IT struck the ancient Romans as very odd indeed that although the noon day sun reaches its highest point in the sky around June 21st each year, the warmest weather invariably occurs several weeks after that event. So they began to look around for some other source of heat that might explain this strange phenomenon.
They noticed that for a 40 day period around this time of year, Sirius, normally the brightest star in the sky, rises and sets with the sun, and they assumed that it was the extra radiant power of this heavenly body, added to that of the sun, that provided the extra heat to make July and August so unbearable.
Sirius lies in the constellation can is major, and at those times of year when it is above the horizon after dark, it can be located by a downward extrapolation of Orion's belt. Indeed the two are related Orion was the giant son of Jupiter, and Sirius was his faithful hound and thus the star Sirius became "the Dog Star".
But the Dog Star, when above the horizon during the day, did more than just provide a little extra heat. The Romans believed that it had an evil influence on human affairs, and a malign effect on the mental health of members of the canine population. Thus the caniculares dies, the "Dog Days" as they called them, became the doldrums of the year, a period when the Roman population had neither the inclination nor the nerve to embark on any worthwhile, enterprise. By a strict astronomical interpretation, the "Dog Days" can begin anytime from early July until mid August, and vary in length from 30 to 54 days. Traditionally, however, they are reckoned to begin today, July 3rd, and to last for 40 days until August 11th.
We, of course, are much wiser than the Romans were. Firstly we know that Sirius has no effect at all on dogs or human beings. And secondly we are aware that all the stars together, never mind the much maligned Sirius alone, radiate in our direction only a minute fraction of the amount of energy we receive every day from the sun, and could have no effect whatever upon our weather.
And thirdly, we know the real reason why it is hotter at this time of year than near the summer solstice. The earth, like a vast storage heater, retains much of the solar warmth absorbed in May and June for several weeks after the solstice, the ground, on average, is warmer than the air above it, allowing a net flow of heat from ground to air so that the average temperature of the air, for a few weeks at least, continues to increase.