Answer me this...

[Ebme Geek]

Not only the length of day but more the angle the radiation is hitting the ground, another version of what has been already said >>  shine a torch verticaly down onto the floor and you get a nice circle, now angle the torch over and you get a stretched circle covering a much larger area, but the same amount of energy in total, so each bit gets less heating effect.

I think i know. 

Could always express that in Polar notation
  And please nobody mention the 'frequencey domain'
  otherwise we will get into Fourier transforms  [img alt=:groan]http://foc-u.co.uk/Smileys/efocicon/groan.gif[/img]