On Friday, 18 March 2016 10:48:09 UTC, Bernard Burton wrote:
"Vidcapper" wrote in message
...
On 18/03/2016 04:14, Tudor Hughes wrote:
On Friday, 18 March 2016 00:40:43 UTC, jumper wrote:
Why is the spring equinox on the 20th March, when at my location
today we had 12 hrs daylight / nighttime? On the 20th March we
have 12hrs 12 mins daylight.

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Refraction. When the sun appears to be on the horizon it's
geometrically below it by just over half a degree because the rays
are bent downwards due to the higher density of the atmosphere at
lower levels.

But the earth rotates at 15°/hr, so it would only take 2 minutes to turn
half a degree, not enough to account for the 12 minute difference
noted above...

Paul Hyett, Cheltenham



This link may help explain. One of the main factors is that the earth's
orbit around the sun is not a circle but an elipse.


http://physics.stackexchange.com/que...rent-every-day

--
Bernard Burton

Wokingham Berkshire.

Weather data and satellite images at:
http://www.woksat.info/wwp.html

That explains why apparent noon (on the meridian) is not usually 12 o'clock UT but has no effect on the time the sun is above the horizon each day. It makes the spring equinox early and the autumn one late, by about 2 days, compared with a "mean" earth or one in a circular orbit. The earth is closest to the sun on about January 3rd but you wouldn't know it, at least thermally. The eccentricity of the orbit is about one part in 60 each way.

Tudor Hughes