A blood moon, which occurs during a total lunar eclipse when the moon takes on a reddish color, generally happens about 2 to 4 times a year worldwide, but not all are total eclipses. Total lunar eclipses, the type that cause the classic blood moon effect, happen roughly 4 to 5 times per decade for a given location. On average, a total lunar eclipse can be seen every 2 to 3 years from any one place on Earth. The reddish color is caused by sunlight filtered and refracted through Earth's atmosphere during the eclipse. More specifically, total lunar eclipses happen when the moon moves into Earth's full shadow (umbra), which occurs typically every six months during full moon phases, but the exact frequency of observable blood moons varies due to the moon's orbit and the observer's location on Earth.
