Lunar Eclipse

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  • Topic: Lunar eclipse, Solar eclipse, Eclipse
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Lunar eclipse

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The June 2011 total eclipse
A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the Sun, Earth, and Moon are aligned (in "syzygy") exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a full moon. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place, due to the smaller size of the moon's shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are no brighter (indeed dimmer) than the full moon itself.

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Types of lunar eclipse

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Schematic diagram of the shadow cast by the Earth. Within the central umbra shadow, the Moon is totally shielded from direct illumination by the Sun. In contrast, within the penumbrashadow, only a portion of sunlight is blocked. [pic]

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As seen by an observer on Earth on the imaginary celestial sphere, the Moon crosses the ecliptic every orbit at positions called nodes twice every month. When the full moon occurs in the same position at the node, a lunar eclipse can occur. These two nodes allow two to five eclipses per year, parted by approximately six months. (Note: Not drawn to scale. The Sun is much larger and farther away than the Moon.) [pic]

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A total penumbral lunar eclipse dims the moon in direct proportion to the area of the sun’s disk blocked by the earth. This comparison shows the southern shadow penumbral lunar eclipse ofJanuary 1999 (left) to the same moon outside of the shadow (right) demonstrates this subtle dimming. The shadow of the Earth can be divided into two distinctive parts: the umbra andpenumbra. 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 4 hours.[1] 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 appreciably within the changes in the orbital distance of the moon. Thus, a totally eclipsed Moon occurring near apogee will lengthen the duration of totality.

The timing of total lunar eclipses are determined by its contacts:[2]

P1 (First contact): Beginning of the penumbral eclipse. The Earth's penumbra touches the Moon's outer limb. U1 (Second contact): Beginning of the partial eclipse. The Earth's umbra touches the Moon's outer limb. U2 (Third contact): Beginning of the total eclipse. The...
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