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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eclipse cycle | 4/12 | https://en.wikipedia.org/wiki/Eclipse_cycle | reference | science, encyclopedia | 2026-05-05T11:12:34.827205+00:00 | kb-cron |
=== Notes === Fortnight Half a synodic month (29.53 days). When there is an eclipse, there is a fair chance that at the next syzygy there will be another eclipse: the Sun and Moon will have moved about 15° with respect to the nodes (the Moon being opposite to where it was the previous time), but the luminaries may still be within bounds to make an eclipse. For example, the penumbral lunar eclipse of May 26, 2002 is followed by the annular solar eclipse of June 10, 2002 and penumbral lunar eclipse of June 24, 2002. The shortest lunar fortnight between a new moon and a full moon lasts only about 13 days and 21.5 hours, while the longest such lunar fortnight lasts about 15 days and 14.5 hours. (Due to evection, these values are different going from quarter moon to quarter moon. The shortest lunar fortnight between first and last quarter moons lasts only about 13 days and 12 hours, while the longest lasts about 16 days and 2 hours.) For more information see eclipse season. Synodic month Similarly, two events one synodic month apart have the Sun and Moon at two positions on either side of the node, 29° apart: both may cause a partial solar eclipse. For a lunar eclipse, it is a penumbral lunar eclipse. Pentalunex 5 synodic months. Successive solar or lunar eclipses may occur 1, 5 or 6 synodic months apart. When two solar eclipses are one month apart, one will be seen near the Arctic Circle and the other near the Antarctic Circle. When they are five months apart, they are both seen near the Arctic Circle or both near the Antarctic Circle. Semester Half a lunar year. Solar eclipses will repeat exactly one semester apart at alternating nodes in a cycle that lasts for 8 eclipses. Because it is close to a half integer of anomalistic, draconic months, and tropical years, each solar eclipse will (usually) alternate between hemispheres each semester, as well as alternate between total and annular. Hence there is usually a maximum of one total or annular eclipse each in a given lunar year. (However, in the middle of an eight-semester series the hemispheres switch, and there is a switch during the series between whether the odd ones or the even ones are total.) It is possible to have two eclipses separated by a semester and a third eclipse one month before or after, so that two of the three are separated by seven months, but this only happens during certain centuries (see graph of inex versus date below). Lunar eclipses will repeat exactly one semester apart at alternating nodes in a cycle that lasts for 8 or 9 eclipses. Because it is close to a half integer of anomalistic, draconic months, and tropical years, each lunar eclipse will usually alternate between edges of Earth's shadow each semester, as well as alternate between Lunar eclipses with the moon’s penumbral and umbral shadow difference less or greater than 1. Hence there is usually a maximum of one Lunar eclipse with Moon’s penumbral and umbral shadow difference less or greater than 1 each in a given lunar year. Between June 29, 1946 and August 3, 2054, all solar eclipse semester series sets have 8 solar eclipses, except the 1990-1992 solar eclipse semester series set, which had only 7. Lunar year Twelve (synodic) months, a little longer than an eclipse year: the Sun has returned to the node, so eclipses may again occur. Hexon 6 eclipse seasons, and a fairly short eclipse cycle. Each eclipse in a hexon series (except the last) is followed by an eclipse whose saros series number is 8 lower, always occurring at the same node. It is equal to 35 synodic months, 1 less than 3 lunar years (36 synodic months). At any given time there are six hexon series active. Hepton 7 eclipse seasons, and one of the less noteworthy eclipse cycles. Each eclipse in a hepton is followed by an eclipse 3 saros series before, always occurring at alternating nodes. It is equal to 41 synodic months. The interval is nearly a whole number of weeks (172.96), so each eclipse is followed by another that is usually on the same day of the week (moving backwards irregularly by an average of a quarter day). At any given time there are seven hepton series active. Octon 8 eclipse seasons, 1⁄5 of the Metonic cycle, and a fairly decent short eclipse cycle, but poor in anomalistic returns. Each octon in a series is 2 saros apart, always occurring at the same node. It is equal to 47 synodic months. At any given time there are eight octon series active. Tzolkinex Includes a half draconic month, so occurs at alternating nodes and alternates between hemispheres. Each consecutive eclipse is a member of preceding saros series from the one before. Equal to about one day less than ten 260-day tzolk'ins (the basis of the Maya calendar). Every third tzolkinex in a series is near an integer number of anomalistic months and so will have similar properties. Hibbardina An eclipse "cycle" of at most 3 eclipses, but in fact meant as a period separating a pair of similar eclipses with opposite gamma values. Adding 1 lunation (for 112 synodic months) gives another period with the same property, the other half of a saros. The two surround a sar (half-saros). Named for William B. Hibbard who identified it in 1956. One lunar year less than a Hibbardina, 99 lunations, is only about a day and a half more than eight years. Sar (half saros) Includes an odd number of fortnights (223). As a result, eclipses alternate between lunar and solar with each cycle, occurring at the same node and with similar characteristics. A solar eclipse with small gamma will be followed by a very central total lunar eclipse. A solar eclipse where the Moon's penumbra just barely grazes the southern limb of Earth will be followed half a saros later by a lunar eclipse where the Moon just grazes the southern limb of the Earth's penumbra. Tritos Equal to an inex minus a saros. A triple tritos is close to an integer number of anomalistic months and so will have similar properties. Saros The best known eclipse cycle (described in the Almagest but not given this name), and one of the best for predicting eclipses, in which 223 synodic months equal 242 draconic months with an error of only 51 minutes. It is also very close to 239 anomalistic months, which makes the circumstances between two eclipses one saros apart very similar. Being a third of a day more than a whole number of days, each succeeding eclipse is centered about 120° further west over the Earth. If the Earth's orbit around the sun were circular, the saros cycle would be very close to a periodic orbit that would repeat exactly every 223 months.ὤ