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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 penumbra shadow, only a portion of the sunlight is blocked.
As seen by an observer on Earth, 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, separated by approximately six months. (Note: Not drawn to scale. The Sun is much larger and farther away than the Moon.)
A lunar eclipse occurs whenever the Moon passes through some portion of the Earth\'s shadow. 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 last lunar eclipse was total, and occurred on February 21, 2008. The next eclipse--a partial one--occurs on August 16, 2008. The next total lunar eclipse occurs on December 21, 2010.
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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 does not cause any noticeable darkening of the Moon\'s surface, though some may argue it turns a little yellow. 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.Hannu Karttunen. Fundamental Astronomy. Springer. 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.
A selenelion or selenehelion is a type of lunar eclipse when, due to the Moon\'s proximity to the ecliptic, both the Sun and the eclipsed Moon can be observed at the same time. This particular arrangement has led to the phenomenon being referred to as a horizontal eclipse. It can only be observed just prior to sunset or just after sunrise. The specific arrangement is not common, and last occurred on May 16, 2003 over Europe.John Hammond. "Weather Centre: Astronomical event threatened by the Great British weather", BBC News, 15 May 2003. Retrieved on 2008-02-20.
The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth\'s atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red colouring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth\'s atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish colour; an alternative way of considering the problem is to realise that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind the Earth.
The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red colour. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting colour.
The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:Paul Deans and Alan M. MacRobert. Observing and Photographing Lunar Eclipses. Sky and Telescope.
Every year there are usually at least two partial lunar eclipses, although total eclipses are significantly less common. If one knows the date and time of an eclipse, it is possible to predict the occurrence of other eclipses using an eclipse cycle like the Saros cycle. 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.
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February 20, 2008. The full moon lunar eclipse over Seattle\'s world famous Space Needle in Seattle, Washington |
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March 2007. The advancing shadow of Earth brings out detail on the lunar surface. The huge ray system emanating from Tycho is shown as the dominant feature on the southern hemisphere. |
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August 2007. A total lunar eclipse allows more stars to be seen near the Moon. |
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August 2007. A total lunar eclipse and Milky Way. |
August 28, 2007 lunar eclipse, a collage of photos taken from the Oregon coast by Randall Scholten. |
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Total lunar eclipse taken from Melbourne, Australia on August 28, 2007 |
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A sequence of images taken at 3 minute intervals showing the 28 August 2007 lunar eclipse |
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Multiple exposure composite photo of the lunar eclipse of October 2004 as seen from Northern California. |
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The lunar eclipse as seen from Kerman, Iran. |
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3 March 2007 Lunar Eclipse as seen from Leeds, England. |
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Images taken in 3-5 minute Intervals - Lunar Eclipse as seen from Bradley, Illinois on February 20, 2008. |
20 February 2008 Lunar Eclipse as seen from Winnipeg, Manitoba, Canada. |
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20 February 2008 Lunar Eclipse as seen from Regina, Saskatchewan, Canada. |
Look up lunar eclipse in
Wiktionary, the free dictionary.
Wikimedia Commons has media related to:
| The Moon | ||
|---|---|---|
| General | Calendar · Month · Moon in art and literature · Moon in mythology · Moon illusion · Lunar effect |  |
| Orbit | Orbit of the Moon · Phases of the Moon · Solar eclipse · Lunar eclipse · Tides | |
| Physical characteristics | Internal structure · Gravity field · Topography · Magnetic field · Atmosphere | |
| The lunar surface | Selenography · Near side · Far side · Lunar mare · Impact crater · South Pole-Aitken basin · Shackleton crater · Ice · Peak of eternal light · Space weathering · Transient lunar phenomenon · | |
| Lunar science | Geology · Lunar geologic timescale · Giant impact hypothesis · Moon rocks · Lunar meteorites · KREEP · ALSEP · Lunar laser ranging · Late Heavy Bombardment | |
| Exploration | Exploration of the Moon · Project Apollo · Robotic exploration · Future missions · Lunar colonization · Moon Landing hoax accusations | |
| See also Solar system, natural satellite | ||
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