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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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.

Tudor Hughes.

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.

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

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

Joe Egginton Wrote in message:
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.
The spring equinox occurs when the sun moves north of the equator,
not when a location has equal dark/light in a 24 hour
period.

--
Freddie
Pontesbury
Shropshire
102m AMSL
http://www.hosiene.co.uk/weather/
http://twitter.com/PontesburyWx for hourly reports


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"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.

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

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

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.

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

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

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.

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

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

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.

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

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

"Tudor Hughes" wrote in message
...
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.

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

That's right.
The other 5 minutes or so is accounted for because published sunrise and sunset times refer to
when the sun's upper limb (rather than its centre) is (or appears to be ...) on the horizon.
So there is roughly another quarter of a degree to add to the half degree that comes from
refraction.