When the sun, the Earth and the moon are aligned in space (nearly or perfectly), with the Earth between the sun and moon, then Earth's shadow falls on the moon's face. That's when people on Earth see the shadow gradually turn a bright full moon dark in a lunar eclipse.
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Explanation:
When the sun, the Earth and the moon are aligned in space (nearly or perfectly), with the Earth between the sun and moon, then Earth's shadow falls on the moon's face. That's when people on Earth see the shadow gradually turn a bright full moon dark in a lunar eclipse.