclipses coincides with the moment the Moon’s umbra starts to sweep across the Earth’s surface. The period of centrality begins when the axis of the Moon’s shadow cone starts to sweep across the Earth’s surface, and the eclipse’s maximum occurs at the moment when the axis of the Moon’s shadow comes closest to the centre of the Earth. Centrality ends when the axis of the Moon’s umbra finishes its sweep across the Earth’s surface, and total or annular eclipses end when the Moon’s umbra leaves the Earth’s surface. The general eclipse is over when the Moon’s penumbra leaves the Earth’s surface.

Stages of a total lunar eclipse (Luc Viatour)

Stages of a total lunar eclipse (Luc Viatour)

During a Solar eclipse, first contact (also called first exterior contact) refers to the instant when the Moon’s disc starts to cover the Sun’s. Second contact (also called first interior contact) occurs at the instant when the Moon’s disc is entirely surrounded by the Sun’s (for an annular eclipse) or the instant when the Sun’s disc disappears completely behind the Moon’s (for a total eclipse). Third contact (also called second interior contact) is the instant when the Moon’s disc starts to come out of the Sun’s (for an annular eclipse) or the instant when the Sun’s disc reappears from behind the Moon’s (for a total eclipse). Lastly, fourth contact (also called second exterior contact) is the instant when the Moon’s disc clears the Sun’s.

Lunar eclipses
There are three types of lunar eclipses: penumbral, when the Moon crosses only the Earth’s penumbra; partial, when the Moon crosses partially into the Earth’s umbra; and total, when the Moon crosses entirely within the Earth’s umbra. During a lunar eclipse, first contact (also called first exterior contact) is the instant when the Moon starts to enter into the Earth’s umbra. Second contact (also called first interior contact) is the instant when the Moon enters completely into the Earth’s umbra, and is the beginning of totality. The maximum of the eclipse occurs when the angular distance between the centre of the Moon’s disc and the centre of the shadow cone is at its smallest value. Third contact (also called second interior contact) occurs at the instant when the Moon starts to come out of the Earth’s umbra, which is the end of totality. Lastly, fourth contact (also called second exterior contact) is the instant when the Moon clears the Earth’s umbra completely.

Lunar Eclipse Types (Daniel M. Short)

Lunar Eclipse Types (Daniel M. Short)

The eclipse in mythology
Before modern astronomy arose, many cultures explained eclipses in terms of onflicts between mythic forces. For example, in Hindu mythology, the two demons Rahu and Ketu were believed to be responsible for eclipses. Similarly in China, at the Imperial observatory in Beijing, is a carved stone with the following inscription:
“This carved stone chart explained the cause of solar eclipses. The centre of the golden bird (the symbol of the sun) was covered by the toad (the symbol of the Moon). The people of the Han Dynasty called the phenomenon a good combination of the Sun and the Moon.”
In this explanation we see recognition of the celestial realities and a cheerful outlook regarding the event. In other cultures an eclipse could be both a surprising and a terrifying event.

Mars
On Mars, only partial eclipses are possible, because neither of its moons (Deimos and Phobos) is large enough to cover the Sun’s disc as seen from the surface of the planet. Martian eclipses have been photographed from both the surface of Mars and from orbit.

Jupiter, Saturn, Uranus and Neptune

The gas giants, which have many moons, frequently display eclipses. The most striking involve Jupiter, which has four large moons and a low axial tilt, making eclipses more frequent. It is common to see the larger moons casting circular shadows upon Jupiter’s cloudtops.

Io shadow on Jupiter (NASA)

Io shadow on Jupiter (NASA)

Pluto
Pluto, with its large moon Charon, is also the site of many eclipses, although these have never been directly observed from Earth.

NASA maintains a list of past and future eclipses, which can be found at:
http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

There are 6 eclipses due for 2009 – 4 Lunar and 2 Solar
Annular Solar Eclipse: Jan 26
Penumbral Lunar Eclipse: Feb 09
Penumbral Lunar Eclipse: Jul 07
Total Solar Eclipse: Jul 22
Penumbral Lunar Eclipse: Aug 06
Partial Lunar Eclipse: Dec 31

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