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...


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Paul Hyett, Cheltenham