Lunar Eclipse

A lunar eclipse is a eclipse occurring with each time the Moon is in the shade of the ground. This occurs only when the Moon is full, and when the Sun, the Ground and the the Moon are aligned perfectly or close to be it. The type and the size of a lunar eclipse depend on the relative position of the Moon compared to its orbital nodes. The last lunar eclipse took place the August 28th, 2007. The next one will take place the February 21st, 2008.

Description

A lunar eclipse occurs when the shade of the Earth is projected on the Moon. Two conditions are necessary so that arrives. Initially, the Moon must be full, i.e., compared to the Sun, the Moon must be just behind the Earth. However, like the orbital plan of the Moon is tilted of 5° compared to the orbital plan of the Earth (the ecliptic ), the majority of full moons occur when the Moon is in the North or the South of the shade of the Earth. Then, a second condition so that a lunar eclipse occurs is that the Moon must be near one of the two points of intersection that its orbit makes with the ecliptic. These two nodal points are respectively called ascending Nœud lunar and downward Nœud lunar.

The terrestrial shade can be broken up into two distinct parts: shade and half-light. In the shade, there is no direct solar radiation. However, because of importance of the angular diameter of the Sun, solar illumination is partially stopped in the external part of the terrestrial shade which one names half-light.

A eclipse pénombrale occurs when the Moon crosses the half-light of the Earth. The half-light does not cause any notable obscuration of lunar surface, however certain people affirm that it yellows a little. Certain eclipses pénombrales are total, during which the Moon is entirely in the zone of half-light of the Earth. The total eclipses pénombrales are rare, and when they occur, the part nearest of the shade can appear darker than the remainder of the Moon.

A partial lunar eclipse occurs only when part of the Moon enters the shade. When the Moon crosses the terrestrial shade completely, one can observe a total lunar eclipse. The speed of the Moon through the shade is about the kilometer a second, and totality can last until nearly 107 minutes (a little more 1H45). Nevertheless, the total duration between the first and the last contact of the Moon with the shade is much longer (up to 6 a.m.). The longest total lunar eclipse over the period of -1000 and 3000 lasted one 1:47 m14s, and took place on May 31st, 318. The relative distance from the Moon of the Earth during an eclipse can affect the duration of an eclipse. In particular, when the Moon is close to sound Apogée (i.e., the most distant point compared to the Earth on its orbit) its orbital velocity is slower. The diameter of the shade does not decrease more with the distance. Thus, the completely eclipsed moon occurring close to the apogee will prolong the duration of totality.

Appearance

With the difference in the solar eclipses, the lunar eclipses are completely inoffensive to observe with the naked eye.

The Moon does not disappear completely when it passes through the shade because of the Réfraction of the Solar light of the terrestrial atmosphere in the cone of shadow. The quantity of refracted light depends on the quantity of clouds or dust in the atmosphere, and this light made rougeoyer the Moon with a coloring rusts which varies from one eclipse to another.

The scale below (the scale of Danjon) was established by André Danjon to evaluate the residual luminosity and the coloring of the Moon at the time of such an eclipse.

L=0 : Very dark eclipse. The almost invisible moon, particularly in the middle of totality.

L=1 : Dark eclipse, with gray or brownish coloring. The lunar details are difficult to distinguish.

L=2 : Eclipse red dark or of color rusts, with, generally, a very dark spot in the center of the shade, while the external zone is rather clear.

L=3 : Eclipse red-brick. The shade is often bordered of an grey area or yellow.

L=4 : Eclipse color coppers or orange very clearly. The external zone is bluish and very brilliant.

Periodicity

Each year there are at least one or two lunar eclipses. By knowing the date and the hour of an eclipse, one can envisage of them others thanks to the cycles of eclipses, such as the cycle Saros. With the difference in a solar eclipse, which can be seen on a very restricted zone of the world, a lunar eclipse is visible anywhere on the Earth in its side harms.

Notes and references of the article

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