Editing Saros (astronomy) (section)

==Description==
[[Image:Lunareclipsediagram.svg|240px|thumb|left|Lunar eclipses occurring near the Moon's descending node are given ''odd'' saros series numbers. The first eclipse in such series passes through the southern edge of the Earth's shadow, and the Moon's path is shifted northward each successive saros, while lunar eclipses occurring near the Moon's ascending node are given ''even'' saros series numbers. The first eclipse in such series passes through the northern edge of the Earth's shadow, and the Moon's path is shifted southward each successive saros.]]

The saros, a period of 6585.3211 days (15 common years + 3 leap years + 12.321 days, 14 common years + 4 leap years + 11.321 days, or 13 common years + 5 leap years + 10.321 days), is useful for predicting the times at which nearly identical eclipses will occur. Three periodicities related to lunar orbit, the [[Month#Types of months in astronomy|synodic month]], the [[Month#Types of months in astronomy|draconic month]], and the [[Month#Types of months in astronomy|anomalistic month]] coincide almost perfectly each saros cycle. For an eclipse to occur, either the Moon must be located between the Earth and Sun (for a [[solar eclipse]]) or the Earth must be located between the Sun and Moon (for a [[lunar eclipse]]). This can happen only when the Moon is [[new moon|new]] or [[full moon|full]], respectively, and repeat occurrences of these [[lunar phase]]s result from solar and lunar orbits producing the Moon's ''synodic period'' of 29.53059 days. During most full and new moons, however, the shadow of the Earth or Moon falls to the north or south of the other body. Eclipses occur when the three bodies form a nearly straight line. Because the plane of the lunar orbit is inclined to that of the Earth, this condition occurs only when a full or new Moon is near or in the [[plane of the ecliptic|ecliptic plane]], that is when the Moon is at one of the two [[lunar node|nodes]] (the ascending or descending node). The period of time for two successive lunar passes through the ecliptic plane (returning to the same node) is termed the ''draconic month'', a 27.21222 day period. The three-dimensional geometry of an eclipse, when the new or full moon is near one of the nodes, occurs every five or six months when the Sun is in conjunction or opposition to the Moon and coincidentally also near a node of the Moon's orbit at that time, or twice per [[Year#Draconic year|eclipse year]]. Two eclipses separated by one saros have very similar appearance and duration due to the distance between the Earth and Moon being nearly the same for each event: this is because the saros is also an integer multiple of the ''anomalistic month'' of 27.5545 days, the period of the moon with respect to the [[apse line|lines of apsides]] in its orbit.
[[File:Saros15122015.gif|thumb|Visualization of a period of one saros cycle in 3D.]]
After one saros, the Moon will have completed roughly an integer number of synodic, draconic, and anomalistic periods (223, 242, and 239) and the Earth-Sun-Moon geometry will be nearly identical: the Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the saros is close to 18 years in length (about 11 days longer), the Earth will be nearly the same distance from the Sun, and tilted to it in nearly the same orientation (same season).<ref name="totality">{{Cite book| last = Littmann| first = Mark |author2=Fred Espenak |author3=Ken Willcox | title = Totality: Eclipses of the Sun | publisher = Oxford University Press | date = 2008 | isbn = 978-0-19-953209-4}}</ref> Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. During this 18-year period, about 40 other solar and lunar eclipses take place, but with a somewhat different geometry. One saros equaling 18.03 years is not equal to a perfect integer number of lunar orbits (Earth revolutions with respect to the fixed stars of 27.32166 days [[sidereal month]]), therefore, even though the relative geometry of the Earth–Sun–Moon system will be nearly identical after a saros, the Moon will be in a slightly different position with respect to the stars for each eclipse in a saros series. The axis of rotation of the Earth–Moon system exhibits a [[Lunar precession|precession]] period of 18.59992 years.

The saros is not an integer number of days, but contains the fraction of {{frac||1|3}} of a day. Thus each successive eclipse in a saros series occurs about eight hours later in the day. In the case of an eclipse of the Sun, this means that the region of visibility will shift westward about 120°, or about one third of the way around the globe, and the two eclipses will thus not be visible from the same place on Earth. In the case of an eclipse of the Moon, the next eclipse might still be visible from the same location as long as the Moon is above the horizon. Given three saros eclipse intervals, the local time of day of an eclipse will be nearly the same. This three saros interval (19,755.96 days) is known as a '''triple saros''' or ''[[exeligmos]]'' ([[Greek language|''Greek'']]: "turn of the wheel") cycle.