Autumnal equinox.

The equinox, like the solstice (see post dated 21/6/08), is a precise moment in time when the sun is directly over the equator - and this year that's around 9 o'clock tonight. It's often said that the day of the equinox is when day and night are of equal length, and indeed the word comes straight from the Latin - aequus - equal and nox - night. And yet this isn't quite so. It is the day when sunrise and sunset are 12 hours apart, give or take, but what you have to remember is that it gets light about half and hour before sunrise, and doesn't get dark until about half an hour after sunset, so in fact there's about 13 hours of daylight today and only 11 hours of darkness. The day when we get 12 hours of each isn't for about another fortnight. The word twilight, by the way, although we use it pretty well exclusively to mean the time after the sun has set when it's still light(ish), equally applies to the time before sunrise when it's getting light - it actually means "half-light". And that's enough pedantry for today!