Equinox and Solstice Calculator

An equinox occurs twice a year (around March 20 and September 22–23) when the Sun crosses the celestial equator, resulting in nearly equal day and night lengths worldwide. A solstice occurs when the Sun reaches its farthest point north or south of the equator — the June solstice brings the longest day in the Northern Hemisphere, and the December solstice brings the shortest.

This calculator uses Jean Meeus's astronomical algorithms, which are accurate to within one minute for years between 1951 and 2050. Accuracy decreases slightly for years farther outside that range due to unpredictable variations in Earth's rotation.

TDT (Terrestrial Dynamical Time) is a uniform timescale used for astronomical calculations and is not affected by the irregular slowing of Earth's rotation. UTC (Coordinated Universal Time), which most civil clocks follow, includes leap seconds to stay aligned with Earth's actual rotation. This calculator outputs times in UTC adjusted for your selected timezone offset.

Earth's orbit around the Sun takes approximately 365.25 days, not exactly 365. This causes the astronomical events to shift by about 6 hours each year, which is why we add a leap day every 4 years. The Gregorian calendar keeps these events relatively stable, but slight variation of a day or so is normal.

Yes — an equinox or solstice is a single moment in time and occurs simultaneously everywhere on Earth. However, because of timezone differences, the local calendar date of that moment may differ by one day depending on where you are. For example, an equinox at 23:00 UTC is experienced on the same day in Europe but the next calendar day in parts of Asia.

Astronomically, the seasons are defined by the solstices and equinoxes. However, meteorological seasons (used by climatologists) are defined by fixed calendar months: spring is March–May, summer is June–August, autumn is September–November, and winter is December–February in the Northern Hemisphere.

The timing of the events is the same globally, but the associated seasons are reversed. When the Northern Hemisphere experiences the June solstice (its longest day and start of summer), the Southern Hemisphere experiences its shortest day and the start of winter.

Yes — this calculator works for any year from 1000 to 3000. Keep in mind that accuracy is highest for years between 1951 and 2050, and results for very distant past or future years carry a somewhat larger margin of error.