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