An eclipse happens when one astronomical object moves into the shadow of another, or passes directly between another object and an observer, blocking it from view. The two most familiar kinds, from Earth’s perspective, are solar eclipses and lunar eclipses.
A solar eclipse occurs when the moon passes directly between the Earth and the sun, briefly blocking some or all of the sun’s light from reaching a narrow path on Earth’s surface. A lunar eclipse is the reverse, when the Earth passes between the sun and the moon, casting its shadow across the moon’s surface and often turning it a deep red.
Solar eclipses have played an outsized role in the history of physics, since a total solar eclipse in 1919 provided the first real evidence for general relativity, when starlight bending around the darkened sun matched Einstein’s predicted values almost exactly.