Why do we talk about ‘Dog days’?

At a recent visit to the local observatory I was interested to find out the origin of the French expression canicule, and consequently the English equivalent the ‘dog days of summer’.

In Greek and then Roman times Sirius, the dog star, was associated with the hottest days of summer, and also marked the flooding of the Nile in Ancient Egypt. The name ‘Sirius’ is derived from the Ancient Greek Σείριος (Seirios) which means “glowing” or “scorcher”. Sirius is called the dog star because it is the brightest star in the Canis Major (‘Large Dog’) constellation; canicula means ‘small dog’ and is the diminutive of the Latin canis. Canicule corresponded to the period of the year when Sirius and the sun rose at the same time, which for the Romans was between 23rd/24th July and 23rd/24th August, hence the association between Sirius and hot weather. The Romans sacrificed a red dog in April to appease the star’s rage, believing Sirius was the cause of the heatwaves (ancient astronomers saw Sirius as red; it’s now commonly perceived as white).


Canis Major as depicted in Urania’s Mirror, a set of constellation cards published in London in c.1825.

In his Natural History, Pliny the Elder (AD 23-79) wrote:

The Dog-star rises in the hottest time of the summer, when the sun is entering the first degree of Leo; this is fifteen days before the Calends of August. … The most powerful effects are felt on the earth from this star. When it rises, the seas are troubled, the wines in our cellars ferment, and stagnant waters are set in motion … There is no doubt that dogs, during the whole of this period, are peculiarly disposed to become rabid … Canine madness is fatal to man during the heat of Sirius, and, as we have already said, it proves so in consequence of those who are bitten having a deadly horror of water.

‘Dog’ is also referenced in the word for this sultry time of year in many other European languages (for example Hundstage in German, or Canícula in Spanish).

Just as the appearance of Sirius in the morning sky marked summer in the southern hemisphere, so it marked the chilly onset of winter here in the southern hemisphere. It was an important star for navigation around the Pacific Ocean, and the Maori called the star and the season Takurua.  Its culmination at the winter solstice was marked by celebration in Hawaii, where it was known as Ka’ulua, “Queen of Heaven”. Many other Polynesian names have been recorded, including Ta’urua-e-hiti-i-te-tara-te-feiai “Festivity who rises with prayers and religious ceremonies” in Tahiti.


Observatoire Astronomique des Makes, St Louis, Reunion Island

Further reading: