Equinox Conundrum

[Puffy MacCloud]

I’ve always recognised the 21st of September as the Autumn Equinox. Until now. Because it isn’t. Or not here.

According to the BBC Weather App for my village, sunrise this morning was at 07:00 (06:00 GMT) and sunset will be at 19:20 (18:20 GMT), which means that the noxes are not equi. That doesn’t happen until the 25th of September when sunrise will be at 07:08 and sunset will be at 19:10. (That’s as close as it gets.)

So why, oh why, is there an offset from the expected result? I guess that the ten-minute slide is due to the longitudinal westerly component from Greenwich to here, and I further hypothesise that the four-day delay is due to the fifty-five and a bit degrees of latitude and a diminishing horizon, but I’m not at all convinced that I’m right.

Anyone know how and why I must wait four days for my village Equinox?

 

[Nick L]

The equinox is different to the equilux. The term for when the days and nights are equal length is the equilux:

The Equinox and Solstice

WWW.METOFFICE.GOV.UK

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Equinox and solstice are vital parts of the astronomical calendar which determine the transitions between the seasons.

The equinox very rarely occurs on the 21st Sept as it happens. It's a date that has somehow got drilled into our mindset but it's extremely rare and is only occurring twice this century.

[Weather-history]

That's why the meteorological seasons based on the solstices and equinoxes is not IMO the best one, certainly when it comes to comparing seasons.

Any case why use astronomical definitions for meteorology? 

Further the astronomical calendar does not make sense to me, anyway. Astronomical summer should be the period of the year when the sun is above the horizon for longest ie early May to early August for northern hemisphere and winter early November to early February when the sun is above the horizon for the least amount of time.

 

[reef]

Its also worth noting that day and night being exactly 12 hours each happens after the Autumn equinox (and before in Spring) as the sun is visible on the horizon before sunrise and after sunset for a few extra minutes due to refraction in the atmosphere.