Planets and moons do not give off their own light. They reflect light from a star. They also cast shadows, just like we do when we are in sunlight.
An eclipse takes place when one celestial body passes through the shadow of another. An eclipse may be partial, with only some rays hidden, or total, when all the sun’s rays are cut off. Remember, there cannot be an eclipse without syzygy.
Syzygy, from a Greek word meaning “yoked,” or bound together, refers to the alignment of celestial bodies.
Without the exact lining up of the Sun, Earth, and the Moon, there can be no eclipse of any kind: solar, lunar, or annular.