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Can someone explain why today is the equinox when sunrise is 06.03 and sunset is 18.13 in London.

Tessa K wrote: Fri Mar 20, 2026 9:58 am Can someone explain why today is the equinox when sunrise is 06.03 and sunset is 18.13 in London.

Atmospheric refraction, and the fact that sunrise is defined as being the moment when the top edge of the sun first appears above the horizon, and sunset when the top edge of the sun disappears below the horizon.

Atmospheric refraction is about twice the effect of the second. If there was no atmosphere, the sun would appear to be 1/2° lower in the sky near the horizon, and sunrise/sunset would be 4 minutes later/earlier. The time taken for the center of the sun to travel the 1/4° to where the upper limb is 2 minutes. So you get an extra 6 minutes at each end of the day.

Tessa K wrote: Fri Mar 20, 2026 9:58 am Can someone explain why today is the equinox when sunrise is 06.03 and sunset is 18.13 in London.

What you're looking for is an equilux.

The timing in relation to an equinox depends on how far from the equator you, whether it's spring or autumn and which hemisphere you're in. I don't think it's a very useful thing, but it gives the newspapers something to write about every so often.

There is a third material effect, on the same scale as the two dyqik described, that also affects the day length of the "equinox day". But the effect varies from year to year, and can be either positive or negative - on average it is zero. It also varies by latitude. At the latitude of London, the maximum amount of the variation from this cause is somewhere between 3 and 4 minutes, from one year to another.

This effect is that the equinox is a precise moment in time, when the earth crosses from one side to the other side of its orbit. Whereas a day is a whole day long. Day length is continuously changing. If the moment of the equinox is in the later part of the day assigned as the equinox day, then the daylength of the equinox day is shorter, whereas it is longer if it occurs in the earlier half of the day. The maximum size of this effect occurs when the moment of the equinox is close to local midnight.

In London, daylength is increasing by about 3-4 mins per day around the spring equinox, so that is the maximum size of the effect in London. In Svalbard, in the high arctic, daylength changes by about 15-16 minutes per day around the equinox. So that can make a more appreciable difference to the length of the day assigned as the equinox day.

There is a further small adjustment, on the scale of a few seconds, from the so-called "equation of time". This is the fact that time from one midnight to the next midnight ("true" daylength) is not precisely 24 hours, it is only 24 hours on average. True daylength varies over the year by a few seconds. It is set ("equated") to precisely 24 hours in modern time-keeping. In traditional time-keeping, where a day was reckoned from one true midnight to the next, this variation was allowed to occur naturally. But when we fixed our clock, to enable train timetables, etc, we equated all the days to precisely the same length long, hence "equation of time". The variation in true day length is a matter of seconds, but accumulates to quite a few minutes with the seasons - the precise amount varies by latitude - the biggest effects are at lower latitudes. It has a rather odd pattern over the year, because there are two contributors to the variation, that operate on different cycles. These are the tilt of the earth's axis (about 2/3 of the effect), which for geometric reasons changes the moment of sunrise and sunset as the earth rotates around the sun; and the elliptic nature of the earth's orbit, meaning that the earth does not rotate around the sun at a constant angular velocity (about 1/3 of the effect).

There are yet other very small adjustments, sub-second, from wobbles in the earth's orbit and rotation.