Lunar eclipses only occur at the full moon.
Lunar eclipses happen at the full moon.
A lunar eclipse can only occur at Full Moon. Technically, the central moment of the eclipse must be the precise moment of Full Moon.
That's a true statement, if you're referring to a 'full phase' of the moon.
A full moon happens when the Moon is in the Earth's shadow, so naturally, at that moment, the Moon is opposite the Sun in the sky.
Solar eclipses happen during NEW moons, when the Moon blocks the light of the Sun. Lunar eclipses happen during FULL moons, when the Earth blocks the light of the Sun.