Sirius Ruddy

Books VII and VIII of Ptolemy’s Almagest (~150 CE) addresses the positions and motions of the fixed stars, including precession of the equinoxes, a 25,800-year cycle of the earth’s poles movement against the stars.. They also contain a star catalog of 1022 stars, described by their positions in the constellations. The brightest stars were marked of the first magnitude (m = 1), while the faintest were of sixth magnitude (m = 6), the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered to be twice the brightness of the following grade (a logarithmic scale).

The brightest star in this catalog is Sirius, the Dog Star. Ptolemy (90-168 CE) assigned his celestial coordinate system, our current equivalent of the Greenwich Meridian, starting at Sirius. In 1718, Edmond Halley (1656-1742 CE) used Ptolemy’s star positions to determine that in 1500 years, Sirius had moved 30 arc minutes, the approximate diameter of the moon, southward. Ptolemy’s star catalog was that good.

The controversy starts because Ptolemy states Sirius is one of six red stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux. Aratus, Cicero, Germanicus, and Seneca also commented on Sirius’ redness, with Seneca even describing it as being redder than Mars. Today, five of the six are still red, but Sirius is a blue white star, with a known white-dwarf companion. The white dwarf was a red giant at one time after being the more massive blue-white star of this binary system, but the red giant stage is thought to be 120-million years ago, and progression from red giant to white dwarf thought to take 100,000 years. Thus, the suggestion that Sirius B, the white dwarf, was a red giant two thousand years ago, is considered highly improbable.

Interesting controversy, but I’ve seen only this one explanation of how Ptolemy, a professional astronomer/observer, could be right.