Season dates (equinoxes and solstices)
Get the approximate dates and times of the two equinoxes and two solstices for a given year, for the Northern or Southern Hemisphere. All calculation is done locally, and no data is sent to a server.
How it works
Equinoxes and solstices don't fall on exactly the same date every year: the length of the tropical year isn't a whole number of days, which shifts these moments slightly from one year to the next, in a cycle that repeats roughly every four years. This tool uses a standard polynomial formula from astronomy (from the work of Jean Meeus) to estimate the exact moment, expressed in Coordinated Universal Time (UTC).
Why are the dates only approximate?
The full calculation of equinoxes and solstices requires adding dozens of periodic correction terms related to the gravitational perturbations of the Moon and the other planets. This tool applies only the main polynomial formula, which gives a result accurate to within a few hours — more than enough to plan an event or know the exact day, but not for high-precision scientific use.
Why are the seasons reversed between the Northern and Southern Hemispheres?
Seasons are defined by the tilt of the Earth relative to the Sun, which is opposite between the two hemispheres at any given moment: when the Northern Hemisphere leans toward the Sun (summer), the Southern Hemisphere leans away from it (winter). The four astronomical moments stay the same in absolute terms, and only the name of the associated season changes depending on the hemisphere you choose.
This calculation is purely mathematical and runs entirely in your browser — no information is sent to a server.