Eclipse seasons

The Moon circles Earth on an orbit inclined by about 5° to the plane of Earth's orbit around the Sun. Most new moons and full moons therefore pass slightly above or below the alignment required for an eclipse. The Moon's orbit intersects the ecliptic at two points called nodes. When the Sun is near one of these nodes, a new or full moon can align closely enough for an eclipse.

An eclipse season is the interval during which the Sun is near a lunar node and an eclipse can occur. NASA describes the Sun crossing a zone about 34° wide around each node in roughly 34.5 days; the European Space Agency describes the window as about 35 days. The season midpoint occurs when the Sun is closest to the node. Successive midpoints are separated by about 173.3 days, just under half a year, because the lunar nodes slowly move.

A solar eclipse requires a new moon in the alignment zone, while a lunar eclipse requires a full moon. Since the season lasts longer than the Moon's roughly 27.3-day orbit around Earth, it usually contains opportunities for one solar and one lunar eclipse, and occasionally three eclipses in total. The precise count and types depend on the timing and geometry of the new and full moons within that season; an eclipse season does not mean every location can see every event.

Eclipse seasons explain the short-term clustering of eclipses. A separate, longer recurrence pattern is the Saros: about 6,585.3 days, or 18 years 11 days 8 hours. After a Saros, the Sun, Earth, and Moon return to similar relative geometry, so an eclipse of similar character can recur. The extra roughly eight hours mean Earth has rotated by about 120° more, shifting the visible path of a solar eclipse westward; the recurrence is not an exact repeat at the same place.