I'm replying from work via Google and was going to reply to Janet but
for some reason she doesn't show up in google groups !?

( I'm not completely wrong, but definitely missing a big part of the
picture )

The sun does set later as you go west but because of the angle of the
earth's axis, it's not quite as simple as I suggested and is affected
by latitude too. That's sort of irrelevent though because as others
pointed out, it's the day length and strength of light that matters,
not what the time of day is.

So some other questions:

Is it better for plants to have long days of weak sun, or shorter days
of strong sun? At which latitude is the greatest total amount of
energy received from the sun each day?

How does it affect plants which are regulated by day length?

Is it the angle to the sun or is it the extra atmosphere that the
light goes through? If it's really just the angle, then I should be
able to get tropical levels of sunlight by rigging up a suitably
angled mirror to reflect the light. eg. at the poles, a mirror of
1m*1.41m at 45 degrees would catch 1 sq m of light and reflect it to
the ground as if it had come from directly above. Maybe this could be
designed into greenhouse roofs so that they had one side of the roof
angled with mirrors instead of glass to give the effect of direct
sunlight from above?