I went for my workout this afternoon with my trainer and asked him if he knew the meaning of 'the dog days of summer'. Instantly he rattled of a complicated meaning. Here's the gist:
"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 Osirus, 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.
The 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). "
August 2002, Jagadheep D. Pandian (more by Jagadheep D. Pandian), Karen Masters (more by Karen Masters)
I've read some gruesome stories about brown dogs being sacrificed to the dog star. Downright evil. If you have a brown dog, hide him until mid September.