A lunar eclipse happens when Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon. This can happen only when the Moon is full.
Why the Moon may look red
During a total lunar eclipse, the Moon can appear reddish because Earth's atmosphere bends some sunlight toward the Moon and filters out much of the blue light.
Lunar versus solar eclipse
In a solar eclipse, the Moon blocks sunlight from reaching part of Earth. In a lunar eclipse, Earth blocks direct sunlight from reaching the Moon.
Lunar eclipses are generally easier and safer to watch than solar eclipses because they do not require looking at the Sun.