The "dog days" of summer officially came to an end this week, but few people know what the expression really means. The phrase actually has a celestial origin.

Some will say that summer's "dog days" signify hot sultry days "not fit for a dog," while others suggest it?s the weather in which dogs go mad.

But the "dog days" are actually defined as the period from July 3 through Aug. 11 when the Dog Star, Sirius, rises in conjunction (or nearly so) with the sun.

(This sky map shows where to find Sirius at sunrise in mid-August. Sirius is the brightest star in the night sky.)

As a result, some felt that the combination of the brightest luminary of the day (the sun) and the brightest star of night (Sirius) was responsible for the extreme heat that is experienced during the hieght of the summertime. Other effects, according to the ancients, included droughts, plagues and madness.

A more sensible view was put forward by the astronomer Geminus around 70 B.C.  He wrote: :"It is generally believed that Sirius produces the heat of the 'dog days,' but this is an error, for the star merely marks a season of the year when the sun?s heat is the greatest."

Dog star, Nile days

In ancient Egypt, the New Year began with the return of Sirius. It was, in fact, the "Nile Star" or the "Star of Isis" of the early Egyptians. 

Interestingly, some 5,000 years ago, this ?heliacal rising? (appearing to rise just prior to the Sun) occurred not in August, as is the case today, but rather on, or around June 25. When they saw Sirius rising just before the Sun, they knew that the "Nile Days" were at hand. Its annual reappearance was a warning to people who lived along the Nile River. 

The star always returned just before the river rose, and so announced the coming of floodwaters, which would add to the fertility of their lands. People then opened the gates of canals that irrigated their fields. 

Serious about star Sirius

Priests, who were the calendar keepers, sighted the first rising of the Dog Star from their temples.  At the temple of Isis-Hathor at Denderah is a statue of Isis, which is located at the end of an aisle lined by tall columns. 

A jewel was placed in the goddess? forehead. The statue was oriented to the rising of Sirius, so that the light from the returning Dog Star would fall upon the gem. When the priests saw the light of the star shining upon the gem for the first time, they would march from the temple and announce the New Year.

In the temple appears the inscription:  ?Her majesty Isis shines into the temple on New Year?s Day, and she mingles her light with that of her father Ra on the horizon.? 

This week, just before sunrise, Sirius might again be glimpsed rising just above the southeast horizon for those living in mid-northern latitudes. At more southerly latitudes, Sirius is already conspicuous, twinkling above the horizon at dawn.

Sirius is the brightest star of the constellation Canis Major, the ?Greater Dog? in Latin. According to Burnham?s Celestial Handbook other names for it include ?The Sparkling One? or ?The Scorching One.?

The star appears a brilliant white with a tinge of blue, but when the air is unsteady, or when it is low to the horizon as it is now, it seems to flicker and splinter with all the colors of the rainbow.  At a distance of just 8.7 light years, Sirius is the fifth-nearest known star.  Among the naked-eye stars, it is the nearest of all, with the sole exception of Alpha Centauri.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.