An equinox is one of two moments each year when the Sun is directly above the celestial equator, resulting in roughly equal lengths of day and night everywhere on Earth. There are two equinoxes: the vernal (spring) equinox, around March 20–21, and the autumnal (fall) equinox, around September 22–23. At these times, the tilt of Earth's axis is such that it is not leaning toward or away from the Sun, and the Sun’s path crosses the celestial equator. Key points:
- The term comes from Latin aequinoctium, meaning “equal night,” though the day and night are not exactly 12 hours due to atmospheric refraction and the Sun’s apparent size.
- An equinox also corresponds to the points where the Sun’s apparent path (the ecliptic) intersects the celestial equator in the sky.
- In the Northern Hemisphere, the vernal equinox signals the start of spring; the autumnal equinox signals the start of autumn. In the Southern Hemisphere, these seasons are reversed.
If you’d like, I can add quick diagrams or explain how the equinox differs from the solstice.
