A lunar eclipse is an eclipse of the moon. The sun, Earth, and the Moon are in a straight line during a lunar eclipse. Except the Moon is not in the middle, Earth is. Since Earth is in the middle, Earth casts a shadow on the Moon. This darkening is visible from the side of Earth facing the Moon, which is night time. That is why a lunar eclipse can only be seen at night.
A lunar eclipse will only occur when there is a full Moon. That is because the bright side of the Moon is facing Earth's dark side when the sun, Earth, and the Moon line up. The full Moon starts to look less and less full as the eclipse begins.
At first, the Moon just appears a little dim. That is because Earth's penumbra, a partial shadow, reaches the Moon 30 minutes before a large shadow, or umbra, does. When the umbra meets with the Moon, a dark curve slipping across the Moon can be seen.
Lunar eclipses are easier to see than solar eclipses. That's because lunar eclipses last up to 4 hours. Looking at a lunar eclipse or total eclipse of the Moon will not harm your eyes. But looking at an eclipse of the sun can permanently damage your eyes.