Editing Eclipse cycle (section)

== Recurrences ==
[[Image:Lunar eclipse diagram-en.svg|240px|thumb|A symbolic orbital diagram from the view of the Earth at the center, showing the Moon's two nodes where eclipses can occur.]]
Up to three eclipses may occur during an [[eclipse season]], a one- or two-month period that happens twice a year, around the time when the Sun is near the nodes of the Moon's orbit.

An eclipse does not occur every month, because one month after an eclipse the relative geometry of the Sun, Moon, and Earth has changed.

As seen from the Earth, the time it takes for the Moon to return to a node, the [[draconic month]], is less than the time it takes for the Moon to return to the same ecliptic longitude as the Sun: the [[synodic month]]. The main reason is that during the time that the Moon has completed an orbit around the Earth, the Earth (and Moon) have completed about {{frac|13}} of their orbit around the Sun: the Moon has to make up for this in order to come again into conjunction or opposition with the Sun. Secondly, the orbital nodes of the Moon [[lunar precession|precess]] westward in ecliptic longitude, completing a full circle in about  18.60 years<!--- 18.59948 a --->, so a draconic month is shorter than a [[sidereal month]]. In all, the difference in period between synodic and draconic month is nearly {{frac|2|1|3}} days<!--- 2.31837 d --->. Likewise, as seen from the Earth, the Sun passes both nodes as it moves along its ecliptic path. The period for the Sun to return to a node is called the [[eclipse year|eclipse or draconic year]]: about 346.6201 days, which is about {{frac|20}} year<!--- 0.05102 ---> shorter than a [[sidereal year]] because of the precession of the nodes.

If a solar eclipse occurs at one new moon, which must be close to a node, then at the next full moon the Moon is already more than a day past its opposite node, and may or may not miss the Earth's shadow. By the next new moon it is even further ahead of the node, so it is less likely that there will be a solar eclipse somewhere on Earth. By the next month, there will certainly be no event.

However, about 5 or 6 [[lunation]]s later the new moon will fall close to the opposite node. In that time (half an eclipse year) the Sun will have moved to the opposite node too, so the circumstances will again be suitable for one or more eclipses.