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.

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.

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.

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 due to the fact that 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.

A lunar eclipse occurs when the moon passes behind the earth such 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, 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 will occur on December 21, 2010. 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. Some lunar eclipses have been associated with important historical events.

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 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.

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 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:^[3]

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

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 cycle.

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 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]
• There was a partial eclipse of the Moon on December 31, 2009.
• There was a partial eclipse of the Moon on June 26, 2010.
• The next total eclipse of the Moon will occur on December 21, 2010.

1995-1998

Lunar eclipse series sets from 1995-1998

Ascending node				Descending node
Saros Photo	Date Viewing	Type Chart		Saros Photo	Date Viewing	Type Chart
112	1995 Apr 15	Partial		117	1995 Oct 08	Penumbral
122	1996 Apr 04	Total		127	1996 Sep 27	Total
132	1997 Mar 24	Partial		137	1997 Sep 16	Total
142	1998 Mar 13	Penumbral		147	1998 Sep 06	Penumbral
Last set	1994 May 25			Last set	1994 Nov 18
Next set	1999 Jan 31			Next set	1998 Aug 08

1998-2002

Lunar eclipse series sets from 1998-2002

Descending node				Ascending node
Saros	Date Viewing	Type Chart		Saros	Date Viewing	Type Chart
109	1998 Aug 08	Penumbral		114	1999 Jan 31	Penumbral
119	1999 Jul 28	Partial		124	2000 Jan 21	Total
129	2000 Jul 16	Total		134	2001 Jan 09	Total
139	2001 Jul 05	Partial		144	2001 Dec 30	Penumbral
149	2002 Jun 24	Penumbral
Last set	1998 Sep 06			Last set	1998 Mar 13
Next set	2002 May 26			Next set	2002 Nov 20

2002–2005

Lunar eclipse series sets from 2002–2005

Descending node				Ascending node
Saros Photo	Date View	Type Chart		Saros Photo	Date View	Type Chart
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

2006–2009

Lunar eclipse series sets from 2006–2009

Descending node				Ascending node
Saros Photo	Date Viewing	Type Chart		Saros Photo	Date Viewing	Type Chart
113	2006 Mar 14	penumbral		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

2009–2013

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

2013–2016

Lunar eclipse series sets from 2013–2016

Ascending node				Descending node
Saros	Date Viewing	Type		Saros	Date Viewing	Type
112	2013 Apr 25	Partial		117	2013 Oct 18	Penumbral
122	2014 Apr 15	Total		127	2014 Oct 08	Total
132	2015 Apr 04	Total		137	2015 Sep 28	Total
142	2016 Mar 23	Penumbral		147	2016 Sep 16	Penumbral
Last set	2013 May 25			Last set	2012 Nov 28
Next set	2017 Feb 31			Next set	2016 Aug 08

2016–2020

Lunar eclipse series sets from 2016–2020

Descending node				Ascending node
Saros	Date	Type Viewing		Saros	Date Viewing	Type Chart
109	2016 Aug 18	Penumbral		114	2017 Feb 11	Penumbral
119	2017 Aug 07	Partial		124	2018 Jan 31	Total
129	2018 Jul 27	Total		134	2019 Jan 21	Total
139	2019 Jul 16	Partial		144	2020 Jan 10	Penumbral
149	2020 Jul 05	Penumbral
Last set	2016 Sep 16			Last set	2016 Mar 23
Next set	2020 Jun 05			Next set	2020 Nov 30

See also

• 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
• Solar eclipse
• Umbra

References

• 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

• 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
• Shadow and Substance for animation of future and past eclipses
• Tips on photographing the lunar eclipse from New York Institute of Photography
• E-mail notification on upcoming eclipses

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