The first dates (7,000–8,000 years ago) were determined by radiocarbon analysis of organic material in the sediment. The second date (6,000 years ago) was determined by calculating the precession of the equinoxes. That is, over time stars appear to change position in the sky because of a slight wobble in the earth’s axis of rotation. The star (Dubhe) marking north 6,000 years ago has since moved to another position.