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How does a lunar eclipse occur?

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near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

A lunar eclipse occurs when the Moon moves into the shadow cast by Earth as it blocks sunlight. This phenomenon can only happen during a full moon when the Sun, Earth, and Moon are aligned.21

The process involves two primary parts of Earth's shadow:
* Penumbra: The outer, partial shadow. When the Moon passes through this area, it dims only slightly, often making the event difficult to notice.21
* Umbra: The darkest, central part of the shadow. A lunar eclipse is considered partial if the Moon enters only part of the umbra, and total if it passes completely through it.21

During a total eclipse, the Moon does not turn completely black. Instead, sunlight refracted through Earth’s atmosphere reaches the lunar surface. Because redder light penetrates the atmosphere most effectively, the Moon typically takes on a reddish-brown or coppery appearance, sometimes referred to as a "blood moon".341