A partial lunar eclipse occurred at the Moon’s ascending node of orbit on Monday, August 6, 1990,[1] with an umbral magnitude of 0.6766. 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 about 6.2 days after apogee (on July 31, 1990, at 9:20 UTC), the Moon's apparent diameter was smaller.[2]
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.
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.[5]
The lunar eclipses on June 27, 1991 (penumbral) and December 21, 1991 (partial) occur in the next lunar year eclipse set.
This eclipse is a part of Saros series 138, repeating every 18 years, 11 days, and containing 82 events. The series started with a penumbral lunar eclipse on October 15, 1521. It contains partial eclipses from June 24, 1918 through August 28, 2026; total eclipses from September 7, 2044 through June 8, 2495; and a second set of partial eclipses from June 19, 2513 through August 13, 2603. The series ends at member 82 as a penumbral eclipse on March 30, 2982.
The longest duration of totality will be produced by member 48 at 105 minutes, 24 seconds on March 24, 2369. All eclipses in this series occur at the Moon’s ascending node of orbit.[6]
Greatest
First
The greatest eclipse of the series will occur on 2369 Mar 24, lasting 105 minutes, 24 seconds.[7]
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.
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.
A lunar eclipse will be preceded and followed by solar eclipses by 9 years and 5.5 days (a half saros).[8] This lunar eclipse is related to two total solar eclipses of Solar Saros 145.
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