This image explains how the Earth's shadow causes a lunar eclipse. Notice the difference between the umbra and the penumbra.
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Windows to the Universe original image
A lunar eclipse is when the Earth's shadow blocks sunlight from reaching the Moon. It can only happen when there is a full moon. The Sun, Earth and Moon all need to be in a straight line too.
The Earth's shadow has two parts called the penumbra and the umbra. The umbra is much darker than the penumbra. When the Moon passes through the umbra, it is called a total eclipse.
When only part of the Moon passes through the umbra, it's called a partial eclipse. And when the Moon goes through the penumbra, it's called a penumbral eclipse. Have you seen an eclipse?
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