A lunar eclipse does NOT occur at every full Moon because the:

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Quizplus · Accepted Answer

A lunar eclipse occurs when the Moon passes through the Earth's shadow, which can only happen when the Earth, Moon, and Sun are all lined up in a specific way. However, the plane of the Moon's orbit is tilted at an angle of about 5 degrees to the plane of Earth's orbit around the Sun. This means that during most full moons, the Moon is above or below the Earth's shadow, and no eclipse occurs. Only when the Moon is at or near one of the two points in its orbit where it crosses the ecliptic (the plane of Earth's orbit) can a lunar eclipse occur.

A Lunar Eclipse Does NOT Occur at Every Full Moon