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.
Not every solar eclipse looks the same. A total eclipse happens when the moon completely covers the sun’s disk, briefly turning day into an eerie twilight and revealing the sun’s faint outer atmosphere, the corona, that is normally too dim to see. An annular eclipse occurs when the moon is slightly farther from Earth in its elliptical orbit and appears too small to fully cover the sun, leaving a bright ring, or annulus, visible around its edge. Total solar eclipses are visible from any given spot on Earth only once every 375 years or so on average, which is why eclipse chasers will travel across continents to stand inside a narrow path of totality that might be only a few hundred kilometers wide.
Eclipses are not random events, they follow a predictable pattern called the Saros cycle, roughly 18 years and 11 days long, after which the sun, moon, and Earth return to nearly the same relative positions and a similar eclipse recurs. Ancient astronomers in Babylon had already identified this cycle over two thousand years ago, letting them predict eclipses well before anyone understood the actual mechanics behind them.