Yesterday (21 March) was my half-birthday. Is that worth celebrating? We didn’t notice this kind of thing when we were young but my own children do. Soon they will be 11 and a half and 13 and a half, and I can now add “and a half” to my own age.

There are many months when the 21st day has significance, although it’s not quite as clear-cut as I thought. We were brought up to believe that the equinoxes (the generally-accepted plural of equinox) were 21 March and 21 September. Those are the days when there are exactly 12 hours of daylight and 12 hours of night-time. it turns out that they’re not the same every year. In 2018 the vernal equinox is 20 March and the autumn equivalent has shifted to 23 September. My pocket diary tells me that 20 March was the first day of spring and 23 September is the first day of what our American friends call fall. At least the solstices are more straightforward, most years. This year the longest day (the summer solstice) is still 21 June and the shortest (the winter solstice) is 21 December.

Looking at this table, with equinox and solstice dates many years ahead, I see that even the solstices shift around a bit. Every four years, from 2012 through to 2044, the longest day falls on 20 June rather than 21 June. From 2011 through to 2043, again every four years, the shortest day is on 22 December.

It also turns out that that my birthday, 21 September, has not been the autumnal equinox since way before 1900. It will next happen in 2092. I might not be around to see that one, but I hope my children will. They would both be in their 80s by then. Discovering that the equinox took place two days after I was born has put paid to a phrase I have used often, and not entirely seriously: “I was born on the autumnal equinox; that’s why I’m so well-balanced.” But it turns out that I wasn’t; and maybe I’m not.