15 December 2019

The Long Solstice Season Begins

Image courtesy calaware.org 
This time of year, as darkness sets in early and the cold of winter envelops the northern hemisphere, I savor the changes that make each day different from the previous one. The Winter Solstice takes place this year on December 22nd and represents the shortest day at all locations in the northern hemisphere. Literally, Solstice means that "the Sun stands still" and in the context of the Sun's annual meandering (from our point of view) higher or lower above the horizon at solar noon, the Winter Solstice is when the Sun ceases to drift southward as measured at mid-day and begins the six month journey north. In my view, however, the Solstice is not just that one day and moment in time but rather a season, approximately one month in duration, during which the changes to the time of sunrise and sunset as well as the total length of the day go through rather fascinating changes.

As the Earth orbits the Sun every 365 1/4 days, its orbital speed is higher in December and January. The orbit of the Earth is elliptical and that ellipse brings the Earth slightly closer to the Sun in December and January and the speed of our orbit around the Sun goes up just enough that it has a measurable influence on the length of the day. The Earth always takes the same amount of time to go around once on its axis (one day), but because we are moving more rapidly around the Sun as we are at the inside of our elliptical orbit, the time period from one local noon (exactly when the Sun is at its highest point of the day) to the next varies by a few minutes, plus or minus, as we travel through this faster period of time. The consequence of this changing speed of the Earth means that the earliest sunset of the year happens one to two weeks before the Winter Solstice, and the latest sunrise one to two weeks after. The total effect of the early sunset and late sunrise meet 'in the middle' and create the shortest day on December 22nd, so the Solstice is very much still the shortest day. But the two other effects are quite noticeable.

Depending on your latitude north of the equator, the exact date of the earliest sunset and latest sunrise vary a bit. Using the tables on the timeanddate.com website, I entered my location here in Munich Germany and can see that the earliest sunset (16:19 or 16:20) is happening from December 5th through the 18th. Skipping ahead to January, I can also see that the latest sunrise (8:04) happens from January 1st through the 3rd. The exact timing depends not only on your latitude north of the equator, but also your location in your timezone, and varies if you are more to the east or west of the center of your timezone. Plug in your city and see how things look, and you can start to get a sense of your own Solstice Season.

Due to the nuances of the Earth's elliptical orbit around the Sun, the same effect happens in June but is much less pronounced around the Summer Solstice. So this is the best time of year to be attentive to the subtle changes that are happening in our sky and on our Earth, and by the time is it early January you can already start to celebrate the very gradual return of light to the daily rhythms of the northern hemisphere.

If you want to learn more, here are two good articles on the subject, one from Bruce McClure at EarthSky.org, and another by Aparna Kher on TimeandDate.com.

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