Lunar eclipse

For other uses, see Lunar eclipse (disambiguation).

Not to be confused with Solar eclipse.

The 2011 total eclipse

A lunar eclipse occurs when the Moon passes behind the Earth so that the Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and Moon are aligned 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. The most recent total lunar eclipse occurred on June 15, 2011; it was a central eclipse, visible over Europe and South America after sunset, over Africa and most of Asia, and Australia before sunrise. It lasted 100 minutes. The previous total lunar eclipse occurred on December 21, 2010, at 08:17 UTC.^[1]

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.

Types of lunar eclipse

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 sunlight is blocked.

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 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.

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.)

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 of January 1999 (left) to the same moon outside of the shadow (right) demonstrates this subtle dimming.

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.^[2] 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:^[3]

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 Moon's surface is entirely within the Earth's umbra.

Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of the Earth's umbra.

U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits the Earth's umbra.

U4 (Fifth contact): End of the partial eclipse. The Earth's umbra leaves the Moon's surface.

P2 (Sixth contact): End of the penumbral eclipse. The Earth's shadow no longer makes any contact with the Moon.

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.^[4]

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 coloring 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 air molecules and 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 color; an alternative way of considering the problem is to realize 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 color. 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 color.

Danjon scale

The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:^[5]

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

L=1: Dark eclipse, gray or brownish in coloration. 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.

Eclipse cycles

See also: Saros (astronomy) and Eclipse cycle

Every year there are at least two lunar eclipses, although total lunar 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.

Recent and forthcoming lunar eclipse events

• March 3, 2007, lunar eclipse ― The first total lunar eclipse of 2007 occurred on March 3, 2007, and was partially visible from the Americas, Asia and Australia. The complete event was visible throughout Africa and Europe. The event lasted seventy-five minutes, began at 20:16 UTC, and reached totality at 22:43 UTC.^[6]
• 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.^[7]
• 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.^[8]
• There was a partial eclipse of the Moon on December 31, 2009.
• There was a partial eclipse of the Moon on June 26, 2010.
• There was a total eclipse of the Moon on December 21, 2010.
• There was a total eclipse of the Moon on June 15, 2011.
• The next total eclipse of the Moon will occur on December 10, 2011.

Gallery

• 

  December 21, 2010 Lunar Eclipse

• 

  This diagram shows how the moon appears reddish orange during a lunar eclipse.

• 

  Painting by Lucien Rudaux, showing what a lunar eclipse might look like when viewed from the surface of the moon. The moon's surface appears red because the only sunlight available is refracted through the Earth's atmosphere on the edges of the earth, as shown in the sky in this painting.

See also

Moon portal

• Lunar eclipses in history
  • May 1453 lunar eclipse - Fall of Constantinople
  • March 1504 lunar eclipse - Columbus’ lunar eclipse
  • December 1573 lunar eclipse - Tycho Brahe
• Eclipse
• Moon illusion
• Orbit of the Moon

References

1. ^ "Total Lunar Eclipse of December 21, 2010". Outer Space Universe. http://www.outerspaceuniverse.org/total-lunar-eclipse-of-december-21-2010.html. Retrieved 20 December 2010. 
2. ^ Hannu Karttunen. Fundamental Astronomy. Springer. http://books.google.com/books?id=DjeVdb0sLEAC&pg=PA139&lpg=PA139&dq=lunar+eclipse+%22maximum+duration%22. 
3. ^ Clarke, Kevin. "On the nature of eclipses". Inconstant Moon. Cyclopedia Selenica. http://www.inconstantmoon.com/cyc_ecl1.htm. Retrieved 19 December 2010. 
4. ^ 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 2008-02-20. 
5. ^ Paul Deans and Alan M. MacRobert. "Observing and Photographing Lunar Eclipses". Sky and Telescope. http://skytonight.com/observing/objects/eclipses/3304036.html. 
6. ^ "Total Lunar Eclipse: March 3, 2007". NASA Eclipse Page. NASA. 2008. http://sunearth.gsfc.nasa.gov/eclipse/OH/OH2007.html. Retrieved 2008-02-20. 
7. ^ "Total Lunar Eclipse: August 28, 2007". NASA Eclipse Page. NASA. 2008. http://sunearth.gsfc.nasa.gov/eclipse/LEmono/TLE2007Aug28/TLE2007Aug28.html. Retrieved 2008-02-20. 
8. ^ "Total Lunar Eclipse: February 20, 2008". NASA Eclipse Page. NASA. 2008. Archived from the original on 2008-02-18. http://web.archive.org/web/20080218025416/http://sunearth.gsfc.nasa.gov/eclipse/LEmono/TLE2008Feb21/TLE2008Feb21.html. Retrieved 2008-02-20. 

• Bao-Lin Liu, Canon of Lunar Eclipses 1500 B.C.-A.D. 3000, 1992
• Jean Meeus and Hermann Mucke Canon of Lunar Eclipses. Astronomisches Büro, Vienna, 1983
• Espenak, F., Fifty Year Canon of Lunar Eclipses: 1986-2035. NASA Reference Publication 1216, 1989

External links

• A good video introduction to Lunar Eclipses from NASA
• Lunar Eclipse: how to photograph
• Live Free Lunar eclipse webcast & hands-on lunar eclipse experiments: 2011-06-15
• Live-webcast of the lunar eclipse on June 15th, 2011, University of Applied Sciences Offenburg/Germany
• Worldwide viewing times for the December 2010 Total Lunar Eclipse
• Animated explanation of the mechanics of a lunar eclipse, University of Glamorgan
• Lunar Eclipse time sequence
• U.S. Navy Lunar Eclipse Computer
• NASA Eclipse home page
• Search among the 12,064 lunar eclipses over five millennium and display interactive maps
• Lunar Eclipses for Beginners
• Eclipse Geeks
• Shadow and Substance for animation of future and past eclipses
• Tips on photographing the lunar eclipse from New York Institute of Photography
• Total Lunar Eclipse, December 2010 - slideshow by Life magazine