An equinox is when day and night are roughly equal, happening twice a year in March and September. A solstice is an extreme — the longest or shortest day — happening in June and December. Equinoxes mark the balance points; solstices mark the turning points.
The core difference
Four moments in the year, defined by the Earth's tilt relative to the Sun.
- equinox — around 20 March and 22 September; day and night nearly equal.
- solstice — around 21 June and 21 December; the longest and shortest days.
What the words say
Equinox is Latin aequus (equal) plus nox (night). Solstice is sol (sun) plus sistere (to stand still) — at the solstice the Sun's daily path stops moving north or south and reverses, appearing to stand still before turning back.
The equal-day detail
Day and night are not exactly equal at the equinox. Because the atmosphere refracts sunlight and sunrise is measured from the Sun's upper edge, the day runs slightly longer. The exact date of equal day and night is called the equilux and falls a few days off the equinox.
Frequently asked questions
How many equinoxes and solstices are there?
Two of each per year — equinoxes in March and September, solstices in June and December.
What does solstice mean literally?
Sun standing still, from Latin sol and sistere — the Sun's path pauses and reverses direction.
Are day and night exactly equal at the equinox?
Not quite. Atmospheric refraction makes the day slightly longer. The date of truly equal day and night is called the equilux.