Regulus

Regulus is Alpha Leonis, apparent magnitude 1.35, at about 79 light years, and it is one of the more physically extreme bright stars.

It rotates extraordinarily fast. The equatorial surface velocity is around 320 kilometres per second, which is roughly 96 percent of the speed at which the star would break up. The consequence is a markedly oblate shape, the equatorial diameter substantially exceeds the polar, and a phenomenon called gravity darkening: the poles are hotter and per unit area about five times brighter than the equator. Regulus is not a sphere and it is not uniformly bright.

It is a quadruple system: Regulus A, itself a spectroscopic binary whose faint companion is probably a white dwarf, together with the more distant pair Regulus B and C.

The fact of greatest astrological consequence is positional. Regulus lies 0.465 degrees from the ecliptic, closer than any other first-magnitude star. It is therefore occulted by the Moon regularly, and it sits almost exactly on the path the Sun and planets follow. A star that the Sun effectively passes over every year, and that the Moon repeatedly covers, is going to be noticed by any tradition watching the ecliptic, and Regulus is prominent in Babylonian, Greek, Persian, Indian and Arabic astronomy alike.

The Latin Regulus, little king, is Copernicus's rendering of the Greek Basiliskos; the Babylonian name means the star of the king, and the Arabic Qalb al-Asad and the Latin Cor Leonis both mean the Lion's heart.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy

Ptolemy gives Regulus the nature of Mars and Jupiter, and the tradition that follows reads it as the most kingly of the fixed stars: power, high office, ambition realised, martial honour, with the standing caveat that its gifts are lost through pride or revenge and end in a sudden fall.

The two components divide cleanly. The Mars half supplies the courage and the military honour, which is the classic profile of a royal significator in a world where kingship meant commanding armies. The Jupiter half supplies the rulership itself, the office and the elevation. The caveat about the fall is not Ptolemy's and belongs to the later tradition, principally Robson.

Regulus is one of Agrippa's Behenian fifteen, paired with garnet and with mugwort, a specific talismanic correspondence with a named Renaissance source, and a rare case where a star, a stone and a plant are all documented as a set by one author. It is also counted among the four Royal Stars, as the Watcher of the North, in the scheme discussed under Aldebaran and best treated as unsourced.

There is a genuine ancient layer beneath all of this. Regulus is the king star in Babylonian celestial omen literature, where its significance is for the state and the monarch rather than for individuals, and that is the oldest attested royal association it has. The star's kingly character is therefore about as well established as any in this material (it appears independently in Mesopotamian, Greek, Persian and Indian traditions), even though the specific delineations printed today are modern.

Regulus is the best available demonstration of why precession matters to this material, because it crossed a boundary within living memory.

The earth's axis wobbles with a period of about 25,800 years, which causes the equinoctial points to slide westward along the ecliptic at roughly 50 arcseconds a year, one degree in about 72 years. Tropical zodiacal signs are measured from the equinox, so the fixed stars drift steadily forward through them. This is not a theory; it was detected by Hipparchus in the second century BC, and he detected it using Spica.

The recorded position of Regulus in this collection is 29 degrees 50 minutes of tropical Leo, an epoch-2000 figure. That leaves ten arcminutes to the boundary of Virgo, which at one degree per seventy-two years is about twelve years. Regulus accordingly entered tropical Virgo around 2012, and it is there now. A star that had been the heart of the Lion, in the sign of the Lion, for the whole recorded history of Western astrology, is no longer in that sign.

Tropical and sidereal astrologers weigh this very differently, and the disagreement is genuine rather than a misunderstanding. Sidereal systems measure from the stars, so Regulus never moves relative to the sign and nothing has happened. Tropical systems measure from the equinox, so the star has changed signs and some tropical practitioners regard the ingress as significant while others treat fixed stars by conjunction to natal points and consider the sign irrelevant.

The general point is unavoidable, whatever one's position: a medieval attribution of a star to a degree of the zodiac refers to a different ecliptic longitude than the same star occupies now, by roughly fourteen degrees since Ptolemy.