Solstice & Equinox Calculator (Exact Dates and Times, Any Year 1000–3000)
Exact date and time of the March and September equinoxes and the June and December solstices for any year and time zone, with the start and length of each astronomical season in the Northern or Southern Hemisphere.
How it works
Mean instant JDE₀ = polynomial in Y = (year − 2000) ÷ 1000 for each event (Meeus table 27.B) · correction: JDE = JDE₀ + 0.00001·S ÷ Δλ, with S = Σ A·cos(B + C·T) over 24 periodic terms and Δλ = 1 + 0.0334 cos W + 0.0007 cos 2W · UT = TT − ΔT · local = UT + offset
Equinoxes and solstices are instants, not days: the moments when the Sun's apparent longitude reaches 0° (March equinox), 90° (June solstice), 180° (September equinox) and 270° (December solstice). Because Earth's orbit is elliptical the four astronomical seasons have different lengths — northern summer is about 93.7 days, winter about 89 — and the dates drift through the 4-year leap cycle. The calculator uses Jean Meeus's algorithm (accurate to about a minute for 1000–3000) and converts Terrestrial Time to clock time with ΔT.
Worked example
In 2026 (UTC) the March equinox is Friday, March 20, 2026 at 14:46, the June solstice Sunday, June 21 at 08:25, the September equinox Wednesday, September 23 at 00:05 and the December solstice Monday, December 21 at 20:50. Northern spring therefore lasts 92.74 days, summer 93.65, autumn 89.86 and winter 88.98 days.
Sources: Meeus, Astronomical Algorithms (2nd ed.), ch. 27 Equinoxes and Solstices · US Naval Observatory — Earth's seasons
Frequently asked questions
When is the first day of summer?
Astronomically, at the June solstice in the Northern Hemisphere (June 20 or 21) and at the December solstice in the Southern Hemisphere (December 21 or 22). Meteorologists instead start summer on June 1 (or December 1).
Is the equinox always on March 21?
No. In the 21st century the March equinox falls on March 19 or 20 (UTC) and the September equinox on September 22 or 23. The Gregorian leap-year rule keeps them within a two-day window.
Are day and night equal on the equinox?
Almost but not quite: refraction and the Sun's disc add several minutes of daylight, so equal day and night (the "equilux") happens a few days before the March equinox and after the September one.
Why is the solstice date different in my time zone?
The event is a single instant worldwide. If it happens at 00:05 UTC on September 23, it is still September 22 in the Americas. Enter your UTC offset to see your local date and time.
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This calculator is for educational purposes. Double-check important results before acting on them.