I have a frustrating time trying to upload this video:
Do have a look at Pete Glastonbury's remarkable photographs showing the sun setting along the slope of Silbury Hill on May 16. There is a theory that the builders of megalithic monuments designed them so that viewers would see this phenomenon from an observation point that was also part of the site design. But presumably, as the angle of the sun's trajectory as it approaches the horizon varies widely according to the seasons, it wouldn't have mattered what the gradient of the Hill was; there would be many pairs of positions in the vicinity of the monument and days in the year which could provide this extraordinary spectacle. I hope that in a later posting I will be able to give an explanation of the theory.