Weeknotes 215

Some beautiful weather this week: clear, sunny, crisp, blue.

It’s no longer dark when I wake up!

I bought a bunch of daffodils.

Project Get Out More achieved medium success: I met Denis for coffee, and I also went to LRUG. I’d intended to be more sociable than that, but I walked around too much and had to dial it back later in the week to let the pain subside.

Going to LRUG made me realise it’s been a year since my last talk there. Rewatching the video of it was a nice reminder of the fun I had with that project, and it made me glad I pulled those endless hours of nonsense together into something brief and presentable.

My foot continues to slowly heal and was barely hurting at all before I overdid it. I need to remind myself to be patient and resist the temptation to, erm, walk before I can walk.

I’m in dire need of exercise and I’d like to return to the gym. It just feels a bit too risky still.

Tuesday was Pancake Day, so I made vegan pancakes with a batter held together by apple sauce and the discard from my sourdough starter.

The recipe was really impressive. They were easily the lightest, fluffiest vegan pancakes I’ve ever had, and that’s important knowledge because I love pancakes. I’ll definitely be making them again, which I suppose is sort of like going to the gym, except the opposite.

Pancake Day was immediately followed by Valentine’s Day. That felt a bit sudden and I tried to recall how much worse it is when they fall on the same day. But I couldn’t remember that happening, and in fact couldn’t easily convince myself it had ever happened.

Has it? Will it? Can it? Maths.

Pancake Day — Shrove Tuesday — is always a Tuesday, so to have any chance of it coinciding with Valentine’s Day, we need 14 February to be a Tuesday too. That generally happens every six years unless an extra leap year shifts it a year earlier or skips it entirely. In my lifetime there have been a handful of such years: 1984, 1989 (’84 & ’88 were leap years), 1995, 2006 (the leap year in ’00 skipped ’01), 2012, 2017 (’12 & ’16 were leap years) and 2023.

That’s only seven candidate years among, well, many more than seven. So, before we even consider the possible dates of Shrove Tuesday, we’re already pretty constrained by the interference pattern produced by the coprime periods of the seven days of the week and the quadrennial rhythm of leap years.

How does the future look? The leap year in 2028 skips the Tuesday in 2029, so the next possibility is 2034, then 2040, then 2045 (’40 & ’44 will be leap years). That gives us ten candidate years overall, which is a nice round number to think about and doesn’t force me to confront my mortality too brutally.

Okay. Given those comparatively rare years when Valentine’s Day falls on a Tuesday, when will Shrove Tuesday fall on the exact same Tuesday?

Shrove Tuesday is the day before the beginning of Lent. Lent lasts for 40 days — as any fan of both history and facts knows, this is how long Jesus spent fasting and resisting Satan’s temptations while wandering in the desert — and finishes the day before Easter Sunday.

In the Anglican tradition these 40 days mysteriously don’t include Sundays, so Shrove Tuesday falls 47 days (Shrove Tuesday itself + 40 non-Sunday days + the 6 intervening Sundays) before Easter Sunday. For Shrove Tuesday to land on 14 February we need Easter Sunday to occur 47 days after that, which is 2 April in a normal year or 1 April in a leap year (because Lent includes 29 February).

So the question becomes: when does Easter Sunday fall on 1 or 2 April?

Well… it’s a bit complicated. Easter Sunday falls by definition on the first Sunday after the first ecclesiastical full moon that happens on or after 21 March, the ecclesiastical equinox.

The dates for the astronomical full moon are pretty wonky because the period of the moon’s orbit relative to the direction of the sun is just over 29½ days, and a year is just over 365¼ days, so the dates shift earlier by an average of 365¼ - (29½ × 12) = 11¼ days each year, again with aliasing errors which get smoothed out by the leap years.

This heavenly shitshow is mitigated by the computus, an algorithm for arranging a pattern of lunar months in a repeating 19-year cycle and thereby defining a completely predictable (albeit inaccurate) date for the first full moon after the spring equinox.

Here are the full moon dates given by the computus for each of our candidate years:

1984: Monday 16 April
1989: Wednesday 22 March
1995: Friday 14 April
2006: Thursday 13 April
2012: Saturday 7 April
2017: Tuesday 11 April
2023: Wednesday 5 April
2034: Monday 3rd April
2040: Wednesday 28 March
2045: Sunday 2 April

There’s no real pattern here, largely because we’re already looking through the completely unrelated filter of years where 14 February happens to be a Tuesday, but we can see that the full moon date bounces around a bit: it most often appears in early April and only occasionally dips down into the final days of March. This is pretty much too late for our purposes since we want the Sunday after the full moon to fall on the first day or two of April.

So when do those Easter Sundays fall?

1984: Sunday 22 April
1989: Sunday 26 March
1995: Sunday 16 April
2006: Sunday 16 April
2012: Sunday 8 April
2017: Sunday 16 April
2023: Sunday 9 April
2034: Sunday 9 April
2040: Sunday 1 April
2045: Sunday 9 April

In almost all of these cases Easter comes far too late for Shrove Tuesday to coincide with Valentine’s Day, with a couple of notable exceptions. In 1989 it was actually a week too early: Easter Sunday on 26 March puts Shrove Tuesday on 7 February.

But in 2040… success! Easter falls on 1 April — it’s a leap year — so Shrove Tuesday will actually be on 14 February. You’ve got sixteen years to prepare.

If this once-in-a-lifetime conjunction isn’t already called Love Tuesday then it should be. ❤️🥞

I wasn’t feeling particularly confident about any of this reasoning so I wrote a Ruby program to confirm it:

require 'date'
FORMAT = '%a %d %b'

(1900..2100).each do |year|
  valentines_day, ecclesiastical_equinox =
    Date.new(year, 2, 14), Date.new(year, 3, 21)

  next unless valentines_day.tuesday?

  golden_number = (year % 19) + 1
  solar_correction = (year - 1600) / 100 - (year - 1600) / 400
  lunar_correction = (((year / 100) - 14) * 8) / 25
  correction = solar_correction - lunar_correction
  paschal_day_number = (3 - (11 * golden_number) + correction) % 30
  paschal_day_number -= 1 if
    paschal_day_number == 29 ||
    (paschal_day_number == 28 && golden_number > 11)

  paschal_day = ecclesiastical_equinox + paschal_day_number
  easter_sunday = paschal_day + (7 - paschal_day.wday)
  shrove_tuesday = easter_sunday - 47

  print "#{year}:"
  print "\t🌝 #{paschal_day.strftime(FORMAT)}"
  print "\t🐰 #{easter_sunday.strftime(FORMAT)}"
  print "\t🥞 #{shrove_tuesday.strftime(FORMAT)}"
  print ' ❤️' if shrove_tuesday == valentines_day
  puts
end

And, yeah, checks out:

It transpires this did happen twice during the twentieth century, in 1956 and 1961 — on two consecutive Valentine’s Tuesdays! — and after 2040 it’ll happen again in 2051, a year which I am unlikely to see if I don’t stop eating pancakes and get some bloody exercise.

I thought about getting tickets to see Dune: Part Two at the Everyman when it comes out in a couple of weeks.

Their site says:

Please be aware that all screenings advertised for 29th February at 00:01 are midnight screenings starting right after midnight on 1st March.

This is the most confusing solution possible. It’s like someone realised they needed to do something weird with a midnight screening time to avoid ambiguity at the boundary between two dates, but didn’t know what weird thing exactly, so they just winged it.

Anyway, it’s like £45 for two tickets, so forget it.

I really dislike Connections’ new animation speed. Until this week: sedate, dignified. Now: unhinged, desperate.