Lunar eclipse |
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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. |
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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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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. |
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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.) |
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| Types of lunar
eclipses |
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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. |
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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. |
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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.[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 much with
distance. Thus, a totally-eclipsed Moon occurring near apogee will
lengthen the duration of totality. |
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A selenelion or selenehelion occurs when
both the Sun and the eclipsed Moon can be observed at the same
time. This can only happen just before sunset or just after
sunrise, and both bodies will appear just above the horizon at
nearly opposite points in the sky. This arrangement has led to the
phenomenon being referred to as a horizontal eclipse. It
happens during every lunar eclipse at all those places on the Earth
where it is sunrise or sunset at the time. Indeed, the reddened
light that reaches the Moon comes from all the simultaneous
sunrises and sunsets on the Earth. Although the Moon is in the
Earth's geometrical shadow, the Sun and the eclipsed Moon can
appear in the sky at the same time because the refraction of light
through the Earth's
atmosphere causes objects near the horizon to appear higher
in the sky than their true geometric position.[2] |
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As viewed from Earth, the Earth's shadow can be imagined as two
concentric circles. As the diagram illustrates, the type of lunar
eclipse is defined by the path taken by the Moon as it passes
through Earth's shadow. If the Moon passes through the outer circle
but does not reach the inner circle, it is a penumbral eclipse; if
only a portion of the Moon passes through the inner circle, it is a
partial eclipse; and if entire Moon passes through the inner circle
at some point, it is a total eclipse. |
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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. |
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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. |
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| Danjon
scale |
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The following scale (the Danjon scale) was
devised by André Danjon
for rating the overall darkness of lunar eclipses:[3] |
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- L=1: Dark Eclipse, gray or brownish in
colouration. Details distinguishable only with
difficulty.
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- L=0: Very dark eclipse. Moon almost invisible,
especially at mid-totality.
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- L=2: Deep red or rust-colored eclipse. Very dark
central shadow, while outer edge of umbra is relatively
bright.
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- L=3: Brick-red eclipse. Umbral shadow usually has
a bright or yellow rim.
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- L=4: Very bright copper-red or orange eclipse.
Umbral shadow has a bluish, very bright
rim.
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| Eclipse
cycles |
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| See also: Saros cycle and
Eclipse
cycle |
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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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| Recent lunar eclipse
events |
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- 28
August 2007 lunar eclipse - 28 August 2007 saw the second total
lunar eclipse of the year. The initial stage began at
07:52 UTC, and reached
totality at 09:52 UTC. This eclipse was viewable form Eastern
Asia, Australia and New Zealand the
Pacific, and the
Americas.[5]
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