I was wondering the same thing!
I found this bit of info in Gardenersworld.com
"Orientation
If you want to grow crops all year round, it's best to line up the ridge of the structure to run east-west, as this will maximise light during the winter. It will also help it to heat up more quickly after cold nights. If you only intend to grow summer crops, aligning the ridge north-south is preferable as it gives an equal amount of sun to each side and helps to reduce overheating on the hottest days."
