In message , anne
writes
If I trim 4 foot from the height of a hedge will this give me 4 foot more
light on my allotment?

The problem is, that during the day the sun travels the length of a hedge
that runs all the way up the side of the allotment and the hedge puts 50% of
the allotments's entire length in the shade. This is now - september. How
much will it decrease in winter and how much will it increase in summer?

My main question though, is does the amount you cut off equal the amount of
sunlight you gain.

You do better than that in winter (but the sun is weak) and worse in
summer. You also potentially lose some protection from wind too.

Maximum altitude of the sun is at transit due south (approx 1pm BST)
Summer 90 - latitude + 23.5 ~60 degrees
Equinoxes 90 - latitude ~37.5
Winter 90 - latitude - 23.5 ~14

(Assuming a latitude of middle England say 53.5 degrees)

So the gain in illuminated ground at midday as a proportion of hedge
lopped off is roughly 58% in midsummer, 130% at the equinoxes and 400%
mid winter (although the sun then is so feeble as to be useless). The
overall influence is about half to two thirds of these values in
practice since the sun isn't permanently due south or at its maximum
altitude.

NB You can always lop the N side with a chamfer to allow more light in
but without chopping as much off the overall height.

Regards,
--
Martin Brown