What are fixed stars in astrology?

In astrology the fixed stars are the stars proper, as opposed to the planets, which wander. The idea is ancient: Ptolemy gave them a chapter in the second century. Here is what he wrote, and why the stars were never quite fixed.

Fixed and wandering

The word planet comes from the Greek for "wandering stars" (opens in a new tab): the planets were named for their motion against the stars that seemed to stay put. Those were the fixed stars, and astrology kept the name.

Ptolemy's chapter

In the Tetrabiblos, book I, chapter 9, "Of the Power of the Fixed Stars" (opens in a new tab), Ptolemy describes the stars group by group, and gives each a nature by comparing it with the planets. In F. E. Robbins's translation, the stars in the head of Aries act like Mars and Saturn mingled; Antares, "tawny and rather bright", like Mars and in some degree Jupiter; the stars of Lyra like Venus and Mercury; the bright star in the Dog's mouth, Sirius, like Jupiter and, less, Mars; and the stars on Orion's shoulders like Mars and Mercury.

How to read that

Ptolemy's method is analogy: a star is described as a blend of planets. This page reports what he claimed. It is not evidence that any star acts on anyone, and Ptolemy's list is his, not a universal key.

The fixed stars move

Two things move the stars against the frame astrologers measure them in. The first is the stars' own motion through space: by this site's calculation from the Bright Star Catalogue, Arcturus has moved about 1.2 degrees since Ptolemy wrote, and Alpha Centauri more still.

The second is the slow wobble of Earth's axis, a cycle of about 25,771 years (opens in a new tab), which slides every star's position in the tropical zodiac forward by about a degree every 72 years: by that arithmetic, some 26 degrees since the second century. A table of fixed-star positions has to be dated to mean anything.

Reflect