Yup, position and height are critical. Click on the pic here - http://silbury-hill.blogspot.c[...]rchive.html#315953661864961496 (click again to enlarge). Silbury can just be seen on the skyline between the stone on the left and the stone on the right. If Silbury were originally higher it would have been more easily seen from the Obelisk. If Waden Hill was slightly higher however (now somewhat reduced due to ploughing?) or wooded in the Neolithic, Silbury wouldn’t have been visible from that point... suggesting there was no grand design to link Silbury and the Henge visually?