Tuesday, March 3, 2009

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.

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[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

See also: Saros cycle and Eclipse cycle

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.

[edit] Recent and upcoming lunar eclipse events

The current lunar year eclipse cycle has 8 events, each event separated by 6 lunations (177 days):

Descending node Ascending node
  1. March 14, 2006 - partial
  2. March 3, 2007 - total
  3. February 21, 2008 - total
  4. February 9, 2009 - penumbral
  1. September 7, 2006 - partial
  2. August 28, 2007 - total
  3. August 16, 2008 - partial
  4. August 6, 2009 - penumbral
These charts show the Moon's path through the Earth's shadow near its ascending and descending nodes.

The next lunar year eclipse cycle has 9 events, each event separated by 6 lunations (177 days):

Descending node Ascending node
  1. December 31, 2009 - partial
  2. December 21, 2010 - total
  3. December 10, 2011 - total
  4. November 28, 2012 - penumbral
  1. July 7, 2009 - penumbral
  2. June 26, 2010 - partial
  3. June 15, 2011 - total
  4. June 4, 2012 - partial
  5. May 25, 2013 - penumbral


[edit] See also

[edit] Lunar eclipse photo galleries

Eclipse are grouped by their Saros series which repeat every 18 years, 10 days. There are currently 48 active Saros series (for lunar eclipses), indexed as 109 to 150.

Lunar eclipses from all 48 active saros series are represented below, grouped by their lunar year cycles (12 lunations), and subgrouped by their ocurrences at either the ascending or descending nodes of the Moon's orbit.

Lunation series 1995-1998
Ascending node
April 15, 1995
Partial
Series 112		 April 4, 1996
Total
Series 122		 March 24, 1997
Partial
Series 132		 March 13, 1998
Penumbral
Series 142
(No photo) (No photo) (No photo) (No photo)
Descending node
October 8, 1995
Penumbral
Series 117		 September 27, 1996
Total
Series 127		 September 16, 1997
Total
Series 137		 September 6, 1998
Penumbral
Series 147
(No photo) (No photo) (No photo) (No photo)
Lunation series 1998-2002
Ascending node
January, 31, 1999
Penumbral
Saros 114		 January 21, 2000
Total
Saros 124		 January 9, 2001
Total
Saros 134		 December 30, 2001
Penumbral
Saros 144
(No photo) (No photo) (No photo) (No photo)
Descending node
August 8, 1998
Penumbral
Saros 109		 July 28, 1999
Partial
Saros 119		 July 16, 2000
Total
Saros 129		 July 5, 2001
Partial
Saros 139		 June 24, 2002
Penumbral
Saros 149
(No photo) (No photo) (No photo) (No photo) (No photo)
Lunation series 2002-2005
Descending node
May 26, 2002
Penumbral
Saros 111		 May 16, 2003
Total
Saros 121		 May 4, 2004
Total
Saros 131		 April 24, 2005
Penumbral
Saros 141
(No photo)
Ascending node
November 20, 2002
Penumbral
Saros 116		 November 7, 2003
Total
Saros 126		 October 28, 2004
Total
Saros 136		 October 17, 2005
Partial
Saros 146
(No photo) (No photo)
Lunation series 2006-2009
Descending node
March 14, 2006
Penumbral
Saros 113		 March 3, 2007
Total
Saros 123		 February 21, 2008
Total
Saros 133		 February 9, 2009
Penumbral
Saros 143
Ascending node
September 7, 2006
Partial
Saros 118		 August 28, 2007
Total
Saros 128		 August 16, 2008
Partial
Saros 138		 August 6, 2009
Penumbral
Saros 148
(Future)
Lunation series 2009-2013
Ascending node
July 7, 2009
Penumbral
Saros 110		 June 26, 2010
Partial
Saros 120		 June 15, 2011
Total
Saros 130		 June 4, 2012
Partial
Saros 140		 May 25, 2013
Penumbral
Saros 150
(Future) (Future) (Future) (Future) (Future)
Descending node
December 31, 2009
Partial
Saros 115		 December 21, 2010
Total
Saros 125		 December 10, 2011
Total
Saros 135		 November 28, 2012
Penumbral
Saros 145
(Future) (Future) (Future) (Future)

[edit] References

  1. ^ Hannu Karttunen. "Fundamental Astronomy". Springer. http://books.google.com/books?id=DjeVdb0sLEAC&pg=PA139&lpg=PA139&dq=lunar+eclipse+%22maximum+duration%22&source=web&ots=2g2ku9x57X&sig=x5J8rF3DEVu4-TkJGhYr9LhW_GQ.
  2. ^ John Hammond (May 15, 2003). "Weather Centre: Astronomical event threatened by the Great British weather". BBC News. http://www.bbc.co.uk/weather/ukweather/daily_review/news/15052003news.shtml. Retrieved on 2008-02-20.
  3. ^ Paul Deans and Alan M. MacRobert. "Observing and Photographing Lunar Eclipses". Sky and Telescope. http://skytonight.com/observing/objects/eclipses/3304036.html.
  4. ^ "Total Lunar Eclipse: March 3, 2007". NASA Eclipse Page. NASA. 2008. http://sunearth.gsfc.nasa.gov/eclipse/OH/OH2007.html. Retrieved on 2008-02-20.
  5. ^ "Total Lunar Eclipse: August 28, 2007". NASA Eclipse Page. NASA. 2008. http://sunearth.gsfc.nasa.gov/eclipse/LEmono/TLE2007Aug28/TLE2007Aug28.html. Retrieved on 2008-02-20.
  6. ^ "Total Lunar Eclipse: February 20, 2008". NASA Eclipse Page. NASA. 2008. http://sunearth.gsfc.nasa.gov/eclipse/LEmono/TLE2008Feb21/TLE2008Feb21.html. Retrieved on 2008-02-20.

[edit] External links

Sister project Look up lunar eclipse in
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