How to Find the Exact Solstice Moment

One source says the solstice is on the 21st, another says the 20th, and both are published by organisations that know what they are doing.

They are both right. The solstice is an instant, not a day, and which day it falls on depends on your time zone.

A solstice is a moment

The solstice is the instant the Sun reaches its greatest declination from the celestial equator. The equinox is the instant it crosses. Both are points in time, precise to the minute.

Near midnight UTC, that instant falls on different calendar dates in different places. A solstice at 03:20 UTC on the 21st is 22:20 on the 20th in New York — same moment, two dates, both correct locally.

This is the same phenomenon that makes people argue about the date of the Moon landing, and it resolves the same way: state the time zone.

Why the date moves between years

The tropical year — one complete cycle of the seasons — is about 365.2422 days, not 365.

That quarter-day surplus means each solstice occurs roughly six hours later by the clock than the previous year’s. After four years the accumulated drift is close to a full day, and the leap day pulls it back.

So the solstice date cycles across a range of two or three dates over a leap cycle, drifting later within a leap cycle and jumping back at each leap year. Over longer periods the Gregorian calendar’s century rules apply a further small correction, which is what keeps the drift bounded over centuries.

The seasons are not equal

The four astronomical seasons differ in length by several days, which surprises people who assume a quarter of a year each.

Earth’s orbit is elliptical, and by Kepler’s second law a planet moves faster when nearer the Sun. Earth is closest to the Sun in early January, so it travels through the segment of its orbit containing northern hemisphere winter more quickly.

The result is that northern summer — from June solstice to September equinox — runs several days longer than northern winter. Southern hemisphere seasons are the mirror image.

Equal day and night is not the equinox

The word suggests twelve hours of each, and the day it happens is not the equinox.

Two effects push it earlier. Atmospheric refraction bends sunlight around the horizon, so the Sun appears to have risen while it is still geometrically below it. And sunrise and sunset are defined by the disc’s upper edge, not its centre, adding a further couple of minutes at each end.

Together those give a few extra minutes of daylight, so equal day and night — the equilux — falls several days before the March equinox and several days after the September one, at a date that depends on latitude.

Meteorological seasons are different

A second set of season boundaries that explains a lot of apparent disagreement.

Astronomical seasons run solstice to equinox, so they start on varying dates around the 20th to 23rd.

Meteorological seasons run in whole months — in the northern hemisphere, winter is December through February, spring is March through May, and so on.

Meteorological seasons exist because climate records need consistent periods to compare. Averaging temperature over a season whose start date moves by three days makes year-on-year comparison harder for no benefit.

So a weather service announcing the first day of summer on 1 June and an almanac announcing it on the 21st are both correct within their own convention. Neither is a mistake, and knowing both exist resolves the annual argument.

Common mistakes to avoid

• Quoting a date without a time zone.
• Assuming the seasons are equal in length.
• Treating the equinox as the day of equal day and night.
• Using last year’s date for this year.
• Confusing astronomical seasons with meteorological ones, which start on fixed calendar dates and are defined for record-keeping.

How to do it with Solstice and Equinox Finder

The Solstice and Equinox Finder gives the exact moments in your time zone.

1. Enter the year and choose local time or UTC.
2. Read the instants, which are precise to the minute.
3. Note the intervals between events — they are not equal.
4. When quoting a date to anyone else, say which time zone it is in.

Other date and time tools are in the tools directory.

Frequently asked questions

Why do solstice dates change from year to year?

Because the tropical year is about 365.2422 days. Each event drifts about six hours later annually until a leap day pulls it back, so the date cycles across a range of two or three.

Why are the seasons different lengths?

Earth’s orbit is elliptical and it moves faster when closer to the Sun. Northern summer, when Earth is furthest away, therefore takes several days longer than northern winter.

Is the equinox exactly twelve hours of daylight?

No. Atmospheric refraction and measuring to the disc’s edge add several minutes, so equal day and night falls a few days before the March equinox and after the September one.

Final thought

Ask for the time, not the date. Half the disagreements about when a solstice falls are two people quoting the same instant in different time zones.

Try the free Solstice and Equinox Finder