Canis Major, the large dog:

A very ancient constellation, Canis Major is the larger of the the two hunting dogs used by Orion. Within the constellation can be found the brightest star in the night sky Sirius, also known as the "Dog Star". The name Sirius is derived from the Greek word seirios which means "scorcher". It was thought that during the hot months of summer when this star is above the horizon during the day time its heat was added to the sun. This is the origin of the term "the dog days of summer". Canis Major can be easily found by following a path leading east from the belt stars of Orion.