The Summer Solstice 2019

The Summer Solstice in the Northern hemisphere happens today, Friday 21st June 2019, at 16.54 Irish Time (15.54 UTC). Among other things, this means that today is the longest day of the year. Days will get shorter from now until the Winter Solstice in December. Saturday June 22nd will be two seconds shorter than today!

This does not mean that sunset will necessarily happen earlier tomorrow than it does today however.  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 2019 summer solstice. This shows that today is indeed the longest day (with a time between sunrise and sunset of 17 hours and 10 seconds), but sunset on 22nd June is actually a bit later than this evening, while sunrise is a bit later.

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 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.

 

 

7 Responses to “The Summer Solstice 2019”

  1. Analemma–new word for me and interesting pic.

  2. Dark Energy Says:

    So in equator two lobes will be exactly same? And in poles
    there will be only one lobe?

    • telescoper Says:

      The analemma is horizontal at the equator. At the poles you only see part of it.

      • Dark Energy Says:

        I thought I understood. In that case, how the earth’s orbit
        and spin axis alignment can be changed so the lobes
        become same in size or the figure of eight collapses to
        a straight-line?

      • telescoper Says:

        You’ll never get funding for such an experiment.

    • Dark Energy Says:

      May not be complicated – may have appeared in some (obscure)
      rigid-body dynamics journals and/or spin-orbit coupling calculations for electrons in theory of atomic spectroscopy.
      Many readers of this blog probably know how to derive the
      shape of the curve. Just curios.

      • Dark Energy Says:

        Need to submit a funding proposal for statistical study of this
        “anal-emma” for exo-planets and its impact on habitability. Who knows.

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