The Winter Solstice 2019

The winter solstice in the Northern hemisphere happens tomorrow, Sunday 22nd December 2019, at 04.19 Irish Time (04.19 UTC). Among other things, this means that tomorrow is the shortest day of the year in the Northern hemisphere. Days will get steadily longer from now until the Summer Solstice next June.  In fact, the interval between sunrise and sunset tomorrow will be a whole second longer tomorrow than it is today. Yippee!

This does not mean that sunrise will happen earlier tomorrow than it did this morning, however. Actually, sunrise will carry on getting later until the new year. 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).

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 shape 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:

The winter solstice is the lowermost point on this curve and the summer solstice is at the top. 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.

Using a more rapid calculational tool (Google), I found a table of the local mean times of sunrise and sunset for Dublin around the 2019 winter solstice. This shows that tomorrow is indeed the shortest day (with a time between sunrise and sunset of 7 hours 29 minutes and 58 seconds).  The table also shows that sunset already started occurring later in the day before the winter solstice, and sunrise will continue to happen later for a few days after the solstice, notwithstanding the fact that the interval between sunrise and sunset gets longer from today onwards.

I hope this clarifies the situation.

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Arrival of Storm Atiyah

I was a bit alarmed when I saw this weather forecast map on Friday. At first I thought it meant that Ireland was about to be swallowed by a black hole but when I realised it was Storm Atiyah I had the lesser but still significant concern that my flight home from Cardiff would be disrupted.

As it happened the flight was on time, though the blustery winds at Dublin Airport ahead of the storm made for a more than slightly bumpy landing.

I was in one of these:

It’s a Bombardier Dash-8 Q400 operated by FlyBe. I had a window seat on the right hand side so had a good view as we bobbled around on the way in to land. The wings being above the level of the cabin and my seat being next to the starboard engine I could see the right undercarriage come down as we approached. We weren’t quite level when we reached the runway though and I felt and heard the left set of wheels touch down while the ones on the right I could see were still in the air. I could also see weren’t moving exactly parallel to the runway but slightly crosswise. We travelled for quite a few seconds on one set of wheels before we had both feet on the ground, so to speak. During that time I thought we might go off the side of the runway. When the right set of wheels did touch down, however, causing a big splash of water, only a slight correction was needed to point us in the right direction and all was well.

Pilots are if course trained to cope with windy conditions and I’m sure everything was always under control but I bet pilots do have to concentrate hard on such occasions.

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Happy Birthday, Quaternions!

Sir William Rowan Hamilton (1805-1865)

Today, October 16th, is Hamilton day! It was on this day 176 years ago, in 1843, that  William Rowan Hamilton first wrote down the fundamental result of quaternions. Apparently he was walking from his residence at Dunsink Observatory into Dublin when he had a sudden flash of inspiration  and wrote the result down on the spot, now marked by a plaque:

 

Picture Credit: Brian Dolan

This episode  is commemorated by an annual Hamilton Walk. Sadly,  Broombridge (Droichead Broome) is near the bridge (Broom Bridge) where Hamilton had his Eureka moment and it is on the main commuter line from Maynooth into Dublin. This is ironic because Quaternion algebra does not commute. (Geddit?)

Although it is quite easy to reach Broombridge from Maynooth, I sadly can’t attend the walk this year because I’m teaching this afternoon.

P.S. Maynooth is also home to the Hamilton Institute which promotes and facilitates research links between mathematics and other fields.

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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:

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 End of the Common Travel Area?

I’m back in Cardiff for a couple of days after flying from Dublin this morning.

When my flight arrived at Cardiff Airport there was yet again a full passport and immigration check on all passengers.

There is supposed to be a Common Travel Area including the UK and Ireland (as well as the Channel Islands and the Isle of Man), and passport checks are not supposed to be made routinely at borders within the CTA.

I have noticed passport checks happening at Cardiff Airport before (e.g. here and here) but I’ve previously imagined there was some specific reason for them. Passport checks have, however, been carried out every time I have arrived in Cardiff recently and it is now abundantly clear that there has been a material change of policy.

When I got to the desk and handed over my passport I asked the Officer whether these checks were being imposed all the time now. She said yes: there are now full passport and immigration checks on all flights to Cardiff from Dublin.

This is from the UK Government’s website:

Well, if they check all passengers on all flights then that sounds like ‘routine’ to me. In other words the British authorities are violating the Common Travel Area agreement just weeks after undertaking to uphold it.

Did someone say ‘Perfidious Albion’?

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A Story of St Stephen’s Green

I had a bit of spare time before the Opera on Friday and the weather was fine so I decided to go for a walk around the park in St Stephen’s Green.

I have walked past St Stephen’s Green many times but have never previously been inside, or if I have it was so long ago that I’ve now forgotten.

On my way around I noticed that there are posters here and there marking the events of the 1916 Easter Rising. Here’s an example:

The artwork is a bit ‘Boy’s Comic’ style but the descriptions are fascinating especially because the Park and the area around it are pretty much unchanged in more than a hundred years since the momentous events of 1916 so it’s not difficult to imagine the scene as it was then. There are still bullet holes in the Fusiliers Arch at the North West Corner of the Green, as there are in a number of other locations around Dublin.

St Stephen’s Green is inside the area marked ‘Citizens Army‘. One look at the map will tell you why this was considered an important location to control as it is at the junction of several main roads. On the other hand if you actually visit the location you will see a big problem, namely that the Green itself is surrounded on all sides by very tall buildings, including the swanky Shelbourne Hotel to the North.

When a contingent of about 120 members of the Citizens Army arrived in St Stephen’s Green on Easter Monday, 24th April 1916, they immediately began erecting barricades outside, and digging trenches inside, the Park. They did not, however, have the numbers needed to seize and hold the buildings around it except for the Royal College of Surgeons building to the West.

The following morning, Tuesday 25th April, the British moved two machine guns into position, one in the Shelbourne Hotel and the other in the United Services club, along with numerous snipers. From these vantage points British soldiers could shoot down into the Park making it impossible for the rebels to move around.

The position inside the Green being untenable the Rebels effected an orderly (but perilous) withdrawal to the Royal College of Surgeons which they had fortified for the purpose. And that’s where they stayed until the end of the Rising.

The British realised that there was no need to assault the RCS building, as the force inside was contained and offered no real threat. From the roof of the building the Rebels watched helplessly as the British systematically reduced the resistance around the Rebel Headquarters in the GPO Building to the North, using artillery fired from College Green. Smoke rose into the sky as one entire side of O’Connell Street went up in flames and the perimeter slowly tightened around the GPO.

In the early hours of the morning of Thursday 27th April the occupants of the RCS were alarmed to see that British soldiers had installed another machine gun on the roof of the University Church on the South side of St Stephen’s Green along with snipers in adjacent buildings.

This was a very dangerous development that required a rapid response. A plan was devised that involved sending a squad of about 30 to break into buildings on the South side of the Green and start fires therein that would force the British to withdraw. This action is the subject of the poster shown above.

This is a photograph of the remarkable Margaret Skinnider, who is shown in the graphic leading the attempted assault. Before the Rising she was a school teacher. During the hostilities she initially acted as a scout and a runner, carrying messages to and from the GPO, but when given the chance she proved herself a crack shot with a rifle and showed conspicuous courage during the heavy fighting in and around St Stephen’s Green.

Skinnider insisted on leading the assault on Thursday 27th April despite the obviously high risk as it would involve running about 30 yards in full view of the machine gun position.

Under the cover of darkness, the detachment made its way out of the RCS to the South West corner of the Green safely enough but when men began breaking windows in Harcourt Street in order to gain entry to buildings there, the sound alerted the British soldiers who realised what was happening and opened fire as the Rebels made their move. Margaret Skinnider was leading from the front and she was inevitably among the casualties: she was hit three times by rifle bullets and very badly wounded.

Such was the volume of fire that the assault was abandoned and the surviving members of the squad retreated to the RCS where they remained until the general surrender on Saturday (29th April).

Skinnider was taken to hospital after the Rising ended but escaped and made her way to Scotland. She later returned to Ireland to take part in the War of Independence and subsequent Civil War. When the latter ended, in 1923, she went back to her job as a teacher in a primary school. She passed away in 1971 at the age of 79.

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Irish Quantum Foundations and Other Matters

So here I am then, at Irish Quantum Foundations (IQF) 2019 which is being held in the Hamilton Building (shown above), and hosted by the School of Mathematics of the College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin, which is sometimes known as Trinity College, Dublin for short.

I got here a bit later than I originally planned as some last-minute things came up this morning to do with next week’s events. I’ll have to skip tomorrow morning too, for similar reasons. When I did get going this morning I had to stand all the way from Maynooth to Connolly because the train was packed. At least it was reasonably on time though.

Anyway, the schedule of IQF 2019 is rather varied and I’m looking forward to the parts of it that I can attend.

Among the things I have been dealing with to do with next week are  submitting the final version of pedagogical piece about the Eclipse Expeditions of 2019 which should be published very soon in Contemporary Physics (at least in the online version) and writing a short piece for RTÉ Brainstorm (which will appear on Monday 27th May), and sorting out an appearance on Newstalk Radio next week. How I’ll get time to finish my exam marking in the middle of all this I don’t know!

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