From Newsgroup: nz.general
The (Efc|Efc+ autumn, Efc4Efco spring) equinox is coming up this weekend <
https://www.itv.com/news/2020-03-17/equinox-and-the-equilux-whats-the-difference>.
That is the point where the angle that the EarthrCOs axis of rotation
makes in the direction of the line to the Sun is 0-#, and you would
think that day and night would be 12 hours long at that point. But
itrCOs not, because the Sun appears as a disc with a nonzero diameter,
so it starts to get light as soon as any part of that disc peeps above
the horizon, and conversely night only descends when it has all
disappeared before the horizon. That takes about two minutes.
(How to work it out? It takes 24 hours for the apparent direction of
the Sun to come back to any given point, i.e. to move through an angle
of 360-#; so each hour (60 minutes) it traverses 15-#. The apparent
diameter of the Sun (and of the Moon, coincidentally enough) is close
to half a degree. The Earth rotates through 0.5-# in 60 |u 0.5 |+ 15 =
two minutes.)
Plus atmospheric refraction extends the length of the day further, by
a few more minutes. So the spring equilux happens a few days before
the spring equinox, instead of at the same time; and correspondingly
the autumn equilux happens a few days after the spring equinox,
instead of on it.
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