You keep hearing about the equinox having equal day and night, but due mainly to refraction, it doesn't. http://www.timeanddate.com/astronomy...not-equal.html

At the latitude of Cornwall, today has equal day & night. (Max possible sunshine 12hrs http://www.sunrecorder.net/googlemap...serial=1000 5 )

I await some astronomer telling me it's tomorrow (which it could just be)

Graham
Penzance

On Saturday, 26 September 2015 09:48:39 UTC+1, Graham Easterling wrote:
You keep hearing about the equinox having equal day and night, but due mainly to refraction, it doesn't. http://www.timeanddate.com/astronomy...not-equal.html

At the latitude of Cornwall, today has equal day & night. (Max possible sunshine 12hrs http://www.sunrecorder.net/googlemap...serial=1000 5 )

I await some astronomer telling me it's tomorrow (which it could just be)

Graham
Penzance

Graham

I have some astronomical algorithms that I use in my programming and today at my latitude (mid-Devon) the day is 12:02 long and tomorrow it's 11:59 long. I think they are well regarded so I, like you await someone to tell us the reason why!

Bruce.

On 26/09/2015 18:14, xmetman wrote:
I have some astronomical algorithms that I use in my programming and today at my latitude (mid-Devon) the day is 12:02 long and tomorrow it's 11:59 long. I think they are well regarded so I, like you await someone to tell us the reason why!

It may be that the Equinox is measured by the point in time that the sun
is directly overhead at the equator whereas the daylength is the point
that the top of the sun's disc touches the theoretical horizon at dawn
and leaves the horizon at dusk.

Therefore the extra few minutes are accounted for by the time it takes
for the whole of the sun's disc from first appearance, to cross the
horizon and become clear of it.

So it's really about the measurement of exactly when do you say that the
day has started, i.e., when the sun first touches the horizon from below
it (at dawn), when it is half-way crossing it or when it is completely
clear of it.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalley.co.uk

It may be that the Equinox is measured by the point in time that the sun
is directly overhead at the equator whereas the daylength is the point
that the top of the sun's disc touches the theoretical horizon at dawn
and leaves the horizon at dusk.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalley.co.uk

That's true, but as I said in my original post, refraction through the Earth's atmosphere is also important. In effect you can sea below the horizon.

I was really looking for someone to show the reltive importance of the 2 aspects.

Graham
Penzance

"Graham Easterling" wrote in message
...

It may be that the Equinox is measured by the point in time that the sun
is directly overhead at the equator whereas the daylength is the point
that the top of the sun's disc touches the theoretical horizon at dawn
and leaves the horizon at dusk.

That's true, but as I said in my original post, refraction through the
Earth's atmosphere is also important. In effect you can sea below the
horizon.

I was really looking for someone to show the reltive importance of the 2
aspects.


I think refraction is quite important and will vary according to vertical
temperature and humidity gradients.

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
---------------------------------------------

"Graham Easterling" wrote in message
...
It may be that the Equinox is measured by the point in time that the sun
is directly overhead at the equator whereas the daylength is the point
that the top of the sun's disc touches the theoretical horizon at dawn
and leaves the horizon at dusk.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalley.co.uk

That's true, but as I said in my original post, refraction through the Earth's atmosphere is
also important. In effect you can sea below the horizon.

I was really looking for someone to show the reltive importance of the 2 aspects.

Graham
Penzance

Refraction raises the sun by approximately 34 minutes of arc, while the solar semi-diameter is
about 16 minutes.

Sunrise and sunset therefore occur when the centre of the sun's disk is about 50 minutes
(of arc) below the horizon, with the effect of refraction being about twice that of the
semi-diameter correction.

On Sunday, September 27, 2015 at 6:49:23 PM UTC+1, Gavino wrote:
"Graham Easterling" wrote in message
...
It may be that the Equinox is measured by the point in time that the sun
is directly overhead at the equator whereas the daylength is the point
that the top of the sun's disc touches the theoretical horizon at dawn
and leaves the horizon at dusk.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalley.co.uk

That's true, but as I said in my original post, refraction through the Earth's atmosphere is
also important. In effect you can sea below the horizon.

I was really looking for someone to show the reltive importance of the 2 aspects.

Graham
Penzance

Refraction raises the sun by approximately 34 minutes of arc, while the solar semi-diameter is
about 16 minutes.

Sunrise and sunset therefore occur when the centre of the sun's disk is about 50 minutes
(of arc) below the horizon, with the effect of refraction being about twice that of the
semi-diameter correction.

Thanks!

Graham
Penzance