Aldebaran

Aldebaran is Alpha Tauri, and the astronomical facts are settled and checkable.

It is an orange giant of spectral type K5 III, apparent magnitude 0.86, at a distance of about 65 light years. It has exhausted core hydrogen and expanded to roughly forty-four times the diameter of the Sun; its surface is cooler than the Sun's, which is why it is orange, and it is nonetheless far more luminous because of its size. A substellar or planetary companion has been proposed on radial-velocity evidence and is not firmly established.

The star is a line-of-sight coincidence with the Hyades cluster, which forms the V of the Bull's face behind it. The Hyades lie at about 150 light years; Aldebaran is less than half that distance and is not a member. The apparent association that gives the constellation its shape is an accident of perspective.

Aldebaran lies close to the ecliptic and is therefore subject to occultation by the Moon, which occurs in series separated by long intervals as the lunar orbit precesses. Such occultations were recorded in antiquity and are useful for dating: an occultation of Aldebaran observed and recorded fixes both a date and a place.

The Arabic name al-dabaran means the follower, from its rising after the Pleiades. The star's redness is the physical basis for the astrological tradition attached to it, and that redness is real: a K5 giant is genuinely orange-red to the eye, as Antares and Betelgeuse are.

Aldebaran is also the star the Pioneer 10 probe is nominally headed toward. It will not arrive for something on the order of two million years, and by then neither the star nor the probe will be where the calculation assumed, since both are moving. The claim is a useful shorthand rather than a destination.

Its brightness varies slightly and irregularly, by a few hundredths of a magnitude, which is typical of red giants and is not detectable by eye.

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

Ptolemy assigns Aldebaran the nature of Mars in the Tetrabiblos, Book I chapter 9, where he goes through the notable fixed stars and gives each a planetary temperament. The reasoning for Aldebaran is not stated but is evident from the pattern: Ptolemy consistently reads the visibly red stars as martial, and gives the same treatment to Antares and to the red shoulder of Orion.

This is worth stating precisely because it is the strongest form of attribution available in this material. Ptolemy is a named author, in a datable second-century text that survives, making an explicit assignment. Most fixed-star attributions in circulation do not have that. Where a star has a Ptolemaic nature, it should be cited as Ptolemy's; where it does not, the source is later and usually much later.

The elaborated readings (honour, courage, eloquence and high office, held only through integrity, with a violent fall for those who fail), are not in Ptolemy. They come from the later compiling tradition, and in English overwhelmingly from Vivian Robson's The Fixed Stars and Constellations in Astrology of 1923, which assembled Ptolemy, Arabic material, Renaissance sources and nineteenth-century writers into a single reference. Robson's book is the proximate source of almost every fixed-star delineation printed in English since, and it is usually quoted without attribution.

Robson was a compiler and did not always distinguish his sources. Reading his entries as ancient tradition, when parts of them are Victorian, is the standard error with this material.

Aldebaran is one of Agrippa's fifteen Behenian stars, paired with ruby or garnet and with milk thistle.

Aldebaran is routinely called one of the four Royal Stars of Persia, the Watcher of the East, with an archangel attached. This scheme deserves more scepticism than it usually receives.

The four are given as Aldebaran, Regulus, Antares and Fomalhaut, and the appeal is obvious: they are bright, widely spaced, and were once near the four cardinal points of the year. The usual claim is that they marked the solstices and equinoxes for the Persians around 3000 BC, which is arithmetically in the right area, precession would have placed them near those points at some period, and this is the strongest part of the case.

The weak parts are numerous. There is no securely identified Persian text setting out four royal stars in this form. The four stars are not equally spaced; Aldebaran and Antares are close to opposite, and Regulus and Fomalhaut are not well placed against them. The set does not correspond to the Babylonian material, which is far better documented and organised quite differently, as Rochberg and others have set out. And the assignment of the archangels Michael, Raphael, Uriel and Gabriel to the four is not ancient Persian at all, it is a Western esoteric elaboration, and the pairing varies between sources, which is what one expects of a modern invention rather than a transmitted tradition.

The scheme is also inconsistent with the better-attested one. Agrippa's Behenian fifteen include Aldebaran, Regulus and Antares but not Fomalhaut, which is difficult to explain if a four-star royal set were a live tradition in the Renaissance.

The honest description is that the Royal Stars are a real and old-feeling idea with a genuine astronomical basis and no identified ancient source, considerably less well founded than Ptolemy's attributions and presented with far more confidence.