"Dog Days of Summer": The Meaning of the Phrase

People around the world use the term "Dog Days" in different ways, usually as a reference to hot weather. It comes from the Dog Star, Sirius, which is part of the constellation Canis Major. Each summer between July and August the constellation appears in the northern hemisphere during what is usually the hottest time of the summer. You would also be correct to refer to the Dog Days as the "canicular days". Sirius is often seen as a twinkling star of different changing colors. The appearance of color is a rainbow effect caused as the light passes through the earth's atmosphere, similar to a rainbow.

Ancient astronomers used the constellations to guide them through the seasons as well as for navigational charts for land and sea. The star named Sirius (the Dog Star) in Canis Major is quite visible in the night sky. Dog Days are named for the appearance of the Dog Star on the eastern horizon. It is the brightest star in Canis Major and is easily recognizable with the naked eye. Depending on which hemisphere you are in and your latitude, the Dog Days occurs at slightly different times. Ancient Egyptians believed that Sirius had an effect on life here on earth, partly because the regular annual rising of Sirius coincided with the days just before the flooding of the Nile River. They watched for the star to mark the coming of the floods, and they used the rising of Sirius to mark the first day of their year. Some say that our New Year's Eve actually marks the return of Sirius to the mid-sky alignment, and that this is the historical reason for the celebration at midnight January 1, since it has been an important ritual celebration date for cultures dating from before the Roman calendar was in use. Ancient Greeks and other civilizations of the early eras offered sacrifices to the star. Ancient coins excavated from the island of Ceos were struck with impressions of dogs and stars with rays emanating from them, which historians believe illustrates the significance of Sirius.

It is interesting to note that Sirius is a binary star system, meaning that there are two stars in the system. The brighter of the stars, which is called Sirius A, is over ten times as bright as our sun and has about three times the mass. Modern astronomers tell us that Sirius is only 8.6 light years away from earth, and moving closer to earth at a speed of several thousand miles per hour. Because Sirius is moving directly towards earth, it creates an axis of rotation that remains constant in relation to the stars from wherever our viewpoint is on earth. So Sirius' visibility as it rises on the earth's eastern horizon exactly corresponds to the earth's rotation of 365.25 days. If you live in a region where Sirius is visible, you will see it on the same date each year in the same part of the sky, although the dates/sky position will differ slightly depending on your location and latitude. Every year Sirius becomes brightly visible on the eastern horizon on the same days in each region. But if you live above 74 degrees latitude, which is much of North America and Europe, you won't see much of Sirius because its position is below your horizon.

Although they appear as one bright star in the sky, the second star in the binary system, Sirius B (also called "The Pup"), orbits Sirius A in an elliptical pattern. Even though it is a dead star, or a "white dwarf", Sirius B spins at an incredible 23 times per minute on its axis. This rate of spin creates an incalculable amount of magnetic pull. As Sirius B reaches its periastron, or the period of closest connection between the two stars, it is estimated that there are immeasurable amounts of visible light, ultraviolet light, gamma rays, x rays, and electromagnetic radiation produced in the Sirius system. Whether or not there are effects from the periastron here on earth, and just what those effects may be, is a mystery.

Perhaps it is a mystery best pondered from a lawn chair under a clear July night sky...

Read more and see positional charts:
http://www.earthsky.org/skywatching/the-skys-brightest-star-sirius-before-dawn

http://en.wikipedia.org/wiki/Dog_Days