The"dog days" of summerofficially came to an end this week, but few people know what theexpressionreally means. The phrase actually has a celestial origin.

Somewill say that summer's"dog days" signify hot sultry days "not fit for a dog," while otherssuggestit?s the weather in which dogs go mad.

Butthe "dog days" are actuallydefined as the period from July 3 through Aug. 11 when the DogStar, Sirius,rises in conjunction (or nearly so) with the sun.

(Thissky map shows where tofind Sirius at sunrise in mid-August. Sirius is the brightest star inthe nightsky.)

Asa result, some felt that thecombination of the brightest luminary of the day (the sun) and the brighteststar of night (Sirius) was responsible for the extreme heatthat isexperienced during the hieght of the summertime. Other effects,according tothe ancients, included droughts, plagues and madness.

Amore sensible view was putforward by the astronomer Geminus around 70 B.C. He wrote::"It is generallybelieved that Sirius produces the heat of the 'dog days,' but this isan error, for the star merely marks a season of the year when the sun?sheat isthe greatest."

Dogstar, Nile days

Inancient Egypt, the New Year began with the return of Sirius. It was, infact,the "Nile Star" or the "Star of Isis" of the early Egyptians.

Interestingly,some 5,000 years ago, this ?heliacal rising? (appearing to rise justprior tothe Sun) occurred not in August, as is the case today, but rather on,or aroundJune 25. When they saw Sirius rising just before the Sun, they knewthat the "Nile Days" were at hand. Its annual reappearance was awarning topeople wholived along the Nile River.

Thestar always returned just before the river rose, and so announced thecoming offloodwaters, which would add to the fertility of their lands. Peoplethenopened the gates of canals that irrigated their fields.

Seriousabout star Sirius

Priests,who were the calendarkeepers, sighted the first rising of the DogStarfrom their temples. At the temple of Isis-Hathor at Denderahis a statue ofIsis, which is located at the end of an aisle lined by tallcolumns.

Ajewel was placed in the goddess?forehead. The statue was oriented to the rising of Sirius, so that thelightfrom the returning Dog Star would fall upon the gem. When the priestssaw thelight of the star shining upon the gem for the first time, they wouldmarchfrom the temple and announce the New Year.

Inthe temple appears theinscription: ?Her majesty Isis shines into the temple on NewYear?s Day, andshe mingles her light with that of her father Ra on thehorizon.?

Thisweek, justbefore sunrise,Sirius might again be glimpsed rising just above the southeast horizonforthose living in mid-northern latitudes. At more southerly latitudes,Sirius isalready conspicuous, twinkling above the horizon at dawn.

Siriusis thebrightest star of the constellationCanis Major, the ?Greater Dog? in Latin. According toBurnham?s CelestialHandbook other names for it include ?The Sparkling One? or ?TheScorching One.?

Thestar appearsa brilliant whitewith a tinge of blue, but when the air is unsteady, or when it is lowto thehorizon as it is now, it seems to flicker and splinter with all thecolors ofthe rainbow. At a distance of just 8.7 light years, Sirius isthefifth-nearest known star. Among the naked-eye stars, it isthe nearest of all,with the sole exception of Alpha Centauri.

The Brightest Stars in the Sky

Gallery - Stars and Galaxies

Beginner Astrophotography Telescopes

JoeRao serves as an instructorand guest lecturer at New York's Hayden Planetarium. He writes aboutastronomyfor The New York Times and other publications, and he is also anon-camerameteorologist for News 12 Westchester, New York.