I use a free desktop program called TPE,( you can also pay for a smartphone app...) The Photographers Ephemeris, and it shows the path of the sun and moon relative to the place you're researching. It's connected to Google maps, so you can see exactly where the light will be coming from on any day of the year. Photographers find it handy for positioning themselves in the best place for sunrise or sunset, generally...
http://app.photoephemeris.com
The only option I have is to keep moving my plants, so I bought Ikea outdoor benches which can be shifted around as the sunlight shifts. There's a gap of around a metre from where they sit in summer to where they sit in winter.
And yes, roof tops are definitely a great place for extra light!