Ursa Minor, the Phoenicians' Cynosure and the Pole Star

Beside the great bear, a younger sister turns in the sky, smaller and less bright, yet of even greater practical importance. At the tip of its tail shines the Pole Star, the almost motionless point around which the whole celestial vault seems to revolve. For ancient and modern navigators Ursa Minor has been a compass written in the stars, and its story weaves together astronomy, trade and myth.

Two bears for two seafaring peoples

Aratus of Soli, in the Phaenomena of the third century BC, observes that the Greeks steered by Ursa Major, which they called Helice, while the Phoenicians relied on Ursa Minor, called Cynosura. The latter, the poet adds, is smaller but more reliable, because it turns more tightly around the pole. The remark reflects the reputation of the Phoenicians as the most skilled navigators of the ancient Mediterranean.

Diogenes Laertius, in the Lives of the Philosophers, reports a tradition going back to the poet Callimachus according to which it was Thales of Miletus, in the sixth century BC, who made the little bear known to the Greeks, having learnt of it from the Phoenicians themselves. Whether Thales really introduced it cannot be verified, but the story shows how the Greeks themselves acknowledged an Eastern origin for this nautical knowledge.

The nurses of Zeus

Greek myth linked the two bears to the childhood of Zeus. According to the version reported by Aratus, the infant god was hidden on Mount Ida, in Crete, to keep him from his father Cronus, who devoured his children. Two nymphs reared him, Helice and Cynosura, and Zeus later, out of gratitude, placed them in the sky in the form of bears. Other authors instead associated Ursa Minor with Arcas, the son of Callisto.

The name Cynosura means in Greek “dog's tail”, perhaps because of the elongated shape of the figure. It is a name with a long history: in English the word cynosure still denotes something that draws every eye, just as the pole star drew the eyes of pilots. In medieval Italy the Pole Star was also called the “tramontana”, a name that later passed to the north wind.

The star that changes

The Pole Star has not always been the star of the pole. Because of the precession of the equinoxes, the Earth's axis traces a slow circle in the sky, and the celestial pole moves accordingly. In the time of Aratus and Hipparchus the pole did not coincide with any bright star, and Polaris lay several degrees away from it. Today it lies less than a degree away, and its closest approach will come towards the end of the twenty-first century.

The two brightest stars in the bear's body, Kochab and Pherkad, are sometimes called the “guardians of the pole”, because they seem to stand guard as they circle the centre of the sky. In a few thousand years, again because of precession, the role of pole star will pass to other stars, and in the distant future it will fall to bright Vega, in Lyra.

Ursa Minor thus tells a story of divine gratitude and practical knowledge. The nurses who saved the infant Zeus become the guides of sailors, and an observation attributed to the Phoenicians turns into the common heritage of all seafarers. Even today anyone who looks north finds that small star which, for centuries, has made it possible not to lose one's course.