Polaris

Polaris is Alpha Ursae Minoris, at about 447 light years by the Gaia measurement, a yellow supergiant of spectral type F7Ib with two smaller companions, a triple system.

It is a Cepheid variable, and this is its principal astronomical importance. Cepheids pulsate with a period tightly related to their intrinsic luminosity, which makes them standard candles for measuring distances to other galaxies; Polaris is the nearest and brightest Cepheid, so its distance matters disproportionately, and it has been contentious, Hipparcos and Gaia measurements have not always agreed well with other methods. Its pulsation period is about four days, and the amplitude of its variation declined markedly through the twentieth century before partially recovering, which is not fully explained.

The important correction concerns the pole. Polaris is close to the north celestial pole now, about 0.66 degrees in 2018, and it will be closest, at around 0.45 degrees, shortly after the year 2100. It has not always been there and will not remain.

In classical antiquity the pole was about ten degrees from Polaris, and was in fact slightly closer to Kochab, Beta Ursae Minoris. Ptolemy did not have a pole star in any useful sense, and neither did the Greeks who named the constellations. Ancient navigation by the north used the constellation and a judgement of the pole's position within it, not a single marker star. The near-coincidence that makes Polaris practically useful is a temporary condition of the last several centuries, arriving through precession and departing the same way.

Around 2700 BC the pole star was Thuban in Draco; around AD 13,700 it will be Vega.

Polaris is given the nature of Saturn and Venus, with the Saturn half carrying sickness, loss and trouble over inheritance, and the reading is notably grim for a star whose modern significance is entirely positive.

The disproportion is the interesting thing here. Polaris is, for most people, the single most culturally loaded star in the sky, the fixed point, the guide, the constant in Shakespeare and in a thousand metaphors, the practical instrument of navigation in the northern hemisphere. Almost none of that appears in its astrological delineation, which is minor, gloomy and concerned with illness and legacies.

The explanation is the precessional one. Polaris was not the pole star when the fixed-star tradition was being formed. Ptolemy's pole was ten degrees away, nearer Kochab. The star acquired its overwhelming symbolic significance only as precession carried the pole toward it through the medieval and early modern period, by which time the astrological attributions were already fixed and being copied forward. The tradition inherited a delineation for an ordinary circumpolar star and never revised it when the star became the pole.

This is a clean demonstration of how the fixed-star literature actually behaves: it is a copying tradition. Attributions are transmitted, not re-derived, and they do not update when the sky changes. Anyone who supposed the delineations were being freshly observed would expect Polaris to have acquired guidance, constancy and direction as significations. It did not.

Polaris is not among Agrippa's Behenian fifteen. In 1533 it was still some three degrees from the pole and less remarkable than it now is.