The Summer Solstice 2021

The Summer Solstice in the Northern hemisphere happens tomorrow, Monday 21st June 2021, at 04:32  Irish Time (03:32 UTC). Among other things, this means that tomorrow is the longest day of the year around these parts.  Usually I post about the Solstices and Equinoxes close to the time that they occur,  and on the same day, but because I don’t want to get up before 4.32am and few would be awake to read it then, I thought I’d do this one in advance.

Sunrise in the Dublin area is about 04:56 local time tomorrow and sunset is at 21:57: the interval between sunrise and sunset will be just about 1s longer on 21st June than today, 20th June, and 5 seconds longer than Tuesday 22nd June. The longest day will last 17 hours and 11 seconds (approximately) so make the most of it – it’s all downhill from now on!

Days will get shorter from tomorrow until the Winter Solstice in December, although this does not mean that sunset will necessarily happen earlier on 22nd  than it does tomorrow. In fact it is a little later. This is 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).

Using a rapid calculational tool (Google), I found a table of the local mean times of sunrise and sunset for Dublin around the 2021 summer solstice. This shows that the earliest sunrise was actually on 17th June and the latest sunset is on 24th.

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 curve 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:

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The summer solstice is the uppermost point on this curve and the winter solstice is at the bottom. The north–south component of the analemma is the Sun’s declination, and the east–west component is the so-called 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.

P.S. A bit of light googling revealed to me that if you live in York then your sunrise on 21st June 2021 is at 4.32am, precisely the same time as the Solstice.

2 Responses to “The Summer Solstice 2021”

  1. Thanks for posting this. In the first-year astronomy course I studied about 100 years ago (subjective time), we covered – in pretty great detail – RA, Dec, equation of time, mean time etc, as well as a lot of spherical geometry. Is this still taught anymore? Most of the astrophysics modules I am aware of focus now on the physics.

    • telescoper Says:

      I didn’t learn this at University. I was a junior member of the BAA way back when I was a teenager and learnt it then. The MSc course at Sussex had a course on “Introduction to Astronomy” that dealt with this stuff and I had to do that at the start of my DPhil. If I remember correctly I only passed on the second attempt.

      Nowadays I think most courses skip it because even amateur telescopes have built-in computers that mean you don’t really have to think about it much.

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