There’s are few celestial events more beautiful than a total lunar eclipse. (Except, perhaps a total solar eclipse.) But a so-called “blood moon” is a close second.
Visible in the western United States — including Hawaii —and Australia and Asia in the early hours of January 31, a full moon will turn an orangey-red hue as it passes through the Earth’s deep shadow.
That it happens to also be a supermoon and a blue moon makes it a true once-in-a-lifetime event. An all-in-one blue moon, supermoon, and total lunar eclipse has not happened in North America since 1866.
What is a supermoon, a blue moon, and a blood moon?
These are three completely different things that just happen to be occurring simultaneously. A supermoon is when the moon looks slightly bigger than normal, but only by a small margin. It happens because the moon’s orbit around Earth is elliptical, so sometimes it is actually closer. When that coincides with a full moon, it’s called a supermoon.
This isn’t an extraordinarily infrequent event. There was a supermoon on both December 3, 2017 as well as January 1, 2018. But the full moon on January 31 is particularly special, as it also happens to be a blue moon.
A blue moon isn’t a visual spectacle. When there are two full moons within one calendar month, it’s called a blue moon, but it happens rarely. Hence the phrase, “once in a blue moon.” The real visual spectacle on January 31 is the blood moon.
What is a total lunar eclipse?
A blood moon is more accurately known as a total lunar eclipse. Astronomers call it an ‘umbral eclipse’ because the entirety of the moon enters the darkest part of Earth’s shadow, called the umbra. Earth is always projecting a huge shadow into space, but the moon only sometimes passes through it. That can only happen during a full moon, when the Earth is aligned between the sun and moon.
The spectacle begins with what’s called a penumbral eclipse, as the moon crosses into the Earth’s lighter shadow and causes it to lose its usual brightness. About an hour later, the moon enters the umbra and begins to turn orange or pink on its edge.
About 40 minutes later, the whole of the moon is within the umbra — that’s called totality. Unlike during a total solar eclipse, lunar totality lasts for about 40 minutes, during which time the moon is closest to the center of the Earth’s shadow. The physics is the same as for a sunset: sunlight is being bent through the Earth’s atmosphere before it hits the moon. The exact…