The Math Behind Our Solstice & Equinox Dates
This note explains the math behind our solstice and equinox dates. The four turning points of the year — two solstices and two equinoxes — are precise astronomical instants, and the tool computes them to the minute without any lookup table.
Meeus's approximation
We use the method from Jean Meeus's Astronomical Algorithms: a polynomial gives the approximate instant for a given year, then a sum of 24 periodic correction terms refines it to account for the planets' gravitational tugs on Earth's orbit. The result is the exact moment the sun crosses the equator (equinox) or reaches its furthest declination (solstice), accurate to within minutes.
Shown in your time
Those instants are universal, but the tool renders them in your local time, so "the June solstice" shows when it actually happens for you. It is the same engine that powers the Moon and sun-time calculations: everything runs client-side, with no API and no tracking. For the meaning behind the dates, see our guide on solstice vs equinox.
Why the dates drift
The tropical year is about 365.2422 days, not 365, so each solstice falls roughly six hours later in the day than the year before — until a leap day pulls it back by 24 hours. That is why the June solstice lands on the 20th or the 21st depending on the year, and why the pattern shifts slowly across a century as the Gregorian rule drops three leap days every 400 years.
Universal instant, local date
Because the crossing is a single moment for the whole planet, the calendar date can differ by a day between timezones: an equinox at 23:30 UTC is already the next day in Tokyo. The tool always converts the instant to your own timezone, so the date you see is the date it happens where you are.
See this year's solstices and equinoxes for your location.
Open the Sun Times tool