Why Is December 21 The Shortest Day Of The Year?
The bus that takes my 10-year-old to school picks him up at around 7:23 am, but he usually starts fidgeting loudly a good ten minutes before that, so he and I will go outside to wait. In recent weeks, between grumbles about particular classes or the recess monitor who won’t let him go outside in shorts and a T-shirt in 40-degree (F) weather, he’s been asking “Why is it so dark?” The answer, of course, is “astrophysics,” but with a side order of theology and politics.
This Tuesday, December 21, is the winter solstice in the Northern Hemisphere, meaning it’s the shortest day of the year. It’s not the earliest sunset (that was a couple of weeks ago) or the latest sunrise (that’s in early January), but it’s the day with the fewest hours between sunrise and sunset.
(Again, this only applies in the Northern Hemisphere; down south, it’s the longest day of the year, which accounts for the summer-y elements of my favorite Aussie Christmas song.)The change in the relative length of days and nights is due to a combination of the motion of the Earth about the Sun, and the rotation of the Earth on its axis. Specifically, it happens because the Earth’s axis is tilted by about 23 degrees relative to the axis of its orbit. And because angular momentum is conserved, that axis stays pointing in the same direction through the whole orbit, in the same way that a gyroscope on a gimbal mount will remain pointed in the same direction in space as it’s moved around.
Whichever hemisphere is getting the long days gets warmer temperatures, simply because the sun is spending more time each day heating that side of the planet. (The seasonal temperature difference has nothing to do with the distance to the Sun— the moment when the Earth is closest to the Sun actually falls in early January, in the Northern Hemisphere winter.) This is also why the hottest and coldest days lag behind the solstices— falling in July/August and January/February for the Northern Hemisphere. The Earth is very big, it takes a long time for one half to heat up or cool down. The separation between the dates of latest sunrise and earliest sunset is also a result of this interaction between orbital motion and axial tilt.
In addition to understanding why the days get shorter in winter, we can also say with a high degree of certainty when this will happen. The solstices fall on very consistent dates in the Gregorian calendar used as a standard in most countries and for international trade— both December and June solstices fall within a day or so of the 21st of their respective months. This is not an accident— fixing the dates of the important astronomical events was one of the key goals of the Gregorian calendar reform back in the 1580’s.
The key issue here is that the span from one winter solstice to the next, the “tropical year” is not an integer number of days. It’s 365.24217 mean solar days, and the calendar, as a model of time, has to account for that. The Gregorian calendar is a remarkably good approximation of this, with an average year length of 365.2425 days, a difference of less than 30 seconds.
How were they able to achieve such accuracy in 1582? Thanks to the sheer depth of the history of timekeeping. The oldest calendar with a fixed number of days in a year is probably the Egyptian civil calendar, which had 12 months of 30 days each, plus a five-day holiday period, and was (most likely) introduced around 2782 BCE, and keyed to the appearance of the star Sirius in the sky just before sunrise. This nearly coincided with the annual flooding of the Nile, the most important event of the agricultural year. Because that 365-day calendar is a little less than a quarter of a day short, though, it slowly slipped out of synch with the season. The drift was very slow, but the Egyptian civilization lasted an incredibly long time, so they were able to track the rising date of Sirius through nearly two full repetitions of the “Sothic cycle.”
The direct ancestor of the Gregorian calendar reform was the Julian calendar, introduced by Julius Caesar in 46 BCE, and very likely influenced by knowledge of the Egyptian calendar. The Julian calendar also featured a fixed number of months with set lengths totaling 365 days, but fixed the Sothic cycle drift by adding an extra day every fourth year. The addition of leap years brings the total length to 365.25 years, which comes within 11 minutes of the modern value for the tropical year. The Julian calendar was the civil calendar for the entire Imperial period of Rome, and on through medieval Europe.
That 11-minute gap is small, but again, the sheer length of time involved allows it to pile up. By the mid-1500’s, this had led to a shift in the typical date of the equinoxes and solstices by about 10 days— the March equinox was falling on the 10th or 11th instead of the 21st. This might not seem like a big deal to a modern secular individual, but in the highly religious environment of Europe in Pope Gregory’s day, it was incredibly significant. Religious rules call for Easter to be celebrated at a particular full moon in Spring, and the drift of the calendar was making it harder to calculate the correct date. Celebrating Easter at the wrong full moon in Spring might be disastrous, in a theological sense. As a result, the Pope formed a committee to study the problem, and provide a reformed calendar that would fix the dates in a convenient manner.
The final form of the Gregorian calendar was largely the work of a Calabrian doctor whose name was Latinized to Aloisius Lilius, and is elegantly simple: a simple tweak to the leap year rule to drop three days from every 400 years. In the Gregorian system, February gains a day every fourth year, except for century years that are not divisible by 400. So, 1900 was not a leap year, but 2000 was. This gets a much better match to the actual tropical year— it should take a bit more than 3000 years for the average date of the equinoxes or solstices to drift by a single day.
So, that’s the science of why the 21st of December is the shortest day of the Northern Hemisphere year, and also why it’s so readily predictable. Even before the invention of the telescope, careful tracking of the seasons over a period of some 4,000 years enabled scientists to make a calendar good to better than 30 seconds in a year.
Post a Comment