Contrary to the halo displays described in the Bible (St.John of Jerusalem's revelation, Daniel's and Ezechiel's visions), Hildegard's books have the advantage to give hints upon the time when she saw the phenomenon. She had the vision in the beginning of 1163. At that time she probably stayed at her monastry on the Rupertsberq near Bingen. But during the years 1161 to 1163 she also undertook same journeys on which she made sermons, so that the place is not surely known. Place and time are of essential importance in reports an halo displays because the shape of the halos depends on the sun elevation. As Hildegard made her observations in the beginning of the year7 the sun elevation may not have been higher than 20°. But we have to keep in mind that in 1582 the Gregorian Calendar was introduced. October 4,1582, was immediately followed by October 15, 1582. This has the effect that nowadays the maximum sun elevation for example on March 1 is at about 31°. On March 1, 1163, however, it was at about 36°. So in January 1163 the sun could reach a slightly higher elevation than nowadays.
Another point is that during the cold season the formation of freezing fog is possible. Freezing fog causes the brightest and most conspicuous halo displays. Furthermore, in freezing fog there are also halos visible below the horizon.