On 18/03/2016 16:04, Tudor Hughes wrote:
On Friday, 18 March 2016 07:08:24 UTC, vidcapper wrote:
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

The sun's path relative to the horizon is not at right angles, not in
these latitudes so it takes longer than the theoretical 2.2 minutes
for the sun to rise or set. At 52° the figure is 2.2/cos 52 (at the
equinoxes) which is 3.6 minutes at each end of the day, total 7.2
minutes which is about the figure. At the pole the sun would rise
over a day earlier than straightforward geometry would indicate.

Tudor Hughes


Thanks, that's a bit clearer now.



--

Paul Hyett, Cheltenham