Definition

A solar eclipse happens when the Moon moves directly between Earth and the Sun at new moon, casting its shadow across part of Earth and briefly turning day dark for anyone inside that shadow. Viewers in the Moon's dark inner umbra see the Sun fully covered for up to a few minutes, a total eclipse, while those in the lighter penumbra see only a partial one; when the Moon is near its farthest point from Earth it looks too small to cover the Sun, leaving a bright ring instead, an annular eclipse. This can only happen when new moon coincides with the Moon passing near a node of its tilted orbit, during eclipse seasons roughly every $173$ days, and totality is possible only because the Sun is about $400$ times the Moon's diameter and about $400$ times farther away, so both appear roughly $0.5^\circ$ across in the sky. Because the umbra's tip barely reaches Earth, the Moon's orbital eccentricity decides whether an eclipse is total, near perigee, or annular, near apogee, and the whole pattern repeats in the $18$-year, $11$-day saros cycle.