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