Dating the pyramids
In addition, pharaohs always started building their tombs at the starts of their rules.
Until now, we dated the pyramids by working out how long each individual Egyptian Pharaoh held power, and then added all the years, and worked our way backwards.
The team believes the ancient Egyptians observed the star from July 17 to 19, and the inundation period began 35 days later—on August 23.
(Related: "Great Pyramid Built Inside Out, French Architect Says.") Pharaohs Reset the Clock Using Sothis's arrival to keep track of the annual Nile floods made sense, said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago who was not involved in the work.
A team of Egyptian researchers arrived at the date based on calculations of historical appearances of the star Sothis—today called Sirius.
Every year around the time of the Nile River floods, Sothis would rise in the early morning sky after a long absence.
The most accurate is the Pyramid of Khufu, also called the Great Pyramid.