About the Solstice and Equinox Calculator
This solstice and equinox calculator gives the exact moment of all four astronomical season markers — the March equinox, June solstice, September equinox and December solstice — for any year between 1000 and 3000. Enter a year and your UTC offset to see the local date, time and weekday of each event, plus how long each astronomical season lasts.
Use it to find the first day of summer or winter, plan a solstice gathering or sunrise photo, label a garden or school calendar, or settle the classic “is the equinox on the 20th or 21st?” question. Choose your hemisphere and the results name the seasons the way you experience them: the June solstice starts summer in New York but winter in Sydney.
Times are computed with Jean Meeus’s method from Astronomical Algorithms, which matches the US Naval Observatory tables to within about a minute for modern years. The time zone is a fixed offset you choose, so add daylight saving time yourself (for example UTC−4 for New York in June instead of UTC−5). All dates use the Gregorian calendar, extended backwards for years before 1582.
How to use the solstice and equinox calculator
- 1Enter the year you are interested in (1000–3000).
- 2Enter your time zone as an offset from UTC, including daylight saving time if it applies on that date.
- 3Pick the event you care most about for the headline result.
- 4Choose your hemisphere so the table names the right seasons.
- 5Read the date, time and weekday of all four events and the season lengths.
Formula and method
For each event the calculator first computes a mean instant JDE₀ from a fourth-degree polynomial in the year (Meeus, Astronomical Algorithms, chapter 27). It then corrects for the real, non-circular motion of Earth and the pull of the Moon and planets by adding 24 periodic terms S, scaled by Δλ = 1 + 0.0334 cos W + 0.0007 cos 2W, where W = 35,999.373·T − 2.47° and T is Julian centuries from J2000.
The result is in Terrestrial Time, a uniform atomic-based scale. Clocks follow Earth’s slightly irregular rotation, so the calculator subtracts ΔT (about 69–75 seconds in the 2020s) using the Espenak–Meeus polynomials published by NASA, converts to a calendar date, applies your UTC offset and rounds to the nearest minute.
- JDE₀
- Mean Julian Ephemeris Day of the event
- S
- Sum of 24 periodic terms A·cos(B + C·T)
- T
- Julian centuries since 1 January 2000, 12:00 TT
- ΔT
- Terrestrial Time minus Universal Time, in seconds
Worked examples
All four events of 2026 in UTC
In 2026 the March equinox falls on March 20 at about 14:45 UTC, the June solstice on June 21 at 08:25, the September equinox on September 23 at 00:05 and the December solstice on Monday, December 21 at 20:50 UTC. The US Naval Observatory lists 14:46, 08:24, 00:05 and 20:50 — the same to within a minute.
First day of summer 2026 in New York (EDT)
New York is on daylight time (UTC−4) in June, so the 08:25 UTC solstice happens at about 04:25 local time on Sunday, June 21. The September equinox at 00:05 UTC is still the evening of September 22 in New York. Astronomical summer lasts about 93.65 days.
Spring equinox 2027 in Sydney
Using a fixed UTC+11 offset (Sydney daylight time), the March 2027 equinox — the start of autumn in the Southern Hemisphere — falls at about 07:25 on March 21, and the September equinox that begins southern spring lands at about 17:01 on Thursday, September 23. The US Naval Observatory gives 20:25 UTC on March 20 and 06:02 UTC on September 23 (07:25 and 17:02 at UTC+11), within a minute. Use +10 instead for dates when Sydney is on standard time.
Frequently asked questions
What is the difference between a solstice and an equinox?+
At a solstice the Sun reaches its farthest point north (June) or south (December) of the celestial equator, giving the longest and shortest days. At an equinox the Sun crosses the equator, so day and night are roughly equal everywhere.
Why does the solstice date change from year to year?+
A calendar year has 365 or 366 days, but a tropical year is about 365.2422 days. Each event drifts about 6 hours later every year, then jumps back roughly a day after each leap day, so in the 21st century the dates wobble between the 19th–21st (March), 20th–21st (June), 22nd–23rd (September) and 21st–22nd (December).
Is the first day of summer June 20 or June 21?+
Astronomical summer starts at the June solstice, which falls on June 20 or 21 depending on the year and your time zone. In 2026 it is June 21 at about 08:24 UTC. Meteorologists instead use June 1 as the start of summer for record-keeping.
Are day and night exactly equal on the equinox?+
Not quite. Because sunrise and sunset are measured from the top edge of the Sun and the atmosphere bends light, daylight is a few minutes longer than 12 hours on the equinox. The day of equal light and dark, sometimes called the equilux, comes a few days earlier in spring and later in autumn.
Why are the seasons different lengths?+
Earth’s orbit is slightly elliptical. It is closest to the Sun in early January and moves faster then, so northern winter is the shortest season (about 89 days) and northern summer, near aphelion in July, is the longest (about 93.6 days).
How accurate are these times?+
Meeus’s method agrees with the US Naval Observatory to within about a minute for years near the present. Far in the past or future the uncertainty in ΔT, the slowing of Earth’s rotation, adds minutes to hours, so treat times before 1600 or after 2200 as approximate.