Equinox and Solstice Dates Calculator

The dates drift by a day or so from year to year, but they cluster around four points. The March equinox falls on March 19, 20, or 21, the June solstice on June 20 or 21, the September equinox on September 22 or 23, and the December solstice on December 21 or 22. Enter a year above to estimate the event time. The variation comes from the calendar year being about a quarter day longer than the leap-year cycle accounts for, which leap years correct in steps.

It uses Meeus' astronomical algorithm to estimate when the Sun reaches each cardinal point of the ecliptic: 0 degrees for the March equinox, 90 for the June solstice, 180 for the September equinox, and 270 for the December solstice. A polynomial gives an approximate time for the year, then a 24-term periodic correction refines it. Treat the result as an estimate rather than an observed event time.

An equinox is a moment when the Sun crosses the celestial equator, so day and night are close to equal length everywhere on Earth. A solstice is a moment when the Sun reaches its farthest point north or south, giving the longest or shortest day of the year depending on your hemisphere. Equinoxes mark the start of spring and autumn, while solstices mark the start of summer and winter.

An equinox or solstice is a single instant that happens at the same moment everywhere, but the clock reading for that moment differs by location. The calculator computes the event in Universal Time, then converts it to the wall-clock time in the zone you choose, applying that zone's offset and daylight saving rules. Because of the conversion, an event can even land on a different calendar day in your zone than it does in UTC.

Yes. The dates are the same worldwide, but the seasons they begin are swapped. The June solstice starts summer in the Northern Hemisphere and winter in the Southern Hemisphere, and the December solstice does the reverse. The March equinox begins spring in the north and autumn in the south, and the September equinox flips that. Each result card notes both hemispheres.

Delta-T is the small, slowly changing gap between Terrestrial Time, the smooth timescale astronomers compute with, and Universal Time, which follows the Earth's gradually slowing rotation. The algorithm finds each event in Terrestrial Time, then subtracts Delta-T to report a civil clock time. Near the year 2025 Delta-T is roughly 70 seconds, so leaving it out would shift the reported times by about a minute.