Today I found out why the hottest part of the summer is called the “Dog Days”.

The earliest reference to some aspect of this expression goes all the way back to the Ancient Egyptians.  They noted that the heliacal rising of the star Sirius heralded the hottest part of the summer.  However, it isn’t exactly known why the ancient Egyptians associated this star with a dog (the star’s hieroglyph is a dog).  Sirius would appear in Egypt, after about a 70 day absence, just before the season where the Nile typically floods.  So it is thought the star’s hieroglyphic symbol being a dog symbolized a “watchdog”.

On the other hand, it’s very possible it was for the same reason the Ancient Greeks and Ancient Romans would also eventually associate this star with a dog.  Namely, that it is the brightest star in what is now known as the Canis Major (Latin for “Greater Dog” or “Big Dog”) constellation.  This constellation simply looks a little bit like a dog and Sirius is the brightest star in the constellation, so the star got named the “Dog Star” and it’s heliacal rising marked the start of the hottest part of the year, which then became the “Dog Days”.

The Roman’s expression for Dog Days was diēs caniculārēs (Latin for “Dog Days”).  The Greeks also had a similar expression that literally translated to “Dog Days”.  They both believed that, when Sirius rose around the same time as the Sun, this contributed to that time of year becoming hotter.  As such, they would often make sacrifices to Sirius, including sacrificing dogs, to appease Sirius with the hope that this would result in a mild summer and would protect their crops from scorching.

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Bonus Facts:

• Canis Major is listed in Ptolemy’s 48 constellations all the way back in the 2nd century.   The name Sirius itself means “scorching star” or “scorcher” and comes from the Ancient Greek Σείριος Seirios.
• Sirius-A is the brightest star visible to the naked eye from Earth, being almost twice as bright as Canopus.  Sirius-A and Sirius-B combine to form a binary system and appear as one star to the naked eye, though the vast majority of luminosity to the naked eye comes from Sirius-A, Sirius-B being a white dwarf which is only around 30 AU (astronomical units) away from Sirius-A.  It is also theorized that there is a Sirius-C; but to date, this has not been proven.  Sirius A and B (and possibly C) combine to form a bright point known as Sirius.
• For those who don’t know, one astronomical unit is the average distance between the Earth and the Sun.  A white dwarf is a former star that is nearing the end of its life-cycle, with no further nuclear fusion going on at its core.  Once most stars have used up the fuel at their core, they go into a red-giant or red super-giant phase and expand to incredible sizes.  At the end of this phase, when the star has used up all its fuel that it can use via nuclear fusion, if it has enough mass, it will extremely rapidly collapse and create one of the most powerful explosions in the universe, the supernova.  The burst of energy from a single supernova can briefly out-shine an entirely galaxy full of billions of stars.  If the red giant doesn’t have enough mass, it will simply collapse into a white dwarf and spend a couple billion years slowly dimming as it gives off its stored energy.
• Some star remnants are too small to begin with and so don’t ever go into a red giant phase.  When they do go into their “red” phase, they are called “red dwarfs”.  However, the lifespan of these stars is longer than the universe has existed.  Thus, every star that will eventually become a red dwarf is still around today and has not reached the red dwarf phase.  As a general rule with stars, the bigger and brighter it is, the faster it burns out.  The smaller it is, the longer it lives.
• Our Sun will eventually become a red giant (and would engulf the Earth, if the Earth was still at its present orbit 5 billion years from now or so when that will happen).  Once the red giant phase is over, it will then contract into a white dwarf as it doesn’t have enough mass to go supernova.
• Even though our sun may not engulf the Earth, due to the Earth’s orbit increasing, because of the increased energy coming from the sun as it expands, in about 3 billion years, all water will be burned off the Earth.  So sometime between now and 3 billion years from now, we need to find another home or figure out how to move the entire Earth.
• For reference to the amazing scale of our Sun’s size now vs. as a red giant vs. as a white dwarf, our Sun could hold about 1.3 million Earths right now at its present size.  When it reaches the peak of its red giant phase, it will expand to about 1.3 AU, meaning it could hold about 28 trillion Earths.  Once it contracts down to a white dwarf, it will be about the same size as Earth.
• Sirius-A is a white main sequence star (A1V) that is about twice the size of our Sun.  This isn’t very massive, relatively speaking.  The extreme brightness from our point of view then comes not so much because it is a particularly bright star, but rather because it is only 8.6 light years away (2.6 parsecs).  That’s a really long ways away by standards of distance we normally deal with, but by galactic standards, that’s just next door and, indeed, it is one of our nearest neighbors.
• The French term “canicule” meaning “little dog” is used to refer to heat waves and this period of the summer known as “dog days” in English.
• In Ancient Rome, the Dog Days went from July 24th through August 24.  According to the old Farmer’s Almanac, the Dog Days go from July 3rd to August 11th.  It is during this period that rainfall is at its lowest in the Northern Hemisphere.
• The Egyptians also based their calendar on the heliacal rising of Sirius when it would first become visible before sunrise after a 70 day absence from the skies.
• People from the Greek island of Ceos in the Aegean Sea offered regular sacrifices to Sirius and Zeus in hopes that this would bring about cooling breezes during the Dog Days.  When the star would first rise, if it would rise looking misty or faint, they would expect pestilence to follow.  If it arose clear, they would expect good fortune that year.
• The Ancient Romans also sacrificed a dog upon the setting of Sirius around April 25th.  They sacrificed this dog along with other animals, to the goddess Robigo, in the hopes of having good wheat crops in the coming summer.
• A feast day is celebrated on August 16th, in honor of Saint Roch, the patron saint of dogs.
• Sirius Black, in Harry Potter, is thought to have been named after the white dwarf, Sirius B.  The relationship is further extended by the fact the character Sirius Black was an animagus that could change into a dog.
• The dog days are mentioned in Dickens’ A Christmas Carol (passage referring to Scrooge): “A frosty rime was on his head, and on his eyebrows, and his wiry chin. He carried his own low temperature always about with him; he iced his office in the dogdays; and didn’t thaw it one degree at Christmas.”
• It is also mentioned in Homer’s Iliad:  Sirius rises late in the dark, liquid sky.  On summer nights, star of stars, Orion’s Dog they call it, brightest Of all, but an evil portent, bringing heat And fevers to suffering humanity.