"A giant 1 acre mound, resembling a grass topped temple, built around 3200 BC. At the winter solstice, the rising sun enters the mound through a specially built roofbox above the main entrance and shines directly along the 19m long passage, gradually widening until illuminating the inner chamber for about 17 minutes. Today the first light enters about four minutes after sunrise, but calculations based on the precession of the Earth show that 5,000 years ago first light would have entered exactly at sunrise."