The Science of the Solstice

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These shorter periods of sunlight during the winter are something to get used to for this Florida girl; having lived near the equator my whole life, to be in an area where the tilt in the earth's axis is so stark comparatively has been a real adjustment. I know I'm looking forward to sunlight's return, which makes Yule one of my favorite holidays!

Last night at 12:30am marked the peak of the winter solstice,the longest night of the year. It's all uphill from here, and the sun will continue to climb higher in the sky until the summer solstice six months from now. But what makes these times so significant, scientifically-speaking?

The solstice, as the first day of astronomical winter in the Northern Hemisphere, the sun is shining directly overhead at 23.5 degrees south latitude, making it the furthest it will get from the North pole and the sun take its lowest and shortest path across the northern sky. If you've been observing sunrises and sunsets these past six months, you'll notice that they've each been drifting lower and lower towards the skyline, and if you're really paying attention, the sun rose at 120 degrees and set at 240 degrees, both the southernmost points along the horizon from due north.

In the MD/DC area, where Matt and I live, the sun rose yesterday morning at 7:23am and set at 4:49 yesterday evening while reaching its minimum height of 27.7 degrees above the horizon. Today? It'll be less than a minute better on either end (7:24am and 4:50pm, respectively), but at least it's getting better!

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The funny thing, though, is that those were not the earliest sunset or latest sunrise of the year. These were instead between 2 and 13 December (sunset was at 4:46pm) and won't happen until 31 December through 10 January (sunrise will be at 7:27am). Why are these so different from the actual solstice itself?

There are actually two forces that determine this. The first is the change in the sun's declination, or its height above the horizon throughout the year. The second is solar noon, which is when the sun reaches its maximum height in the sky on a given day, slowly oscillates back and forth by several minutes throughout the year, so a sundial wouldn't consistently show noon occurring at the same time every day. These shifts are attributed to the earth's elliptical orbit and axial tilt; these tend to wobble rather than follow a strict circular pattern, and can be easily called the "solar noon effect".

As explained by Washington Post's Justin Grieser:

The reason the earliest sunset occurs before the winter solstice has to do with the later shift in solar noon “outweighing” the effect of the sun’s decreasing height and length of time above the horizon. For example, in D.C. solar noon is at 11:57 a.m. on December 1, but drifts 14 minutes later – to 12:11 p.m. – by December 31. This forward shift means that it takes a few seconds more than 24 hours for the sun to complete a full circle between its maximum noontime height from one day to the next. Meanwhile, as we approach the solstice, the sun’s declination is no longer changing as rapidly, which causes the days to shorten at a slower pace.

In late November, the effect of a later-shifting solar noon begins to counteract the effect that the sun’s lowering declination has on pushing sunset earlier. Eventually, sunset reaches a minimum during the first week of December. While we would expect the earliest sunset to occur closer to the winter solstice, the rapid forward shift in solar noon causes sunset to creep later more than a week before then.

So the reason why the sun keeps rising later after the winter solstice? Like above, solar noon's time still moves later for several more weeks, and only by mid-January does the sun's increasing height over the horizon "accelerate" enough to bring earlier sunrises, even as solar noon continues moving later until early February. Which explains why January morning commutes are still in the darkness while December evening commutes are already growing lighter.

Because of these phenomena, meteorologists and climatologists actually define winter as 1 December through the last day of February, despite "official" winter beginning on the solstice (this year, on 22 December) and lasting until the spring equinox (around 21 March).

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I've always wondered why the days grew colder as winter pressed on, even despite the fact that the sun is making its slow and steady return to the Northern Hemisphere. (This year has been rather strange, as the average temperatures this year have been a bit warmer than of record, but still.) The coldest part of the year actually lags about a month behind the solstice, and the same with the hottest in the summer. Simply speaking, assuming the air stays in a particular place, the air in such a place would act kind of like a bank account. If you add heat, it warms up, and since the Earth is always losing heat, that bank account of air is losing it, too. If the amount of heat arriving from the sun is equal to the amount leaving, the temperature stays the same.

Even as the days grow shorter, the amount of heat arriving (because of the period beforehand when the air grows warmer and warmer) is more than the amount leaving until a point in late summer or the fall, depending on your location north of the equator, when it's pretty much in balance. After that point, the "withdrawals" from the heat account grow greater and are greatest in December. Even after the days are growing longer, there is more heat leaving than arriving, and the offset means the air is still growing cooler. This balance doesn't start to shift in the opposite direction until late winter or even spring, again depending on your location.

Some pretty cool stuff! I hope you all enjoyed your longest night last night, and now you know why it is! Solstice blessings today, a very happy Yule, and here's to chillier weather!