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by Craig Crossen 10/21/09 | Email Author

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[Author*s Note: The following is a slightly abridged version of a chapter of a book in progress about the constellations and star-names of Greece and Mesopotamia. The problem of the apparent color change in Sirius between ancient times and the present was first raised about 250 years ago and has gotten worse instead of better because of the findings and hypotheses of modern astrophysics. The observations of the Graeco-Roman authors quoted below have been long known; but the evidence from ancient Mesopotamia is seldom offered. The most famous〞and notorious〞discussion of the matter was by the controversial American astronomer T. J. J. See in the 1926 volume of the German periodical Astronomische Nachrichten.]

One of the most vexing problems of the history of astronomy is the fact that, though today Sirius is conspicuously blue-white in color, every Classical writer that says anything at all about the matter describes the star in no uncertain terms as red or ruddy. First, Ptolemy, at the end of his star-catalogue in Books VII-VIII of the Almagest (written c 140 AD), describes Sirius as hypocirros, ※fiery red.§ This shocking statement might be dismissed as a copyist*s error (after all, the earliest surviving text of the Almagest was written hundreds of years after the original), or even as a slip-of-the-pen by Ptolemy himself except for the fact that in the same section of the Almagest Ptolemy describes five other stars as ※fiery red§: Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse. One might quibble with the inclusion of Pollux in this list〞it looks a deep yellow to me〞but it certainly is not merely white, let alone bluish.

If Ptolemy was alone among ancient authors in calling Sirius red, we still could say that the great astronomer, or one of his copyists, had been caught napping. Unfortunately other ancient writers on astronomy saw Sirius in the same light as Ptolemy.

In his description of Sirius (Phaenomena 326-34), the Greek poet Aratos (early 3rd century BC) uses the term poikilos, for the star. This word is somewhat ambiguous, and has been variously translated as ※brightly colored§ or ※scintillating§ or ※variegated.§ To help us out, fortunately we have two ancient translations into Latin of the Phaenomena by competent literary men, the orator and statesman Cicero (106-43 BC), and Germanicus Caesar 16/15 BC 每 19 AD), Rome*s greatest general under Augustus. Cicero, in his translation of Phaenomena 326-34, writes, rutilo cum lumine claret fervidus ille Canis, ※with ruddy light fervidly glows that dog.§ Germanicus, in a rather freer rendering of the passage, nevertheless uses the term rutilis, and says that the Dog*s mouth (marked by Sirius) ※vomits flame§.

In his Natural Questions (I i 6), the Stoic philosopher Seneca (4 BC 每 65 AD), another master of Latin prose, compares the relative redness of three specific celestial bodies, writing, [I]n coelo quoque non unus appareat color rerum , sed acrior sit caniculae rubor, Martis remissior, Jovis nullus: ※[I]n the heavens also there does not appear one color of things, but the redness of the Dog Star is deeper, that of Mars milder, that of Jupiter nothing at all . . .§ Seneca is certainly right about the color of Jupiter, which has no ruddiness whatsoever, and about the color-contrast between Jupiter and Mars, so he sounds like a reliable witness concerning the redness of Sirius at that time.

Of the ancient astronomers besides Ptolemy, only Geminos of Rhodes (early 1st century BC) says anything about Sirius* color. But his comments support Ptolemy. In his remarks criticizing the popular belief that the heat of the ※Dog Days§ of summer is the result of the combination of Sirius* light with that of the Sun, Geminos states, ※For this star is of the same nature as all the other stars. And whether the stars be fiery, or whether they be merely bright, they all have the same power.§ Geminos uses the word pyrina, ※fiery,§ to describe Sirius, which is comparable to Ptolemy*s hypocirrus, ※fiery red.§

The natural philosophers Columella (4-70 AD) and Pliny (23-79 AD) also describe Sirius* color as reddish in no uncertain terms. In writing about roses Columella compares their color to Tyrian purple, the rising Sun, Sirius, and Mars. (De Cultu Hortorum X 286.) Pliny*s monumental Natural History is very long, and includes many astronomical facts, but he calls only three celestial objects ardens or igneus: the rising Sun, Mars, and Sirius. (Nat. Hist. II 18 47.)

Indirect evidence concerning the ancient redness of Sirius comes from the Roman fertility festival of the Floralia, which was held in May during the Dog Star*s helical setting. According to the Roman grammarian Sextus Pompeius Festus (late 2nd century AD), one of the rites of that festival was the sacrifice of ruddy dogs to the Dog Star. The sacrifice of dogs to the Dog Star by the early Greeks is mentioned by Apollonius Rhodius (295-215 BC) in his Argonautica (II 516-27), though he does not explicitly state that the sacrificial victims were ruddy. In both cases the sacrifices were considered necessary to avert the evil influences of what was considered a most sinister star.

Mesopotamian astronomy also provides indirect evidence for the ancient redness of Sirius. The evidence is mostly in the Sumerian name for the star: Kak-si-s芍, the ※Arrow.§ At the time the Sumerians gave 汐 Canis Majoris this name, around 3000 BC, stone arrowheads had just been superseded by ruddy-colored copper arrowheads. Alternatively, a ruddy 汐 CMa might have suggested the blood of an enemy on an arrowhead. However, the Sumerian constellation of the ※Bow§ in southern Canis Major and northern Puppis (百-汍-考-汛-而-灰 CMa plus 缶-老 Puppis) is so well-defined and conspicuous, and 汐 CMa is oriented so suggestively with respect to it, that the star probably would have ended up being named ※The Arrow§ whatever its color.

As a weapon, the Sumerian celestial Arrow naturally was associated with the old Sumerian war-god Ninurta. From the Royal Library of Ashurbanipal (668-626 BC) in the ruins of his palace at Nineveh, has survived a text that calls itself a ※Prayer to Kak-si-s芍 when it stands at sunrise§〞in other words, as it heliacally rose. In its opening line it addresses ※Ninurta, mighty one, heroic god, prince of the Anunnaki, commander of the Igigi.§ (The ※Anunnaki§ and ※Igigi§ were groups of gods; in fact ※Igigi§ often referred to the stars and constellations.) In this prayer Ninurta is called a ※consuming fire, burning the wicked . . . Whose title in heaven is Kak-si-s芍; among the host of the Igigi mighty is thy [tablet broken]§. It will be remembered that Ptolemy and Geminos both described the ruddiness of Sirius as ※fiery.§ The association of a red star with the god of war is natural, even inevitable. Antares and Mars were also associated with war in both Greece and Mesopotamia.

Thus the evidence is overwhelming that Sirius looked red from the beginning of recorded history down at least to the end of Classical antiquity (4th century AD. Medieval Arabian texts say nothing about the star being ruddy.) The problem is how the star could have changed from ruddy to blue-white in little more than a millennium and a half〞far too short a time for most astrophysical processes to have effected such a radical change. (Faster processes usually end up with a demolished star.) Sirius is an A1 main sequence object and as such went through an early evolutionary phase during which it was a ruddy pre-main sequence star〞but that, according to theory, must have been tens of millions of years ago. An early suggestion was that perhaps during ancient times Sirius was passing through a thick cloud of interstellar dust that reddened it. But to have reddened the star so much, such a cloud would have seriously dimmed it as well, and Ptolemy*s star-catalogue makes it clear that Sirius was every bit as bright then as it is now. And in any case radio astronomy has detected no heavy dust in the neighborhood of Sirius. Indeed, the direction of Sirius is so poor in interstellar dust that the open cluster M41, located just 4∼ to its south, is dimmed only 0.2 magnitude by dust even though it is 2300 light-years away.

The best hope for an explanation of Sirius* dramatic change in color from ancient times to the present is from its famous white dwarf companion. Here is a star that in the astrophysically-recent past was a red giant〞but ※astrophysically-recent past§ means a whole lot more than 1500 years ago! Moreover, in evolving from a red giant to a white dwarf a star ejects most of the matter in its envelope (usually in the form of a planetary nebula), and in the case of Sirius that material would still be in the star*s vicinity. But no such residual of matter ejection has been detected around Sirius. Furthermore, had Sirius B been a red giant 2000 years ago, the Dog Star would have been several times brighter than the ancients reported it to be.

The sheer number of ancient authorities who testify to the ruddiness of Sirius during their time deny us the luxury of writing this puzzlement off to ※copyist error.§ And to say that people of the intellectual caliber of Ptolemy, Geminos, Seneca, Pliny, Columella, Cicero, and Germanicus, writers whose statements on other matters are consistently accurate and reliable, could all have been deceived about the color of the brightest fixed star in the sky would be absurd. The only alternative is that there is something wrong with our astrophysics. But is that so very improbable? Who would claim that modern astrophysics, whatever its successes, is error-free? Our understanding of the universe in general, and of the stars in particular, is still very tentative. We must always keep our hearts and our minds open. Only by considering ALL the possibilities when confronted by a problem like that of the color of Sirius will we continue to make intellectual and scientific progress.

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