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Why aren’t the earliest sunsets on the shortest day?

The year’s shortest day comes at the winter solstice. But, for the Northern Hemisphere, the earliest sunsets are in early December.

The winter solstice is the shortest day. It offers the shortest period of daylight. But the earliest sunsets aren’t on the solstice itself. Instead, they come a couple of weeks before the winter solstice. If you reside in the temperate regions of the Northern Hemisphere, your earliest sunsets are in early to mid-December.

Southern Hemisphere? Click here.

Why isn’t the earliest sunset on the year’s shortest day? You have to think about it in terms of solar noon or midday, the time midway between sunrise and sunset, when the sun reaches its highest point for the day.

A clock ticks off exactly 24 hours from one noon to the next. But the actual days – as measured by the spin of the Earth – are rarely exactly 24 hours long.

So the exact time of solar noon, as measured by Earth’s spin, shifts in a seasonal way. If you measured Earth’s spin from one solar noon to the next, you’d find that – around the time of the December solstice – the time period between consecutive solar noons is actually half a minute longer than 24 hours.

So – two weeks before the solstice, for example – the sun reaches its noontime position at 11:52 a.m. local standard time. Two weeks later – on the winter solstice – the sun reaches its noontime position at 11:59 a.m. That’s 7 minutes later.

The later clock time for solar noon also means a later clock time for sunrise and sunset. The result: earlier sunsets before the winter solstice and increasingly later sunrises for a few weeks after the winter solstice.

The exact date of earliest sunset varies with latitude. But the sequence is always the same. For the Northern Hemisphere, earliest sunset in early December, winter solstice, latest sunrise in early January.

Meanwhile, if you’re in the Southern Hemisphere, take nearly everything we say here and apply it to your winter solstice in June. For the Southern Hemisphere, assuming you’re at a mid-temperate latitude, the earliest sunsets come prior to the winter solstice, which is typically around June 21. The latest sunrises occur in late June.

In early December, the Southern Hemisphere is approaching its summer solstice.  Sunset on that part of Earth will continue coming later until early July.  Photo of sunset with crepuscular rays by Phil Rettke Photography in Ipswich QLD Australia.  Visit Phil Rettke on Facebook.

In early December, the Southern Hemisphere is approaching its summer solstice. Sunset on that part of Earth will continue coming later until early July. Photo of sunset with crepuscular rays by Phil Rettke Photography in Ipswich QLD Australia. Visit Phil Rettke on Facebook.

What’s happening with Southern Hemisphere sunrises and sunsets in December? During the month of December, it’s nearly summer in the Southern Hemisphere; the summer solstice comes this month for that hemisphere. So sunsets and sunrises are shifting in a similar way. For both hemispheres, the sequence in summer is: earliest sunrises before the summer solstice, then the summer solstice itself, then latest sunsets after the summer solstice.

As always, things get tricky if you look closely. Assuming you’re at a mid-temperate latitude, the earliest sunset for the Northern Hemisphere – and earliest sunrise for the Southern Hemisphere – come about two weeks before the December solstice, and the latest sunrise/latest sunset happen about two weeks after.

But at the other end of the year, in June and July, the time period is not equivalent. Again assuming a mid-temperate latitude, the earliest sunrise for the Northern Hemisphere – and earliest sunset for the Southern Hemisphere – comes only about one week before the June solstice, and the latest sunset/latest sunrise happens about one week after.

The time difference is due to the fact that the December solstice occurs when Earth is near its perihelion – or closest point to the sun – around which time we’re moving fastest in orbit. Meanwhile, the June solstice occurs when Earth is near aphelion – our farthest point from the sun – around which time we’re moving at our slowest in orbit.

View larger. Computed position of the sun looking eastward at the same time each morning from the Northern Hemisphere.  December solstice point at lower right and June solstice point at upper left. Solar days are longer than 24 hours long at the solstices, yet less than 24 hours long at the equinoxes.  Roughly midway between a solstice and an equinox, or vice versa, the solar day is exactly 24 hours long.

View larger. Computed position of the sun looking eastward at the same time each morning from the Northern Hemisphere. December solstice point at lower right and June solstice point at upper left. Solar days are longer than 24 hours long at the solstices, yet less than 24 hours long at the equinoxes. Roughly midway between a solstice and an equinox, or vice versa, the solar day is exactly 24 hours long.

In short, the earliest sunset/winter solstice/latest sunrise and earliest sunrise/summer solstice/latest sunset phenomena are due to the fact that true solar days are longer than 24 hours long for several weeks before and after the solstices. At and around the solstices, the Earth must rotate farther on its axis for the sun to return to its daily noontime position, primarily because the sun is appreciably north or south of the Earth’s equator.

However, perihelion accentuates the effect around the December solstice, giving a day length of 24 hours 30 seconds. And aphelion lessens the effect around the June solstice, giving a day length of 24 hours 13 seconds.

Geocentric ephemeris for the sun: 2015

Bottom line: The earliest sunsets and latest sunrises don’t come on the winter solstice, the shortest day of the year.

Here’s more detail about the earliest sunsets.

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