basis for their calendar. The coincidence of a heliacal rising of Sirius and the New Year reported by Censorinus occurred about 20 July, that is a month after the summer solstice. In fact Sirius moves completely in step with other stars as a result of precession. It therefore moves with respect to the equinoxes; the precise agreement of 365.25 days is with the Julian Calendar. The equinoxes drift in the Julian calendar, and that was the reason for the Gregorian calendar reform. Problems and criticisms Determining the date of a heliacal rise of Sirius has been shown to be difficult, especially considering the need to know the exact latitude of the observation. Another problem is that because the Egyptian calendar loses one day every four years, a heliacal rise will take place on the same day for four years in a row, and any observation of that rise can date to any of those four years, making the observation imprecise. A number of criticisms have been levelled against the reliability of dating by the Sothic cycle. Some are serious enough to be considered problematic. Firstly, none of the astronomical observations have dates that mention the specific pharaoh in whose reign they were observed, forcing Egyptologists to supply that information on the basis of a certain amount of informed speculation. Secondly, there is no information regarding the nature of the civil calendar throughout the course of Egyptian history, forcing Egyptologists to assume that it existed unchanged for thousands of years; the Egyptians would only have needed to carry out one calendar reform in a few thousand years for these calculations to be worthless. Other criticisms are not considered as problematic, e.g. there is no extant mention of the Sothic cycle in ancient Egyptian writing, which may simply be a result of it either being so obvious to Egyptians that it didn't merit mention, or to relevant texts being destroyed over time or still awaiting discovery. Marc Van de Mieroop, in his discussion of chronology and dating, does not mention the Sothic cycle at all, and asserts that the bulk of historians nowadays would consider that it is not possible to put forward exact dates earlier than the 8th century BCE. Some have recently claimed that the Theran eruption marks the beginning of the Eighteenth Dynasty, due to Theran ash and pumice discovered in the ruins of Avaris, in layers that mark