In the Dark

So it comes around again. The winter solstice in the Northern hemisphere happens today, Tuesday 21st December, at 15.59 Irish Time. Among other things, this means that tomorrow is the shortest day of the year in the Northern hemisphere. Days will get steadily longer from then until the Summer Solstice next June.  The  the shortest day – defined by the interval between sunrise and sunset – is today. Tomorrow will be two whole seconds longer. Make the most of it!

This does not, however,  mean that sunrise will happen earlier tomorrow than it did this morning.  Actually, sunrise will carry on getting later until the new year, the length of the day nevertheless increasing because sunset occurs even later. Sunrise yesterday morning (20th December) was at 08.37 Dublin Time while today it was 08.38; the latest sunrise will be on 29th December (08.40). Sunset last night was at 16.07 and tonight it will be at 16.08. The earliest sunset this year was actually on 13th December (16:06).

These complications arise because there is a difference between mean solar time (measured by clocks) and apparent solar time (defined by the position of the Sun in the sky), so that a solar day does not always last exactly 24 hours. A description of apparent and mean time was given by Nevil Maskelyne in the Nautical Almanac for 1767:

Apparent Time is that deduced immediately from the Sun, whether from the Observation of his passing the Meridian, or from his observed Rising or Setting. This Time is different from that shewn by Clocks and Watches well regulated at Land, which is called equated or mean Time.

The discrepancy between mean time and apparent time arises because of the Earth’s axial tilt and the fact that it travels around the Sun in an elliptical orbit in which its orbital speed varies with time of year (being faster at perihelion than at aphelion).

In fact if you plot the position of the Sun in the sky at a fixed time each day from a fixed location on the Earth you get a thing called an analemma, which is a sort of figure-of-eight shape whose shape depends on the observer’s latitude. Here’s a photographic version taken in Edmonton, with photographs of the Sun’s position taken from the same position at the same time on different days over the course of a year:

The winter solstice is the lowermost point on this curve and the summer solstice is at the top. These two turning points define the time of the solstices much more precisely that the “shortest day” or  “longest night”.

Anyway, the north–south component of the analemma is the Sun’s declination, and the east–west component arises from the  equation of time which quantifies the difference between mean solar time and apparent solar time. This curve can be used to calculate the earliest and/or latest sunrise and/or sunset.

Using a more rapid calculational tool (Google), I found a table of the local mean times of sunrise and sunset for Dublin around the 2020  winter solstice. This shows that today is indeed the shortest day (with a time between sunrise and sunset of 7 hours 29 minutes and 57 seconds).

P.S. As usual, crowds gathered at the spectacular neolithic monument at Newgrange in County Meath to observe the sunrise at the Solstice. As usual it was cloudy.

This entry was posted on December 21, 2021 at 11:43 am and is filed under The Universe and Stuff with tags Longest Night, Newgrange, Shortest Day, Winter Solstice. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.