The time of the year when day and night are each about 12 hours long and the Sun is at the midpoint of the sky.
This happens twice a year:
• Spring equinox
• Fall (Autumn) equinox
Equinox happens around March 21 and September 21 each year, when the Sun appears to be directly above the equator.
See this animation for why it happens:
../measure/images/earth.js