I looked up the meaning to ‘Dog Days of Summer‘ in Wikipedia. Got some good results which make sense, even to me. My search for ‘Indian Summer’ did not have as good a set of results.
The Dog Star, also known as Sirius, rises just after and sets just before the sun, during the summer months that, in the Northern Hemisphere, is typically the hottest days of the year. The brighter sun hides the star, until their orbits change and Sirius can be seen again. The Romans believed that Sirius produced heat and, when it disappeared, it added its heat to the sun, making the air hotter here on earth.
In doing further research into how the term is used and all that, I came across a debunking site: Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP). They had a page dedicated to a book called The Sirius Mystery by Robert Temple. I found the article to be interesting (and not at all flattering for Mr. Temple). I like the website, or at least the group.
The interesting part is that when the article was written (Fall of 1978), it was believed there were only two stars that made up Sirius. They now have evidence (.pdf – link opens into new window) there may be a third afterall according to an article from July 1995.
Investigating the meaning of “Indian Summer” was a little more difficult due to no one seems to know just how it started and why. Even the National Weather Service has an opinion.
Researching ‘Indian Summer’ led me to a site called Word Origins.org. Their interpretation runs along the same lines as Wikipedia and the NWS. And no, Word Origins did not have ‘Dog Days of Summer’ listed.
It is interesting to read on how various folks believe the same phrase came about. Colloquialisms make it even more difficult, especially for the same word or phrase.