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