Partial lunar eclipse in 1991
A partial lunar eclipse occurred at the Moon’s descending node of orbit on Saturday, December 21, 1991, with an umbral magnitude of 0.0876. A lunar eclipse occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. A partial lunar eclipse occurs when one part of the Moon is in the Earth's umbra, while the other part is in the Earth's penumbra. 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. Occurring only about 23 hours before perigee (on December 22, 1991, at 9:25 UTC), the Moon's apparent diameter was larger.
This eclipse was the last of four lunar eclipses in 1991, with the others occurring on January 30 (penumbral), June 27 (penumbral), and July 26 (penumbral).
Visibility.
The eclipse was completely visible over northeast Asia, much of North America, and much of the Pacific Ocean, seen rising over central, south, and east Asia, and Australia and setting over South America and northern Europe.
Eclipse details.
Shown below is a table displaying details about this particular solar eclipse. It describes various parameters pertaining to this eclipse.
Eclipse season.
This eclipse is part of an eclipse season, a period, roughly every six months, when eclipses occur. Only two (or occasionally three) eclipse seasons occur each year, and each season lasts about 35 days and repeats just short of six months (173 days) later; thus two full eclipse seasons always occur each year. Either two or three eclipses happen each eclipse season. In the sequence below, each eclipse is separated by a fortnight.
Related eclipses.
Lunar eclipses of 1991–1994.
This eclipse is a member of a semester series. An eclipse in a semester series of lunar eclipses repeats approximately every 177 days and 4 hours (a semester) at alternating nodes of the Moon's orbit.
The penumbral lunar eclipses on January 30, 1991 and July 26, 1991 occur in the previous lunar year eclipse set.
Metonic series.
The Metonic cycle repeats nearly exactly every 19 years and represents a Saros cycle plus one lunar year. Because it occurs on the same calendar date, the Earth's shadow will be in nearly the same location relative to the background stars.
Saros 115.
This eclipse is a part of Saros series 115, repeating every 18 years, 11 days, and containing 72 events. The series started with a penumbral lunar eclipse on April 21, 1000. It contains partial eclipses from July 6, 1126 through September 30, 1270; total eclipses from October 11, 1288 through July 20, 1739; and a second set of partial eclipses from July 30, 1757 through February 13, 2082. The series ends at member 72 as a penumbral eclipse on June 13, 2280.
The longest duration of totality was produced by member 36 at 99 minutes, 47 seconds on May 15, 1631. All eclipses in this series occur at the Moon’s descending node of orbit.
Eclipses are tabulated in three columns; every third eclipse in the same column is one exeligmos apart, so they all cast shadows over approximately the same parts of the Earth. 
Tritos series.
This eclipse is a part of a tritos cycle, repeating at alternating nodes every 135 synodic months (≈ 3986.63 days, or 11 years minus 1 month). Their appearance and longitude are irregular due to a lack of synchronization with the anomalistic month (period of perigee), but groupings of 3 tritos cycles (≈ 33 years minus 3 months) come close (≈ 434.044 anomalistic months), so eclipses are similar in these groupings.
Half-Saros cycle.
A lunar eclipse will be preceded and followed by solar eclipses by 9 years and 5.5 days (a half saros). This lunar eclipse is related to two partial solar eclipses of Solar Saros 122.
References.
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