[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¡ªand notorious¡ªdiscussion 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¡ªit looks a
deep yellow to me¡ªbut 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 ¨C 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 ¨C 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¨¢, the
¡°Arrow.¡± At the time the Sumerians gave ¦Á Canis
Majoris this name, around 3000 BC, stone arrowheads had just been
superseded by ruddy-colored copper
arrowheads. Alternatively, a ruddy ¦Á 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 (¦Ê-¦Å-¦Ò-¦Ä-¦Ó-¦Ç CMa
plus ¦Î-¦Ñ Puppis) is so well-defined and conspicuous, and ¦Á
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¨¢
when it stands at sunrise¡±¡ªin 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¨¢;
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¡ªfar 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¡ªbut 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¡ã
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¡ªbut ¡°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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