What is an equinox?
Quick Answer
An equinox is one of two moments each year, around 20 March and 22–23 September, when the Sun is directly above the equator and day and night are roughly equal everywhere.
The Full Story
Earth's axis is tilted about 23.4°, so for most of the year one hemisphere leans towards the Sun. At the equinoxes, neither hemisphere is tilted towards the Sun, and the Sun appears to cross the celestial equator. The March equinox marks the start of astronomical spring in the Northern Hemisphere and autumn in the Southern Hemisphere; the September equinox does the reverse. On these dates the Sun rises almost due east and sets almost due west. Day and night are not exactly equal on an equinox, because atmospheric refraction and the size of the Sun's disc add a few minutes of daylight.
Key Facts
YouTube Angle
Suggested video title for this topic:
"Equal day and night? Not quite — the surprising truth about the equinox."