Heliacal Rising of Stars

the day or, more precisely, the moment of a star’s first appearance of the year above the eastern horizon against the background of daybreak. (For several months before its heliacal rising a star is in the daytime sky but is invisible.) The moment of the heliacal rising of a star depends on its coordinates and the geographic coordinates of the place of observation. The moments of the heliacal rising of a star (Sirius) enabled astronomers in ancient Egypt to predict the times of the Nile’s spring floodings, which were important for determining when agricultural work should be done.