Lunar Eclipses
Just as the Moon has a shadow, Earth also has a shadow. You experience Earth’s shadow every day as the planet rotates into the darkness of night (the darkest part of Earth’s shadow, the umbra).
A lunar eclipse happens when the Moon moves through Earth’s shadow. When the Moon moves through Earth’s penumbra, the Moon’s surface darkens slightly because less sunlight reaches it. However, a penumbral lunar eclipse is not very noticeable.
This composite image shows how the Moon’s appearance changes as it moves through Earth’s umbra. Click to see larger version.
When the Moon passes through Earth’s umbra, as it does during a partial or total lunar eclipse, the change is very noticeable—the Moon darkens and can even look red! Earth’s atmosphere scatters and bends sunlight, allowing some light to reach the Moon. Blue light is more easily scattered by particles in the atmosphere, which is why the sky looks blue and sunsets look reddish. While blue light is scattered in all directions, red light can pass through the atmosphere around Earth and shine on the Moon.
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