The Moon will slip through Earth’s long shadow on Sunday night, creating a total lunar eclipse. It comes exactly one year after another total eclipse — not one calendar year, but one lunar year, which is about 11 days shorter.
A lunar eclipse occurs when the full Moon passes through Earth’s shadow, darkening at least part of the lunar disk. It’s a total eclipse when the shadow covers the entire disk.
Eclipses don’t happen every month, however, because the Moon’s orbit around Earth is tilted. So most months, the Moon passes above or below the shadow, so there’s no eclipse. An eclipse takes place only when the full Moon is crossing the plane of Earth’s orbit.
Eclipses can take place at intervals of one, five, or six “lunations.” A lunation is the period from one full Moon to the next — about 29 and a half days. Six lunations make a semester. And two semesters make a lunar year, which is 354.4 days long. Lunar eclipses can repeat a semester apart, in a series of eight eclipses. They aren’t all visible from the same part of Earth, but there is some overlap from one eclipse to the next.
Sunday’s eclipse comes one semester after the last one, on November 19th, and a full lunar year after another. Another total eclipse will take place one semester from now, on November 8th, and one more eclipse in a lunar year — a barely there eclipse in which the Moon just dips into Earth’s faint outer shadow.
Script by Damond Benningfield