Why the equinox gives twelve hours and seven minutes of daylight

The equinox is the moment the Sun’s apparent longitude reaches 0° in March and 180° in September — the moment it is directly above the equator, and day and night are supposed to be equal.

They are not. On the equinox date, everywhere on Earth gets about seven extra minutes of daylight, and a little more the further you are from the equator. Computed with the standard solar algorithm, for March 20, 2026:

Latitude	Day length	Excess over 12h
0°	12h 07m	+7 min
40°	12h 07m	+7 min
60°	12h 10m	+10 min
66°	12h 12m	+12 min

Two separate causes, both about the Sun not being a point.

Cause one: the Sun has width

Sunrise is defined as the moment the upper limb of the Sun touches the horizon, not the center. The Sun’s angular radius is about 16 arcminutes, so the center is still 16′ below the horizon at sunrise and 16′ below at sunset. That is 32′ of extra arc, spread over the two ends of the day.

Cause two: the atmosphere bends light

Refraction near the horizon lifts the Sun’s apparent position by about 34 arcminutes — roughly its own diameter. The Sun is visible while it is geometrically below the horizon, at both ends of the day. That is another 68′ of arc.

Add them and you get about 100 arcminutes — 1.67° — of extra arc, split between sunrise and sunset. The Sun moves 15° per hour along the celestial equator, so 1.67° is about 6.7 minutes. Which is where the seven minutes come from.

Both numbers matter: skipping refraction gives about 3 minutes, skipping the semi-diameter gives about 4, and only with both do you reproduce the observed day length. This is exactly the kind of detail where a plausible-sounding explanation (“refraction adds a few minutes”) produces a number that does not check out.

Why latitude stretches it

The same 50′ of altitude at each end of the day takes longer to traverse when the Sun crosses the horizon at a shallow angle. Near the equator the Sun rises almost vertically, so the arc is covered in seconds; at high latitude it rises at a slant, and the same altitude takes minutes.

That is also why the excess grows so fast near the poles — at the Arctic Circle the Sun skims the horizon for a long time at the equinoxes, and the day is over twelve hours and twenty minutes long before you get there.

One detail that surprises people

On a given equinox date, the day length is not perfectly monotonic in latitude — the figure at 20° can come out a minute shorter than at the equator. That is not an error. The equinox is an instant (14:46 UTC in March 2026), not a day, and the date either side of it has a slightly different solar declination. Comparing latitudes on the same calendar date is therefore comparing slightly different Sun positions.

Compare them at the equinox instant instead, and the ordering becomes clean.

Why it matters for anything else

Because the same 0.833° convention is baked into every rise time this site publishes. A sunrise table that uses the geometric horizon gives times about four minutes later than reality; one that ignores semi-diameter but keeps refraction is about three minutes late. The sunrise table uses the full convention, which is why its times match the USNO’s rather than disagreeing with them by a few minutes in a way nobody can explain.

For the other half of the seasonal story — why the earliest sunset comes ten days before the shortest day — see that page.

Day lengths computed with the NOAA solar algorithm at 0° longitude on March 20, 2026 and rounded to the minute; the exact values shift by a minute or two with longitude and year.

Frequently asked questions

Why is the equinox not exactly 12 hours of day and 12 of night?

Two effects, both about the Sun's apparent size. Sunrise and sunset are defined on the upper limb, not the center, which adds about 16 arcminutes of the Sun's own radius. And refraction near the horizon lifts the whole disc by about 34 arcminutes, so it is visible while geometrically below the horizon. Together that is about 50 arcminutes of arc, worth roughly seven minutes of time at the equator.

Does the extra time get bigger further from the equator?

Yes, gently. The same 50 arcminutes of altitude corresponds to more time when the Sun crosses the horizon at a shallow angle, which happens at high latitude and near the poles. At 60°N the equinox day is about 12h 10m; at the Arctic Circle, about 12h 12m. The effect is modest until you are very far north or south, where it grows quickly.

Which equinox is the 'official' start of spring?

The March equinox is the astronomical start of spring in the northern hemisphere and of autumn in the southern. Meteorologists use calendar months instead — March to May for northern spring — because monthly statistics are tidier that way. Neither definition changes what the Sun does.

Is the equinox the same instant everywhere?

Yes. It is a single instant, defined by the Sun's apparent longitude reaching 0° or 180°, and it happens at the same moment for the whole planet. Only the local date and clock time differ: in 2026 the March equinox falls on March 20 at 14:46 UTC, which is March 20 in Europe and Africa but still March 20 in the Americas.

Sources

Each source is a primary or official reference, checked before publication. Links open the original.

U.S. Naval Observatory, Astronomical Applications Department — the 2026 season instants (March equinox March 20 14:46 UTC, June solstice June 21 08:25 UTC, September equinox September 23 00:06 UTC, December solstice December 21 20:50 UTC) and the rise/set tables this page's day lengths are checked against (aa.usno.navy.mil)

NOAA Global Monitoring Laboratory, Solar Calculator — the 90.833° zenith definition that folds 34′ of refraction and 16′ of solar semi-diameter into sunrise and sunset (gml.noaa.gov/grad/solcalc/)

Meeus, Astronomical Algorithms, 2nd edition, chapters 15 and 25 — the hour-angle formula and the apparent solar longitude that defines the equinox

National Weather Service — the astronomical versus meteorological definitions of the seasons, and the standard refraction value used at the horizon