when do lunar eclipses happen?
A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the sun, Earth and moon are aligned (in  syzygy ) exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes. A total lunar eclipse has the direct sunlight completely blocked by the earth's shadow.
A lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth's shadow obscures the moon or a portion of it. A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking all or a portion of the Sun. An eclipse can be total, partial, or annular.