In the Dark

The Summer Solstice in the Northern hemisphere takes place later today, Wednesday 21st June 2022, at 15.58 Irish Time (14.58 UTC) or 16.58 local time here in Copenhagen.

Among other things, this means that today is the longest day of the year around these parts. Incidentally, the latitude of Copenhagen is 55.6761° N, which is a little bit South of Edinburgh. I had thought it was further North, but I was wrong.

According to this website, the interval between sunrise and sunset in Copenhagen today will be 17 hours 32  minutes and 18 seconds. which is 5 seconds longer than yesterday while tomorrow will be two whole seconds shorter than that.

It’s all downhill from now on.

In the Northern hemisphere, 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.

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).

Anyway, here’s a  picture of four sixty-somethings – myself, John Peacock, Per Lilje and Ofer Lahav – on our way to dinner last night (including a toast to the memory of Nick Kaiser)

Picture courtesy of Ofer Lahav

The building in the background is Københavns Domhus (the Copenhagen Court House). The restaurant we went to, Puk, is highly recommended.

This entry was posted on June 21, 2023 at 7:37 am and is filed under Uncategorized. 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.