Lunar eclipse

A lunar eclipse occurs when the Moon moves into the Earth's shadow.[1] This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned (in syzygy) with Earth between the other two, which can happen only on the night of a full moon when the Moon is near either lunar node. The type and length of a lunar eclipse depend on the Moon's proximity to the lunar node.

Latter phases of the partial lunar eclipse on 17 July 2019 taken from Gloucestershire, United Kingdom

The reddish color of a totally eclipsed Moon is caused by Earth completely blocking direct sunlight from reaching the Moon, with the only light reflected from the lunar surface has been refracted by Earth's atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of blue light.

Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth. A total lunar eclipse can last up to nearly 2 hours, while a total solar eclipse lasts only up to a few minutes at any given place, because the Moon's shadow is smaller. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than a normal full Moon.

The symbol for a lunar eclipse (or indeed any body in the shadow of another) is (U+1F776 🝶).

For the date of the next eclipse, see § Recent and forthcoming lunar eclipses.

Types of lunar eclipse
A schematic diagram of the shadow cast by Earth. Within the umbra, the central region, the planet totally shields direct sunlight. In contrast, within the penumbra, the outer portion, the sunlight is only partially blocked. (Neither the Sun, Moon, and Earth sizes nor the distances between the bodies are to scale.)

Earth's shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun's diameter appears about one-quarter of Earth's in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.

Penumbral lunar eclipse

This occurs when the Moon passes through Earth's penumbra. The penumbra causes a subtle dimming of the lunar surface, which is only visible to the naked eye when about 70% of the Moon's diameter has immersed into Earth's penumbra.[2] A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth's penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.

Partial lunar eclipse
Timelapse of a partial lunar eclipse

This occurs when only a portion of the Moon enters Earth's umbra, while a total lunar eclipse occurs when the entire Moon enters the planet's umbra. The Moon's average orbital speed is about 1.03 km/s (2,300 mph), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon's limb with Earth's shadow is much longer and could last up to 236 minutes.[3]

Total lunar eclipse

This occurs when the Moon falls entirely within the Earth's umbra. Just prior to complete entry, the brightness of the lunar limb-- the curved edge of the Moon still being hit by direct sunlight-- will cause the rest of the Moon to appear comparatively dim. The moment the Moon enters a complete eclipse, the entire surface will become more or less uniformly bright. Later, as the Moon's opposite limb is struck by sunlight, the overall disk will again become obscured. This is because as viewed from the Earth, the brightness of a lunar limb is generally greater than that of the rest of the surface due to reflections from the many surface irregularities within the limb: sunlight striking these irregularities is always reflected back in greater quantities than that striking more central parts, and is why the edges of full moons generally appear brighter than the rest of the lunar surface. This is similar to the effect of velvet fabric over a convex curved surface which to an observer will appear darkest at the center of the curve. It will be true of any planetary body with little or no atmosphere and an irregular cratered surface (e.g., Mercury) when viewed opposite the Sun.[4]

Central lunar eclipse

This is a total lunar eclipse during which the Moon passes through the centre of Earth's shadow, contacting the antisolar point. This type of lunar eclipse is relatively rare.

The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth's umbra does not decrease appreciably within the changes in the Moon's orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.

Selenelion

A selenelion or selenehelion, also called a horizontal eclipse, occurs where and when both the Sun and an eclipsed Moon can be observed at the same time. The event can only be observed just before sunset or just after sunrise, when both bodies will appear just above opposite horizons at nearly opposite points in the sky. A selenelion occurs during every total lunar eclipse-- it is an experience of the observer, not a planetary event separate from the lunar eclipse itself. Typically, observers on Earth located on high mountain ridges undergoing false sunrise or false sunset at the same moment of a total lunar eclipse will be able to experience it. Although during selenelion the Moon is completely within the Earth's umbra, both it and the Sun can be observed in the sky because atmospheric refraction causes each body to appear higher (i.e., more central) in the sky than its true geometric planetary position.[5]

Timing
Contact points relative to the Earth's umbral and penumbral shadows, here with the Moon near is descending node

The timing of total lunar eclipses is determined by what are known as its "contacts" (moments of contact with Earth's shadow):[6]

  • P1 (First contact): Beginning of the penumbral eclipse. Earth's penumbra touches the Moon's outer limb.
  • U1 (Second contact): Beginning of the partial eclipse. Earth's umbra touches the Moon's outer limb.
  • U2 (Third contact): Beginning of the total eclipse. The Moon's surface is entirely within Earth's umbra.
  • Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of Earth's umbra.
  • U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits Earth's umbra.
  • U4 (Fifth contact): End of the partial eclipse. Earth's umbra leaves the Moon's surface.
  • P4 (Sixth contact): End of the penumbral eclipse. Earth's penumbra no longer makes contact with the Moon.

Danjon scale
The Moon does not completely darken as it passes through the umbra because Earth's atmosphere refracts sunlight into the shadow cone.

The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:[7]

  • 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.

Lunar versus solar eclipse
In a lunar eclipse, the Moon often passes through two regions of Earth's shadow: an outer penumbra, where direct sunlight is dimmed, and an inner umbra, where indirect and much dimmer sunlight refracted by Earth's atmosphere shines on the Moon, leaving a reddish color. This can be seen in different exposures of a partial lunar eclipse, for example here with exposures of 1/80, 2/5, and 2 seconds.

There is often confusion between a solar eclipse and a lunar eclipse. While both involve interactions between the Sun, Earth, and the Moon, they are very different in their interactions.

The Moon does not completely darken as it passes through the umbra because of the refraction of sunlight by Earth's atmosphere into the shadow cone; if Earth had no atmosphere, the Moon would be completely dark during the eclipse.[8] The reddish coloration arises because sunlight reaching the Moon must pass through a long and dense layer of Earth's atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the air molecules and small particles; thus, the longer wavelengths predominate by the time the light rays have penetrated the atmosphere. Human vision perceives this resulting light as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color. An alternative way of conceiving this scenario is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind 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.

Christopher Columbus predicting a lunar eclipse.

Lunar eclipse in culture

Several cultures have myths related to lunar eclipses or allude to the lunar eclipse as being a good or bad omen. The Egyptians saw the eclipse as a sow swallowing the Moon for a short time; other cultures view the eclipse as the Moon being swallowed by other animals, such as a jaguar in Mayan tradition, or a mythical three-legged toad known as Chan Chu in China. Some societies thought it was a demon swallowing the Moon, and that they could chase it away by throwing stones and curses at it.[9] The Ancient Greeks correctly believed the Earth was round and used the shadow from the lunar eclipse as evidence.[10] Some Hindus believe in the importance of bathing in the Ganges River following an eclipse because it will help to achieve salvation.[11]

Inca

Similarly to the Mayans, the Incans believed that lunar eclipses occurred when a jaguar ate the Moon, which is why a blood moon looks red. The Incans also believed that once the jaguar finished eating the Moon, it could come down and devour all the animals on Earth, so they would take spears and shout at the Moon to keep it away.[12]

Mesopotamians

The ancient Mesopotamians believed that a lunar eclipse was when the Moon was being attacked by seven demons. This attack was more than just one on the Moon, however, for the Mesopotamians linked what happened in the sky with what happened on the land, and because the king of Mesopotamia represented the land, the seven demons were thought to be also attacking the king. In order to prevent this attack on the king, the Mesopotamians made someone pretend to be the king so they would be attacked instead of the true king. After the lunar eclipse was over, the substitute king was made to disappear (possibly by poisoning).[12]

Chinese

In some Chinese cultures, people would ring bells to prevent a dragon or other wild animals from biting the Moon.[13] In the 19th century, during a lunar eclipse, the Chinese navy fired its artillery because of this belief.[14] During the Zhou Dynasty (c. 1046–256 BC) in the Book of Songs, the sight of a Red Moon engulfed in darkness was believed to foreshadow famine or disease.[15]

Blood moon
Totality during the lunar eclipse of 15 May 2022. Direct sunlight is being blocked by the Earth, and the only light reaching it is sunlight refracted by Earth's atmosphere, producing a reddish color.

Certain lunar eclipses have been referred to as "blood moons" in popular articles but this is not a scientifically-recognized term.[16] This term has been given two separate, but overlapping, meanings.

The first, and simpler, meaning relates to the reddish color a totally eclipsed Moon takes on to observers on Earth.[17] As sunlight penetrates the atmosphere of Earth, the gaseous layer filters and refracts the rays in such a way that the green to violet wavelengths on the visible spectrum scatter more strongly than the red, thus giving the Moon a reddish cast.[18]

The second meaning of "blood moon" has been derived from this apparent coloration by two fundamentalist Christian pastors, Mark Blitz and John Hagee.[16][19] They claimed that the 2014–15 "lunar tetrad" of four lunar eclipses coinciding with the feasts of Passover and Tabernacles matched the "moon turning to blood" described in the Book of Joel of the Hebrew Bible.[19] This tetrad was claimed to herald the Second Coming of Christ and the Rapture as described in the Book of Revelation on the date of the first of the eclipses in this sequence on April 15, 2014.[20]

Occurrence
See also: Saros (astronomy) and Eclipse cycle

At least two lunar eclipses and as many as five occur every year, although total lunar eclipses are significantly less common. If the date and time of an eclipse is known, the occurrences of upcoming eclipses are predictable using an eclipse cycle, like the saros.

Recent and forthcoming lunar eclipses

Eclipses occur only during an eclipse season, when the Sun appears to pass near either node of the Moon's orbit.

Lunar eclipse series sets from 2002–2005
Descending node   Ascending node
Saros
Photo		 Date
View		 Type
Chart		 Gamma Saros
Photo		 Date
View		 Type
Chart		 Gamma
111 2002 May 26
 penumbral
 1.1759 116 2002 Nov 20
 penumbral
 -1.1127
121
 2003 May 16
 total
 0.4123 126
 2003 Nov 09
 total
 -0.4319
131
 2004 May 04
 total
 -0.3132 136
 2004 Oct 28
 total
 0.2846
141 2005 Apr 24
 penumbral
 -1.0885 146
 2005 Oct 17
 partial
 0.9796
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
Descending node   Ascending node
Saros #
and photo		 Date
Viewing		 Type
Chart		 Gamma Saros #
and photo		 Date
Viewing		 Type
Chart		 Gamma
113
 2006 Mar 14
 penumbral
 1.0211 118
 2006 Sep 7
 partial
 -0.9262
123
 2007 Mar 03
 total
 0.3175 128
 2007 Aug 28
 total
 -0.2146
133
 2008 Feb 21
 total
 -0.3992 138
 2008 Aug 16
 partial
 0.5646
143
 2009 Feb 09
 penumbral
 -1.0640 148
 2009 Aug 06
 penumbral
 1.3572
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 #
Photo		 Date
Viewing		 Type
chart		 Gamma Saros #
Photo		 Date
Viewing		 Type
chart		 Gamma
110 2009 Jul 07
 penumbral
 -1.4916 115
 2009 Dec 31
 partial
 0.9766
120
 2010 Jun 26
 partial
 -0.7091 125
 2010 Dec 21
 total
 0.3214
130
 2011 Jun 15
 total
 0.0897 135
 2011 Dec 10
 total
 -0.3882
140
 2012 Jun 04
 partial
 0.8248 145 2012 Nov 28
 penumbral
 -1.0869
150 2013 May 25
 penumbral
 1.5351
Last set 2009 Aug 06 Last set 2009 Feb 9
Next set 2013 Apr 25 Next set 2013 Oct 18
Lunar eclipse series sets from 2013–2016
Ascending node   Descending node
Saros Viewing
date		 Type Gamma Saros Viewing
date		 Type Gamma
112
 2013 Apr 25
 Partial
 -1.0121 117
 2013 Oct 18
 Penumbral
 1.1508
122
 2014 Apr 15
 Total
 -0.3017 127
 2014 Oct 08
 Total
 0.3827
132
 2015 Apr 04
 Total
 0.4460 137
 2015 Sep 28
 Total
 -0.3296
142 2016 Mar 23
 Penumbral
 1.1592 147
 2016 Sep 16
 Penumbral
 -1.0549
Last set 2013 May 25 Last set 2012 Nov 28
Next set 2017 Feb 11 Next set 2016 Aug 18
Lunar eclipse series sets from 2016–2020
Descending node   Ascending node
Saros Date Type
Viewing		 Gamma Saros Date
Viewing		 Type
Chart		 Gamma
109 2016 Aug 18
 Penumbral
 1.5641 114
 2017 Feb 11
 Penumbral
 -1.0255
119
 2017 Aug 07
 Partial
 0.8669 124
 2018 Jan 31
 Total
 -0.3014
129
 2018 Jul 27
 Total
 0.1168 134
 2019 Jan 21
 Total
 0.3684
139
 2019 Jul 16
 Partial
 -0.6430 144
 2020 Jan 10
 Penumbral
 1.2406
149 2020 Jul 05
 Penumbral
 -1.3639
Last set 2016 Sep 16 Last set 2016 Mar 23
Next set 2020 Jun 05 Next set 2020 Nov 30
Lunar eclipse series sets from 2020–2023
Descending node   Ascending node
Saros Date Type
Viewing		 Gamma Saros Date
Viewing		 Type
Chart		 Gamma
111
 2020 Jun 05
 Penumbral
 1.24063 116
 2020 Nov 30
 Penumbral
 -1.13094
121
 2021 May 26
 Total
 0.47741 126
 2021 Nov 19
 Partial
 -0.45525
131
 2022 May 16
 Total
 -0.25324 136 2022 Nov 08
 Total
 0.25703
141 2023 May 05
 Penumbral
 -1.03495 146 2023 Oct 28
 Partial
 0.94716
Last set 2020 Jul 05 Last set 2020 Jan 10
Next set 2024 Mar 25 Next set 2024 Sep 18
Lunar eclipse series sets from 2024–2027
Descending node   Ascending node
Saros Date Type
Viewing		 Gamma Saros Date
Viewing		 Type
Chart		 Gamma
113 2024 Mar 25
 Penumbral
 1.06098 118 2024 Sep 18
 Partial
 -0.97920
123 2025 Mar 14
 Total
 0.34846 128 2025 Sep 07
 Total
 -0.27521
133 2026 Mar 03
 Total
 -0.37651 138 2026 Aug 28
 Partial
 0.49644
143 2027 Feb 20
 Penumbral
 -1.04803 148 2027 Aug 17
 Penumbral
 1.27974
Last set 2023 May 05 Last set 2023 Oct 28
Next set 2028 Jan 12 Next set 2027 Jul 18
Lunar eclipse series sets from 2027–2031
Descending node   Ascending node
Saros Date
Viewing		 Type
Chart		 Saros Date
Viewing		 Type
Chart
110 2027 Jul 18
 Penumbral
 115 2028 Jan 12
 Partial
120 2028 Jul 06
 Partial
 125 2028 Dec 31
 Total
130 2029 Jun 26
 Total
 135 2029 Dec 20
 Total
140 2030 Jun 15
 Partial
 145 2030 Dec 09
 Penumbral
150 2031 Jun 05
 Penumbral
Last set 2027 Aug 17 Last set 2027 Feb 20
Next set 2031 May 07 Next set 2031 Oct 30

See also

References
  1. McClure, Bruce (July 27, 2018). "Century's Longest Lunar Eclipse July 27". EarthSky. Retrieved August 1, 2018.
  2. H. Mucke, J. Meeus (1992). Canon of Lunar Eclipses -2002 to +2526 (3rd ed.). Astronomisches BĂĽro Wien. p. V.
  3. Karttunen, Hannu (2007). Fundamental Astronomy. Springer. p. 139. ISBN 9783540341444.
  4. "Lunar Limb Magic". Astronomy.com. 27 November 2018.
  5. "Observing Blog - In Search of Selenelion". Sky & Telescope. 2010-06-26. Archived from the original on 2011-12-20. Retrieved 2011-12-08.
  6. Clarke, Kevin. "On the nature of eclipses". Inconstant Moon. Cyclopedia Selenica. Retrieved 19 December 2010.
  7. Deans, Paul; MacRobert, Alan M. (July 16, 2006). "Observing and Photographing Lunar Eclipses". Sky & Telescope. F+W. Archived from the original on May 20, 2007. Retrieved January 7, 2007.
  8. Espenak, Fred; Meeus, Jean. "Visual Appearance of Lunar Eclipses". NASA. The troposphere and stratosphere act together as a ring-shaped lens that refracts heavily reddened sunlight into Earth's umbral shadow.
  9. Littmann, Mark; Espenak, Fred; Willcox, Ken (2008). "Chapter 4: Eclipses in Mythology". Totality Eclipses of the Sun (3rd ed.). New York: Oxford University Press. ISBN 978-0-19-953209-4.
  10. Pollack, Rebecca. "Ancient Myths Revised with Lunar Eclipse". University of Maryland. Retrieved 2 October 2014.
  11. Ani. "Hindus take a dip in the Ganges during Lunar Eclipse". Yahoo News. Retrieved 2 October 2014.
  12. Lee, Jane (14 April 2014). "Lunar Eclipse Myths From Around the World". National Geographic. Retrieved 9 October 2014.
  13. Quilas, Ma Evelyn. "Interesting Facts and Myths about Lunar Eclipse". LA Times. Retrieved 2 October 2014.
  14. "Mythology of the Lunar Eclipse". LifeAsMyth.com.
  15. Kaul, Gayatri (15 June 2011). "What Lunar Eclipse Means in Different Parts of the World". India.com. Retrieved 6 October 2014.
  16. Sappenfield, Mark (13 April 2014). "Blood Moon to arrive Monday night. What is a Blood Moon?". Christian Science Monitor. Retrieved 8 February 2018.
  17. Nigro, Nicholas (2010). Knack Night Sky: Decoding the Solar System, from Constellations to Black Holes. Globe Pequot. pp. 214–5. ISBN 978-0-7627-6604-8.
  18. "All you need to know about the 'blood moon'". theguardian. 28 September 2015.
  19. "What is a Blood Moon?". Earth & Sky. 24 April 2014. Retrieved 30 May 2014.
  20. Bailey, Sarah Pulliam (15 April 2014). "'Blood moon' sets off apocalyptic debate among some Christians". The Washington Post. Religion News Service. Retrieved 8 February 2018.

Further reading
  • Bao-Lin Liu, Canon of Lunar Eclipses 1500 B.C.-A.D. 3000. Willmann-Bell, Richmond VA, 1992
  • Jean Meeus and Hermann Mucke Canon of Lunar Eclipses -2002 to +2526 (3rd edition). Astronomisches BĂĽro, Vienna, 1992
  • Espenak, F., Fifty Year Canon of Lunar Eclipses: 1986–2035. NASA Reference Publication 1216, 1989
  • Espenak, F. Thousand Year Canon of Lunar Eclipses 1501 to 2500, Astropixels Publishing, Portal AZ, 2014
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