13 Lunar Tetrads 1900 to 2100: Rare Four in a Row Total Lunar Eclipses

Summary: Lunar tetrads occur as rare four in a row total lunar eclipses. Eight lunar tetrads occur in the 21st century.

visibility map for fourth and final total lunar eclipse of last tetrad of 20th century, occurring Friday, Oct. 17, 1986: "Permission is freely granted to reproduce this data when accompanied by an acknowledgment, Eclipse Predictions by Fred Espenak and Jean Meeus (NASA's GSFC)," via NASA Eclipse Web Site

A total lunar eclipse requires full phase of the moon and straight-line formation, known as syzygy (Ancient Greek: σύζυγος, suzugos, “yoked together”), of sun, Earth and moon.

Syzygy (siz-i-jee) occurs during the eight-phase cycle that characterizes each complete lunar orbit around the Earth. It accounts for the moon’s complete illumination in the fourth, or full, phase.

Because the Moon’s orbit traces an inclined path angled at 5 degrees to the plane of the Earth’s orbit around the sun, the moon usually avoids monthly eclipses (Ancient Greek: ἔκλειψις, ékleipsis, “eclipse”; from ἐκ, ek, “out” + λείπω, leípō, “I leave behind”) by passing above or below the Earth’s shadow. A completely Earth-shadowed moon requires the occurrence of fullness in conjunction with the moon’s position near one of the two lunar nodes, the points where the lunar orbit ascends or descends with respect to the Earth’s orbital plane.

The straight line formation allows the Earth to block the sun’s light from directly reaching the moon’s surface. Falling completely within the Earth’s shadow, the moon is hidden, or eclipsed, by the Earth. Instead of usual illumination via reflection of direct sunlight, the moon is aglow with a coppery red tinge prompted by all of the sunrises and sunsets rimming the Earth and orchestrated by the sun as the solar system’s dominant light source.

The total number of total lunar eclipses ranges from a low of zero to a maximum of three within a year. The number of total lunar eclipses visible at any point on Earth totals about four or five per decade.

Defined as four-in-row total lunar eclipses with each eclipse spaced six full moons apart, a lunar tetrad surpasses total lunar eclipses in infrequency. Lunar tetrads display centuries of variation in occurrence. Each century may witness a range of lunar tetrads, from a minimum of none to a maximum of eight.

The 20th century, covering dates from Jan. 1, 1901, to Dec. 31, 2000, experienced five lunar tetrads:
1909-1910: June 4; Nov. 27; May 24; Nov. 17
1927-1928: June 15; Dec. 8; June 3; Nov. 27
1949-1950: April 13; Oct. 7; April 2; Sept. 26
1967-1968: April 24; Oct. 18; April 13; Oct. 6
1985-1986: May 4; Oct. 28; April 24; Oct. 17

The 21st century, which runs from Jan. 1, 2001, to Dec. 31, 2100, is slated for the maximum number of lunar tetrads, with eight occurring between 2003 and 2091:
2003-2004: May 16; Nov. 9; May 4; Oct. 28
2014-2015: April 15; Oct. 8; April 4; Sept. 28
2032-2033: April 25; Oct. 18; April 14; Oct. 8
2043-2044: March 25; Sept. 19; March 13; Sept. 7
2050-2051: May 6; Oct. 30; April 26; Oct. 19
2061-2062: April 4; Sept. 29; March 25; Sept. 18
2072-2073: March 4; Aug. 28; Feb. 22; Aug. 17
2090-2091: March 15; Sept. 8; March 5; Aug. 29

Lunar events, especially lunar tetrads, add pizzazz to lunar astronomy. When rare events occur, such as the fourth and final of the 2014-2015 lunar tetrad on Sept. 27-28, the hope is for clear skies over the massive viewing area, which stretches across the Western Hemisphere, with small pockets in the Eastern Hemisphere.

visibility map for fourth final total lunar eclipse of last tetrad of 21st century, occurring Aug. 29, 2091: "Permission is freely granted to reproduce this data when accompanied by an acknowledgment, Eclipse Predictions by Fred Espenak and Jean Meeus (NASA's GSFC)," via NASA Eclipse Web Site

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

visibility map for fourth and final total lunar eclipse of last tetrad (Oct. 17, 1986) of 20th century, occurring Oct. 17, 1986: "Permission is freely granted to reproduce this data when accompanied by an acknowledgment, Eclipse Predictions by Fred Espenak and Jean Meeus (NASA's GSFC)," via NASA Eclipse Web Site @ http://eclipse.gsfc.nasa.gov/5MCLEmap/1901-2000/LE1986-10-17T.gif

visibility map for fourth final total lunar eclipse of last tetrad of 21st century, occurring Wednesday, Aug. 29, 2091: "Permission is freely granted to reproduce this data when accompanied by an acknowledgment, Eclipse Predictions by Fred Espenak and Jean Meeus (NASA's GSFC)," via NASA Eclipse Web Site @ http://eclipse.gsfc.nasa.gov/5MCLEmap/2001-2100/LE2091-08-29T.gif

For further information:

Espenak, Fred; and Jean Meeus. “Total 1986 Oct 17.” NASA Eclipse Web Site > Lunar Eclipses > Lunar Eclipse Page > Lunar Eclipse Catalogs > Catalog of Lunar Eclipse Saros Series > Lunar Eclipses of Saros Series 1 to 180 > Summary of Saros Series 126 to 150 > Catalog of Lunar Eclipses in Saros 136 > 18 -17 1986 Oct 17.
Available via NASA Eclipse Web Site @ https://eclipse.gsfc.nasa.gov/5MCLEmap/1901-2000/LE1986-10-17T.gif

Espenak, Fred; and Jean Meeus. “Total 2091 Aug 29.” NASA Eclipse Web Site > Lunar Eclipses > Lunar Eclipse Page > Lunar Eclipse Catalogs > Catalog of Lunar Eclipse Saros Series > Lunar Eclipses of Saros Series 1 to 180 > Summary of Saros Series 126 to 150 > Catalog of Lunar Eclipses in Saros 139 > 25 -08 2091 Aug 29.
Available via NASA Eclipse Web Site @ https://eclipse.gsfc.nasa.gov/5MCLEmap/2001-2100/LE2091-08-29T.gif

Espenak, Fred; and Jean Meeus. “Lunar Eclipses: 1901 to 2000.” NASA Eclipse Web Site > Lunar Eclipses > Five Millennium Catalog of Lunar Eclipses. Last updated 2011 May 23.
Available via NASA Eclipse Web Site @ http://eclipse.gsfc.nasa.gov/LEcat5/LE1901-2000.html

Espenak, Fred; and Jean Meeus. “Lunar Eclipses: 2001 to 2100.” NASA Eclipse Web Site > Lunar Eclipses > Five Millennium Catalog of Lunar Eclipses. Last updated 2011 May 23.
Available via NASA Eclipse Web Site @ http://eclipse.gsfc.nasa.gov/LEcat5/LE2001-2100.html

ScienceAtNASA. "ScienceCasts: A Tetrad of Lunar Eclipses." YouTube. March 20, 2014.
Available @ https://www.youtube.com/watch?v=5gzgSuJM5O8

Walker, John. “Lunar Prigee and Apogee Calculator.” Fourmilab Switzerland > Astronomy and Space > Earth and Moon Viewer > Viewing the Moon. May 5, 1997.
Available @ https://www.fourmilab.ch/earthview/pacalc.html