Why the earliest sunset is not on the shortest day

wb_twilightThe dates

Earliest sunset, shortest day, latest sunrise

If sunrise and sunset simply followed the length of the day, the latest sunrise and the earliest sunset would both fall on the solstice. They do not.

The gap is wider in New York (27 days between the two extremes) than in Edinburgh (15 days). Keep that in mind: it is a clue to the cause.

timelinePart one

Around the solstice the day length hardly changes

Day length changes fastest at the equinoxes, by about four minutes a day in London, and slowest at the solstices. In the three weeks around 21 December London’s day stays within a few minutes of its minimum:

Between 12 and 30 December the length of the day varies by only five minutes, yet sunrise moves ten minutes later and sunset eight minutes later. Something else is moving them.

schedulePart two

Solar noon is drifting later

Look at the solar noon column. Between 12 and 30 December it moves from 11:54 to 12:03, about half a minute every day. Sunrise and sunset sit either side of solar noon like two hands on a seesaw: when the day is not changing length, the whole day slides later, so sunset gets later while sunrise also keeps getting later.

Before the solstice the day is still shortening, but more and more slowly, and the loss is shared between the two ends. In the morning the drift adds to it, so sunrise keeps getting later. In the evening the drift works against it, and by about 12 December the two cancel: sunset stands still at 15:51 for several days, then begins to move later while the day is still shortening. After the solstice the day lengthens; by 30 December the lengthening finally outpaces the drift in the morning too, and sunrises start getting earlier.

The drift is the equation of time at work. In December it changes faster than at any other time of year, because two effects add up: the Earth is approaching its closest point to the sun, reached in early January, and moving fastest along its orbit; and near a solstice the sun’s daily motion runs parallel to the celestial equator, so all of it adds to the east–west drift that clocks measure.

publicWhy the gap changes

Latitude decides how lopsided it is

Further north the day length changes faster in the weeks either side of the solstice, so it dominates the drift and pulls the extremes close to the solstice, as in Edinburgh. Further south the day length changes more slowly, so the drift wins: New York’s latest sunrise comes in January. On the equator itself the day is almost exactly twelve hours long all year, and the extremes follow the equation of time alone: Singapore’s earliest sunset is on 5 November (18:50) and its latest sunrise on 9 February (07:16), just after the equation of time reaches its extremes on 3 November and 11 February.

Summer has the same pattern, mirrored and weaker, because the equation of time changes more slowly in June. In London the earliest sunrise is on 17 June (04:42) and the latest sunset on 25 June (21:21); in New York they are 14 and 27 June.

helpPractical answer

So when do the evenings start getting lighter?

In London the evenings start drawing out about 12 December, more than a week before the solstice, and by mid-January sunset is already half an hour later. The mornings keep getting darker until the end of December and only really improve in February. If your mood follows the evening light, you can stop waiting for the solstice: the turn has already come.

The sunrise and sunset calculator shows the same extremes for any place, and every city page lists its own earliest sunset and latest sunrise of the year.

For the coming winter, when the days start getting longer lists the earliest sunset, the latest sunrise and the first sunset after 5 pm for cities across the UK and Ireland, North America, Europe and the southern hemisphere.

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