It is not correct that the Egyptians celebrated the heliacal rising of Sirius or marked their years by it. That is an invention of the 18th century based on one thing written is the 2nd century A.D.Quote
I don't believe this is attested until later times but I have read somewhere that the Egyptians celebrated the helical rising of Sirius and marked their years by this. Their calendar was off by some .24 days/ year but these things have been pretty thoroughly investigated.
They understood some pretty complicated concepts to invent the calendar. I believe they were excellent time keepers but can't prove it yet.
They were not the first to invent A calendar, and they weren't the first to observe the starting and end points of a star's rising. The egyptologists are responsible for some of the modern myths regarding the AE's. There is quite a bit of incorrect information and assumption regarding the AE's astronomy.