Archive for Summer Solstice

The Pagan University Year

Posted in Education, Maynooth with tags , , on June 22, 2022 by telescoper

This morning we had the usual end-of-year meeting of the University Examination Board. It’s been a difficult year so it was a longer meeting than usual but it went reasonably smoothly. Marks will be released to students either tomorrow or Friday. That basically concludes the formal business for the academic year.

The proximity of this important event to yesterday’s Summer Solstice got me thinking again about the academic year and how it relates to the old pagan calendar.

In the Northern hemisphere, from an astronomical point of view, the solar year is defined by the two solstices (summer, around June 21st, and winter, around December 21st) and the equinoxes (spring, around March 21st, and Autumn, around September 21st). These four events divide the year into four roughly equal parts each of about 13 weeks.

Now, if you divide each of these intervals in two you divide the year into eight pieces of six and a bit weeks each. The dates midway between the astronomical events mentioned above are (roughly) :

  • 1st February: Imbolc (Candlemas)
  • 1st May: Beltane (Mayday)
  • 1st August: Lughnasadh (Lammas)
  • 1st November: Samhain (All Saints Day)

The names I’ve added are taken from the Celtic/neo-Pagan (and Christian terms) for these cross-quarter days. These timings are rough because the dates of the equinoxes and solstices vary from year to year. Imbolc is often taken to be the 2nd of February (Groundhog Day) and Samhain is sometimes taken to be October 31st, Halloween. But hopefully you get the point.

Incidentally, the last three of these also coincide closely with traditional Bank Holidays in Ireland, though these are always on Mondays so often happen a few days away. The first has not been a holiday but from next year there will be a new Bank Holiday that occurs on or near 1st February, which completes the set of cross-quarter-day holidays.

Anyway, it is interesting (to me) to note the extent that the academic year here in Ireland is defined by these dates.

Usually the first semester of the academic year starts on or around September 21st (Autumnal Equinox) and finishes on or Around December 21st (Winter Solstice). Half term (study week) thus includes the Halloween Bank Holiday (Samhain).

After a break for Christmas and a three-week mid-year exam period Semester Two starts on or around 1st February (Imbolc). Half-term is then around March 21st (Vernal Equinox, which roughly coincides with St Patrick’s Day March 17th) and teaching ends around May 1st (Imbolc). More exams and end of year business take us to the Summer Solstice and the (hypothetical) vacation. Most of us get to take the 1st August holiday (Lughnasadh) off at least!

So we’re basically operating on a pagan calendar.

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The Summer Solstice 2021

Posted in The Universe and Stuff with tags , , , , , , on June 20, 2021 by telescoper

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:

maxresdefault

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.

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The Summer Solstice 2020

Posted in The Universe and Stuff with tags , , , , , , on June 20, 2020 by telescoper

The Summer Solstice in the Northern hemisphere happens this evening, Saturday 20th June 2020, at 22:43 Irish Time (21.43 UTC). Among other things, this means that today is the longest day of the year. This is an earlier day in June than you might expect, primarily because 2020 is a leap year.

Days will get shorter from today until the Winter Solstice in December, although this does not mean that sunset will necessarily happen earlier tomorrow than it does today. 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 2020 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 21st June is actually a bit later than this evening, but sunrise is also bit later so the day is indeed (slightly) shorter.

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:

maxresdefault

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.

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The Summer Solstice 2019

Posted in Uncategorized with tags , , , , , , on June 21, 2019 by telescoper

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:

maxresdefault

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.

 

 

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