I believe that Phil's observation of the eagle low on nest December 11 can be used as the approximate starting date of incubation. This morning at about 9:30 Mary Lou and I stopped by the nest and found no eagles in sight. I decided to watch the nest, and after about 15 minutes the female stood up and turned 180 degrees, looked down into the nest as if rearranging or turning one or more eggs, then watched us for a while before settling down out of sight.
I got several photos, among them: