A blood moon is lit by a ring of sunsets around the Earth
During a total lunar eclipse, anyone standing on the Moon would see the Sun hidden behind a dark Earth circled by a thin red ring. That ring is our atmosphere, bending sunlight into the shadow and scattering away the blues, exactly as at sunset. Its glow is what turns the eclipsed Moon copper instead of black.
A lunar eclipse happens when the Moon slides through Earth's shadow, which is only possible during eclipse seasons, when the Moon's tilted orbit lines up with Earth and Sun. Unlike a solar eclipse, it is visible from the whole night side of the planet, often lasts an hour or more, and is safe to watch without eye protection. Totality can stretch to almost 107 minutes, and from the first brush of shadow to the last, the event may run to 236 minutes. It lasts longest when the Moon is near its farthest point and moving slowest.
Not every eclipse is dramatic. When the Moon grazes only the outer, partial shadow, it merely dims subtly; about a third of lunar eclipses are this penumbral kind. Partial eclipses bite a distinct dark patch from the disc. Over the five thousand years from 2000 BCE to 3000 CE, a little under three in ten are total. There are at least two lunar eclipses every year, and a saros cycle of 18 years and 11 days lets astronomers predict them.
The colour varies from one eclipse to the next, depending on dust and clouds in Earth's air; a big volcanic eruption beforehand can deepen it markedly. The astronomer André Danjon devised a five-step scale, from almost invisible to bright copper-orange. Without an atmosphere, the Moon would vanish completely. Observers on high ridges can even glimpse a selenelion, when refraction lifts both the rising Sun and the eclipsed Moon into view on opposite horizons at once.
People long read meaning into the red disc. Maya and Inca tradition pictured a jaguar devouring the Moon; in China, a three-legged toad or a dragon, driven off with bells, and in the 19th century the Chinese navy fired artillery at one. Mesopotamians seated a substitute king to absorb the demons' attack. Ancient Greeks drew a sharper lesson: the curved shadow proved the Earth was round.
Source: Lunar eclipse
