In another interesting episode of Today I Found Out, host Simon Whistler shares the astronomical reason the hottest days of summer are called the “dog days” that goes all the way back to the ancient Egyptians.
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”.