About Equinox & Solstice Calculator
Equinoxes and solstices mark the four points in Earth's orbit where its 23.4° axial tilt relative to the Sun reaches a notable value: at the solstices, one hemisphere is tilted as far toward or away from the Sun as it gets all year (the longest and shortest days); at the equinoxes, neither hemisphere is tilted toward the Sun, and day and night are close to equal length everywhere on Earth.
Which hemisphere gets summer and which gets winter at a given solstice is just a matter of which one happens to be tilted toward the Sun at that point in the orbit — which is why the seasons are opposite in the Northern and Southern hemispheres, and why 'summer solstice' on its own is genuinely ambiguous without saying which hemisphere you mean.
The calculation here uses Meeus's published mean-date formulas rather than the full periodic-term version, which trades a small amount of precision (see the accuracy note below) for a closed-form calculation that runs instantly for any year without needing to load additional orbital data.
How to find an equinox or solstice date
Enter a year
Any year from 1000 to 3000.
Read all four dates
With the hemisphere each one marks the start of.
Check what's next
The nearest upcoming one is called out separately.