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Today is the astronomical autumn equinox - one of two dates in the year when the Sun's declination is 0 degrees and it crosses the celestial equator.
It's often said that these are the days where night and day are equal - but that's not true.
In fact, depending on latitude, the dates of equal day and equal night aren't at the astronomical equinox.
This is because refraction by the atmosphere allows us to see the Sun slightly earlier at dawn and later at dusk when it is still actually below the visible horizon - lengthening the day beyond what it would be if there was no atmosphere at all.
At Stonehenge, the nearest actual September "equal night" of 12 hours (minus one minute) this year occurs on 25th/26th rather than 22nd.
Interestingly there are precise alignments at Stonehenge for the "equilux" dates, but not for the equinoxes - which is obvious when you realise the alignments are based on observational astronomy rather than our specific coordinate system.
So tomorrow morning when folk will gather at Stonehenge to celebrate the nearest dawn to the astronomical equinox, they'll be celebrating a concept of balanced day and night rather than the actuality of it.
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