Sirius was the favourite dog of Orion, the hunter (and also, apparently, of Orestheus). He was placed in the heavens as Canis Major, the Dog Star, where he was the brightest star in the sky. His name is said to derive from seirios, 'hot', 'scorching', since he appeared in the night sky at the time of greatest heat, July and August, and it was thought that this heat came from Sirius, not the sun.

The Romans called him Canicula, 'little dog', in an affectionate manner, but they also noticed that he appeared at the hottest time of the year, and this gave the English expression 'dog days' (Latin dies caniculares).