Canis Minor

Canis Minor's brightest star, Procyon, rises just ahead of Sirius and gave the constellation its name, before the dog. A second, invisible star travelling with it was worked out mathematically decades before anyone actually saw it.

In 1844 the German astronomer Friedrich Bessel noticed that Procyon's motion across the sky was not perfectly straight but wobbled slightly, as if something unseen were tugging on it, and used the same method that same year to predict an unseen companion for Sirius as well. A faint white dwarf companion, Procyon B, was eventually observed directly by American astronomer John Schaeberle at Lick Observatory in 1896, more than fifty years after Bessel's prediction, confirming that the wobble was real and that a dead, collapsed star was orbiting close beside the brighter one visible to the naked eye.

The delay between prediction and observation is itself part of the story: Bessel worked entirely from small, precise position measurements taken over years, without ever seeing the object his mathematics required to exist, and the confirming telescope had to be considerably more capable than anything available when he made the prediction.

Sirius's own companion followed the identical pattern and was found first, in 1862, but Procyon's case shows the method was not a one-off: the same reasoning applied to two different stars produced two correct, unseen predictions.

Canis Minor, the Lesser Dog, is a small constellation, one of Ptolemy's forty-eight, representing the second of Orion's two hunting hounds, set east of the greater dog near the edge of the Milky Way. It is dominated by the bright star Procyon, one of the nearest stars to the sun, whose name means before the dog, because it rises just ahead of Sirius, the Dog Star. With Sirius and Betelgeuse, Procyon forms the prominent Winter Triangle. In myth the two dogs accompany the hunter Orion across the sky. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Canis Minor as the classical and modern traditions describe it, distinct from any astrological use of the region; despite its small size it holds one of the brightest and closest of stars.

The blank stretch of sky between Canis Minor, Orion and Hydra, left entirely unfigured by Ptolemy, was filled only in the early seventeenth century by Monoceros, the unicorn, one of several constellations introduced in this period specifically to give a name to faint stars that the ancient tradition had judged not worth marking. Procyon's closeness and brightness made it, like Sirius in the neighbouring Greater Dog, a useful marker of the seasons in several independent traditions, though it never carried the calendrical weight in Egypt that Sirius's heliacal rising did for the flooding of the Nile.