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 penumbra shadow, only a portion of the sunlight is blocked.

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.)

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.

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 next total lunar eclipse occurs on December 21, 2010. The next eclipse of the Moon is a penumbral eclipse on July 7, 2009.

[edit] Types of lunar eclipses

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.^[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.

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]

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.

[edit] Danjon scale

The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:^[3]

L=0: Very dark eclipse. Moon almost invisible, especially at mid-totality.

L=1: Dark Eclipse, gray or brownish in colouration. Details distinguishable only with difficulty.

L=2: Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.

L=3: Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.

L=4: Very bright copper-red or orange eclipse. Umbral shadow is bluish and has a very bright rim.

[edit] Eclipse cycles

[edit] Recent and upcoming lunar eclipse events

March 3, 2007, lunar eclipse - The first total lunar eclipse of 2007 occurred on March 03, 2007, and was partially visible from the Americas, Asia and Australia. The complete event was visible throughout Africa and Europe. The event lasted 01h:15m, began at 20:16 UTC, and reached totality at 22:43 UTC.^[4]
August 2007 lunar eclipse - August 28, 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]
February 2008 lunar eclipse - The only total lunar eclipse of 2008 occurred on February 21, 2008, beginning at 01:43 UTC, visible from Europe, the Americas, and Africa.^[6]
The next partial eclipse of the Moon will occur on December 31, 2009.
The next total eclipse of the Moon will occur on December 21, 2010.

Lunar eclipse series sets from 2002-2005
Saros Photo	Date View	Type Chart	Saros Photo	Date View	Type Chart
Descending node			Ascending node
111	2002 May 26	penumbral	116	2002 Nov 20	penumbral
121	2003 May 16	total	126	2003 Nov 09	total
131	2004 May 04	total	136	2004 Oct 28	total
141	2005 Apr 24	penumbral	146	2005 Oct 17	partial

Last set	2002 Jun 24		Last set	2001 Dec 30

Next set	2006 Mar 14		Next set	2006 Sep 7

**Lunar eclipse** series sets from 2006-2009
Saros Photo	Date Viewing	Type Chart	Saros Photo	Date Viewing	Type Chart
Descending node			Ascending node
113	2006 Mar 14	partial	118	2006 Sep 7	partial
123	2007 Mar 03	total	128	2007 Aug 28	total
133	2008 Feb 21	total	138	2008 Aug 16	partial
143	2009 Feb 9	penumbral	148	2009 Aug 06	penumbral

Last set	2005 Apr 24		Last set	2005 Oct 17

Next set	2009 Dec 31		Next set	2009 Jul 07

Lunar eclipse series sets from 2009-2013
Ascending node			Descending node
Saros	Date Viewing	Type chart	Saros	Date Viewing	Type chart
110	2009 July 07	penumbral	115	2009 Dec 31	partial
120	2010 June 26	partial	125	2010 Dec 21	total
130	2011 June 15	total	135	2011 Dec 10	total
140	2012 June 04	partial	145	2012 Nov 28	penumbral
150	2013 May 25	penumbral

Last set	2009 Aug 06		Last set	2009 Feb 9

Next set	2013 Apr 25		Next set	2013 Oct 18