The henge showed up as dark ridges in a wheat field in a 1991 aerial photograph of the site... A magnetometer survey confirmed the anomaly. Analysis shows that the southeast entrance to the henge marked the winter solstice and that the southwest gate was aligned to the summer solstice. Though some archaeologists question the intepretation, Biehl believes the site was a kind of solar observatory. (Courtesy LfA)
There's a very good article on this subject in the July/August edition of Archaeology magazine.