Final answer:
A lunar eclipse can only occur during a full moon when the Earth's shadow falls upon the fully illuminated surface of the moon. This celestial event is visible from any location on Earth where the moon is visible and occurs when the Earth is between the sun and the moon.
Step-by-step explanation:
A lunar eclipse occurs when the Moon moves into the dark shadow of the Earth. This event can only happen when the sun, Earth, and moon align perfectly, which is only at the time of a full moon. During this phase, the side of the moon facing the Earth is fully illuminated, and the Earth is positioned directly between the sun and the moon. As the moon enters the Earth's shadow, the sunlight that normally reflects off the moon's surface is blocked. While the partial shadow of the Earth, called the penumbra, first dims the moon somewhat, it is the Earth's umbra that causes the dramatic darkening effect once the moon is fully engulfed. Even though lunar eclipses can take place about twice a year, the view of this occurrence is far more common than that of solar eclipses, as it can be seen from any place on Earth where the moon is above the horizon at the time of the eclipse.