Best direction for a greenhouse in temperature zone 6, specifically Ohio?

I have heard many conflicting thoughts on specifically which direction an underground greenhouse should face. Of course it MUST be facing south, but some say southeast is better, others say southwest is better. In The Greenhouse of The Future they say that 15 degrees east of true south is best because it warms it up right after night-time. Some people say that southwest is better because it warms it up more before night-time. What are your thoughts?

Many have said it varies from state to state, so for a place in temperature zone 6, Ohio, what would be best?

I think the best would be as close to true solar south as possible. Solar south and magnetic south are not a lot different but they are not exactly the same either. The difference is only a few degrees but still I think if I had opportunity and the option to build one I would face it to solar south. My preference for that would be to collect the maximum possible amount of heat during the day. Facing it off of true in either direction reduces the amount collected.

You can look up how to find solar south for your location. You can probably find the info you need faster by searching on how to install solar panels as they also benefit from proper alignment with the sun.

All that said it probably isn't all that critical, as long as you are not drastically off in your alignment. If your greenhouse is well sealed and insulated it should be fine. Like I said if it was me and I had the option I would go with true solar south. On the other hand if my site didn't lend it's self to that but wasn't off by more than a few degrees I wouldn't worry about it.