Dog days Dog days The dog days or are the hot, sultry days of summer. They were historically the period following the heliacal rising of the star system Sirius, which Greek and Roman astrology connected with heat, drought, sudden thunderstorms, lethargy, fever, mad dogs, and bad luck. They are now taken to be the hottest, most uncomfortable part of summer in the Northern Hemisphere. The English name is a calque of the Latin "" (."the puppy days"), itself a calque of the Greek "kynádes hēmérai" (, "dog days"). The Greeks knew the star α Canis Majoris by several names, including Sirius (,
Sirius indigenous Boorong people of northwestern Victoria named Sirius as "Warepil". In 1717, Edmond Halley discovered the proper motion of the hitherto presumed "fixed" stars after comparing contemporary astrometric measurements with those from the second century AD given in Ptolemy's "Almagest". The bright stars Aldebaran, Arcturus and Sirius were noted to have moved significantly; Sirius had progressed about 30 arc minutes (about the diameter of the Moon) to the southwest. In 1868, Sirius became the first star to have its velocity measured, the beginning of the study of celestial radial velocities. Sir William Huggins examined the spectrum of the star and