This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
" This is a very interesting question. To answer it completely one would need a book. It goes away-way back, farther than history, into many strange beliefs of many ancient peoples. Today, by " dog days" we mean the six weeks of mid-summer from the twenty-fifth of July to the fifth of September. Then the weather is the hottest and driest, and dogs are most liable to become mad. Many peoples of very old times noticed that this heated term began about the time the brightest star in the sky rose with the sun. This star is called Sirius, which means "burning." It is in a group of stars called Canis Major. That is just Latin for Big Dog. You know many groups of stars, or constellations, are supposed to be arranged in the outlines of animals. One group is called the big bear. That is the one we know as the big dipper. One group is Taurus, the bull.
Well, Sirius, the large burning star in the "big dog" group, came to be called the dog star. In Egypt, where stars were very important in every day affairs, many things happened about the time the dog star got up with the sun. The days became the hottest and driest, the Nile river was flooded by melting snows of far-away mountains, there was much sickness, and many dogs went mad. Every one of these things was supposed to be caused by the evil powers of the dog star. The people counted time, making the year begin with the rising of Sirius. This is known in history as the Canicular, or dog-star year.
When men learned more about the stars it was discovered that it was just by accident that Sirius rose with the sun, over Egypt, at that particular time. The time of its rising, on any country, depends upon how far north that country lies, and the time grows a little later every year for all countries, owing to some forward movement, or procession of all the stars through space. In old Egypt "dog days" came in June. In early English almanacs they are recorded as beginning in the first days of July. Today, in northern countries, dog days run from late in July to early September. Sometime, centuries and centuries from now, the dog star will rise with the sun in mid-winter. Perhaps, by that time, all those stories will be forgotten, "dog days" will be no more, and little children will wonder why the burning star Sirius should ever have been called the dog star, and the group of stars in which it stands, the big dog.