What are the "dog days of summer"? (Intermediate)

Are you familiar with the Dog Star? I have heard the star rises and sets with the sun. I am trying to ascertain if it is fact or fiction. Does the term "dog days of summer" come from the star?

Well, the dog star is actually Sirius which is the brightest star in the night sky and it does rise and set with the Sun at some times during the year - when it does this it is said to be in conjunction with the Sun.

Since Sirius is the brightest star that we can see in the sky, it might be thought reasonable to guess that it adds some heat to the Earth when it is in the sky, although that amount is now known to be insignificant.

The name "dog star" came from the ancient Egyptians who called Sirius the dog star after their god Osiris, whose head in pictograms resembled that of a dog. In Egypt, and in ancient Rome, Sirius was in conjunction with the Sun in the summer (ie. it was up in the sky at the same time as the Sun) and ancient Egyptians and Romans argued that it was responsible for the summer heat by adding its heat to the heat from the Sun.

They called the period of time from 20 days before to 20 days after the conjunction "the dog days of summer" because it coincidentally fell at the time of year when it was very hot.

The exact time of conjunction changes with the precession of the equinoxes so that now the conjunction of Sirius with the Sun is a little earlier in the northern summer than it was during Roman times, and as time passes it will move out of the summer season altogether (note: the conjunction is in the southern winter (both now and in Roman times), so ancient civilizations in the southern hemisphere could not have come up with this myth).


This page was last updated on June 27, 2015.

About the Author

Jagadheep D. Pandian

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

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