Equinox and Solstice Calculator
Exact UTC instants of the March and September equinoxes and the June and December solstices for any year, with local times and season lengths.
Season instants for 2026
| Event | Starts | UTC instant | Local time |
|---|---|---|---|
| March equinox | Spring | 2026-03-20 14:45:30 UTC | — |
| June solstice | Summer | 2026-06-21 08:24:49 UTC | — |
| September equinox | Autumn | 2026-09-23 00:05:24 UTC | — |
| December solstice | Winter | 2026-12-21 20:50:07 UTC | — |
Season lengths
| Season | From | Length |
|---|---|---|
| Spring | March equinox | 92.736 days (92d 18h) |
| Summer | June solstice | 93.653 days (93d 16h) |
| Autumn | September equinox | 89.864 days (89d 21h) |
| Winter | December solstice | 88.982 days (88d 24h) |
Equinox to equinox is 365.2356 days. The seasons are unequal because Earth's orbit is an ellipse: it moves fastest near perihelion in early January, which is why the season containing that date is the shortest.
Computed with the mean-instant polynomial and 24 periodic terms from Meeus, Astronomical Algorithms chapter 27, then converted from Dynamical Time to UTC by subtracting ΔT = 75.1 seconds. Meeus quotes a maximum error near 51 seconds for 1951–2050, and this page reproduces published equinox and solstice times for 2000 and 2024–2027 to within one minute.
What is the Equinox and Solstice Calculator?
An equinox or solstice is an instant, not a day — the moment the sun crosses the celestial equator or reaches its furthest declination.
- Meeus chapter 27 method: mean polynomial plus 24 periodic correction terms
- Dynamical Time converted to UTC with an explicit, displayed ΔT
- Both the UTC instant and your chosen time zone's wall-clock time
- Season lengths in days and hours, plus the equinox-to-equinox year length
- Northern or southern hemisphere season naming
- Copy all four instants as CSV
How to use the Equinox and Solstice Calculator
- 1
Enter a year between 1000 and 3000, or step through years with the previous and next buttons.
- 2
Choose your hemisphere — it changes which season each event starts, not the instants themselves.
- 3
Pick a time zone to see each instant as local wall-clock time next to the UTC value.
- 4
Read the season lengths table to see how unequal the four seasons really are.
- 5
Copy the four instants as CSV for a spreadsheet or a calendar.
About the Equinox and Solstice Calculator
An equinox or solstice is an instant, not a day — the moment the sun crosses the celestial equator or reaches its furthest declination. Because that instant moves by roughly six hours a year and jumps back after each leap day, the calendar date shifts around and can even differ between time zones. This tool gives you the exact moment for any year from 1000 to 3000.
The calculation follows chapter 27 of Meeus's Astronomical Algorithms: the mean instant from a fourth-order polynomial, corrected by 24 periodic terms, then converted from Dynamical Time to UTC by subtracting ΔT. It reproduces published equinox and solstice times to within a minute, and the exact ΔT used is shown so you can see the correction.
You also get the length of each astronomical season, which is where the elliptical orbit shows up: northern summer runs almost five days longer than northern winter. Everything is computed in your browser, with local times rendered only after the page has loaded so the results always match your own clock.
Frequently asked questions
When is the summer solstice in 2026?
The June solstice of 2026 falls at 08:24 UTC on 21 June. It marks the start of astronomical summer in the northern hemisphere and of winter in the southern, and it is the same instant everywhere — only the local date and clock time differ.
Why does the equinox fall on a different date each year?
The tropical year is about 365.2422 days, so the instant drifts roughly six hours later each year and then jumps back almost a full day after each 29 February. Over a four-year cycle it wanders across two calendar dates.
Why are the four seasons different lengths?
Earth's orbit is an ellipse, so it moves fastest near perihelion in early January and slowest near aphelion in July. Northern summer, spanning the slow part of the orbit, runs about 93.6 days while northern winter runs about 89 days.
What is ΔT and why does it matter here?
ΔT is the gap between Dynamical Time, which astronomical algorithms work in, and Universal Time, which clocks keep. It is currently around 69 seconds and grows as Earth's rotation slows. The tool subtracts it and shows the value used.
How accurate are these instants?
Meeus quotes a maximum error near 51 seconds for the period 1951 to 2050, and this page lands within a minute of published times for 2000 and 2024 to 2027. Outside that window ΔT becomes a projection, so the drift can grow to several minutes.
Is the equinox the day when day and night are equal?
Not quite. Atmospheric refraction lifts the sun's image and we measure sunrise from the upper edge rather than the centre, so equal day and night — the equilux — falls a few days before the March equinox and a few days after the September one.
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