It struck the ancient Ro mans as being very odd that although the sun reaches its most northerly limit around June 21st, the warmest weather invariably occurs several weeks after that event.
They looked for some other source of heat that might explain this strange phenomenon, and noticed that around this time of year, Sirius, the brightest star in the sky, rises and sets with the sun. Not unreasonably, they assumed that it was the radiant power of this star, added to that of the sun, that provided the extra heat to make July and August down in Rome so totally unbearable.
Sirius lies in the constellation canis major and when it is above the horizon it can be located by downward extrapolation of Orion's belt. Indeed it is related to Orion; the latter was the giant son of Jupiter, chief Roman god, and Sirius was Orion's faithful hound.
Thus the star Sirius became in Roman times "the Dog Star", and no doubt entirely by coincidence, it acquired an evil reputation suited to its name: it was believed, inter alia, that it had a malign influence on the mental health of the Roman canine population.
This evil influence was at a maximum during the 40 days when Sirius was above the horizon during the day, when it supplemented the heat of the July and August sun.
It was during this period, from early July to mid-August, that dogs were most likely to go mad. Moreover, this annual heat-wave, the caniculares dies or the "Dog Days", was, from a human point of view, the doldrums of the year - a period when the Roman population had neither the inclination nor the nerve to embark on any worthwhile enterprise.
Of course, we know better than the ancient Romans. Firstly, we know that Sirius has no effect at all on Roman dogs. Secondly, we are aware that all the stars together, never mind the solitary Sirius, 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 on our weather. And thirdly, we know the real reason why it is hotter at this time of year than near the summer solstice.
Earth, like a vast storage heater, retains much of the solar warmth absorbed in May and June and, for several weeks after the solstice, the ground, on average, is warmer than the air above it.
This allows a net flow of heat from ground to air so that the average temperature of the air, for a week or two at least, continues to increase.