Why does the lunar eclipse happen at full moon?

Answer 1

A lunar eclipse happens when Earth is exactly between the Moon and the Sun (or more correctly, when viewed from the Sun, the Moon passes behind Earth and through its shadow), like this:

(Sun)--------------(Earth)---(Moon)

When you look up from Earth at that time, you see the part of the moon that is in full sunlight.

We don't see a lunar eclipse every full moon because the Moon's orbit around Earth is slightly tilted,so it passes over or under the Earth's shadow most of the time.