What term describes the time or date when day and night are of equal length as the sun crosses the celestial equator?

The term that describes the time or date when day and night are of equal length is equinox. This occurs twice a year, typically around March 20 or 21 in the spring (vernal equinox) and around September 22 or 23 in the fall (autumnal equinox). During these times, the sun crosses the celestial equator, resulting in equal durations of daylight and darkness.

The equinox is a significant astronomical event that marks the changing of seasons and has cultural and historical importance in various societies. In contrast, solstices are points in the year when the sun reaches its highest or lowest point in the sky at noon, leading to the longest and shortest days of the year. Zenith refers to the point in the sky directly above an observer and is not related to day length. Aphelion is the point at which the Earth is farthest from the sun and also does not pertain to equal day and night lengths. Thus, equinox is the precise term that encapsulates the phenomenon of equal day and night durations resulting from the sun's position relative to the celestial equator.