They also possessed lists of the fixed stars, and drew up tables of the times of their heliacal risings.
"The Babylonian Legends of the Creation"
British Museum
They determined the length of their year by the heliacal rising of Sirius, 2782 years before the Christian era, which is the earliest epoch of Egyptian chronology.
"Personal Recollections, from Early Life to Old Age, of Mary Somerville"
Mary Somerville
According to Professor Meyer and Professor Erman, this formal calendar was introduced in 4200 B.C., one of the occasions when the heliacal rising of the star Sothis fell on the first of the month Thoth of the calendar.
"The Egyptian Conception of Immortality"
George Andrew Reisner