30 days hath September,
April, June and November.
All the rest have 31,
Except for February alone,
Which has 28 days clear,
And 29 in each leap-year.
This is a handy little poem that seems to have been phrased in this way just so that it rhymes nicely. No-one talks like that.
“Hey, how many days are there this month?”
“There are 28 days clear”
|But a cool (yet kind of obvious) fact about today in particular is that not only do we only get a February 29th once every four years, but every February 29th has a guaranteed five of one particular day, and just one. Every other year, all the other months get at least two fives of something (because 30 divides by 7 four times with a remainder of 2), and February does not. But in leap years it does. It’s not left out anymore! Hurrah! Unfortunately this month we’ve had five Mondays, but in 2020 we get five Saturdays, so at least we have that to look forward to.
Something a bit more unusual: I wanted to write something on blue moons, and whether or not February can have them (you’d think not because a lunar cycle is 29.53 days, and February can’t go on for longer than 29 days exactly – not even a little bit), but discovered that my understanding of the term “blue moon” was incorrect! This is pretty cool, and should definitely be on QI sometime (are we up to Series ‘M’ yet?)
A Blue Moon isn’t actually the second full moon in a calendar month, as everyone seems to think it is. It dates back to medieval times and refers to an intercalary month (there’s more info on what that word means and on Blue Moons generally here). A season would normally have three moons, but if a season has four moons, the third one is a blue moon – and these occur once every 2.7145 years.
Winter runs officially runs from about 21st December to 21st March (Winter solstice to Spring Equinox), although it varies by year – I did not know this (it makes sense, what with a solar year not quite equalling a calendar year, but I never really thought about it), and so it is actually possible for February to have a blue moon!
Just to confirm the numbers:
21 Dec – 21 Mar = 90 calendar days in non-leap years, and 91 in leap years.
Winter is approximately 89 days in actual scientific terms (position of Earth in relation to Sun, etc.)
3 lunar cycles = 3 x 29.53 = 88.59 days
So it’s possible, for either a year with 365 or 366 days, but it’s less likely than for other seasons, as Winter loses a few days compared to Spring, Summer and Autumn. It also depends on the moon’s cycle lining up with the 89-ish day window, which happens very rarely.
The last time we had a February blue moon was in 2000, and it also happened in 1981 and 1905, as shown in the values below (all times GMT):
Winter 1904/5 Full moons
1904 Dec 22 18.01
1905 Jan 21 07.14
1905 Feb 19 18.52
1905 Mar 21 04.55
1904 Winter solstice 22 Dec 06.14
1905 Spring equinox 21 Mar 06.58
Winter 1980/1 Full moons
1980 Dec 21 18:09
1981 Jan 20 07:40
1981 Feb 18 22:59
1981 Mar 20 15:23
1980 Winter solstice 21 Dec 16.56
1981 Spring equinox 20 Mar 17.03
Winter 1999/2000 Full moons
1999 Dec 22 17:33
2000 Jan 21 04:42
2000 Feb 19 16:28
2000 Mar 20 04:46
1999 Winter solstice 22 Dec 07.44
2000 Spring equinox 20 Mar 07.35
I got my data from timeanddate.com and http://home.hiwaay.net/~krcool/Astro/moon/fullmoon.htm#09
It won’t happen again this century, so it really is a unique thing. While blue moons in any season occur once every 2.7145 years, we’re looking at a little over one per century for the February blue moon. Interestingly, neither of those in the 20th Century fell in leap years.
Curiously, it’s also possible to have a season with only two full moons; this is also dependent on the moon’s cycles lining up so that a full moon happens just before the Winter solstice and one straight after the Spring equinox. These are also super-rare, and can only happen in Winter (because it’s the only season short enough to permit it). We last had this in the Winter of 1961/2, and there won’t be another until 2314/5. Wow!