Therefore, the dog days of summer only refer to the last part of the summer, not the whole season. But why dogs? Well it actually has a little something to do with astronomy. 

You might have heard of a constellation named Orion, often referred to as "The Hunter." Orion is a prominent constellation visible throughout the world. Nearby is the constellation Canis Major, which is Latin for "greater dog." According to constellation lore, Canis Major is one of Orion's hunting dogs.

Located in Canis Major is a star named Sirius (not to be confused with XM), which is also called the "Dog Star." With the exception of our sun, Sirius is the brightest star visible from Earth. The brilliant, blue-white star's name comes from the Greek word for "searing."

Because Sirius is so bright, it was easy to track even for early astronomers. During April and early May, Sirius was visible in the southwest after sunset. But by mid-summer, Sirius would rise and fall with the sun and get lost in the daytime light. 

However, the ancients knew that the "Dog Star" was still there, up in the sky with the sun during the hottest time of the year. They reasoned that since Sirius was so bright and up there with the sun, it must be adding to the heat to produce the hottest time of the year.