A blood moon, which is a total lunar eclipse where the moon takes on a reddish color, generally occurs about two to four times per year. However, total lunar eclipses (the classic "blood moon") are less frequent at any given location, happening about four to five times per decade. Lunar eclipses in general happen between two to five times a year globally, but a full reddish "blood moon" only occurs during total lunar eclipses. The reddish color occurs due to Earth's atmosphere filtering sunlight and casting red light onto the moon. Additionally, there are rare occurrences called lunar tetrads—sets of four total lunar eclipses spaced about six months apart over two years—which happen irregularly over centuries. In summary:
- Lunar eclipses happen 2 to 5 times annually.
- A blood moon (total lunar eclipse with red color) occurs about 2 to 4 times a year worldwide.
- At any single location, blood moons are less frequent, roughly 4-5 times per decade.
- Lunar tetrads (four blood moons spaced six months apart) happen rarely over long periods.
