Summer Solstice 2022

The Summer Solstice in the Northern hemisphere takes place today, Tuesday 21st June 2022, at 10.14am Irish Time (9.14 UTC). Among other things, this means that tomorrow is the longest day of the year around these parts. According to this website, the interval between sunrise and sunset in Dublin today will be 17 hours 5 minutes and 6 seconds. which is 2 seconds longer than yesterday while tomorrow will be four whole seconds shorter than that.

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. Nor does it mean that sunrise will happen later tomorrow; in fact it is a little earlier.

You can find such things out by looking at a table of the local mean times of sunrise and sunset for Dublin around the 2022 summer solstice. This shows that the earliest sunrise was actually on 17th June and the latest sunset is on 25th.

This arises 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).

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.

One Response to “Summer Solstice 2022”

  1. nights drawing in

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