Vertex

Vertex is the name the older astrological star lists give to the position of the Great Andromeda Nebula, catalogued as M31, which is not a star at all but the nearest large spiral galaxy to our own, some two and a half million light years away. It was recorded as a nebulous patch by the Persian astronomer al-Sufi in the tenth century and remained a nebula in the catalogues until the 1920s, when Edwin Hubble resolved Cepheid variables within it and established that it lies far outside the Milky Way, a discovery that enlarged the known universe at a stroke. The astrological tradition, working from lists compiled long before that, treats it as a fixed point of the constellation of the chained princess and gives it the general character of a nebulous object, which in that tradition is read cautiously and with reference to the eyes.

Al-Sufi's tenth-century description in his Book of Fixed Stars calls it a "little cloud", and it is very likely the earliest surviving written record of the galaxy, predating the first known telescopic observation by more than six centuries. The Andromeda Galaxy is, at roughly two hundred and twenty thousand light years across, considerably larger than the Milky Way, and the two are approaching each other at around one hundred and ten kilometres per second, on a trajectory current modelling suggests will bring them into a slow, disruptive merger roughly four billion years from now, long after the Sun has left the main sequence.

Sources The Arabs and the StarsPaul Kunitzsch with AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax)

The name is a source of genuine confusion, because modern astrology also uses Vertex for something entirely different: a calculated point in the birth chart, the western intersection of the prime vertical with the ecliptic, introduced in the twentieth century and read as a point of encounter or fated meeting. The two have nothing to do with one another. One is a fixed position in the constellation of Andromeda, recorded in the star catalogues at a stated ecliptic longitude that shifts slowly with precession; the other is an angle computed from the birth time and geographic latitude and moving through the whole zodiac in a day. Where a chart or a reference lists both, the distinction is worth checking, since the interpretive traditions attached to them are unrelated.

The astrological Vertex was introduced into English-language practice largely through the twentieth-century American astrologer Charles Jayne, who popularised it as a point of fated encounter distinct from anything in the classical apparatus of houses and angles; it has no equivalent whatsoever in Hellenistic, medieval, or Renaissance technique; and because it is calculated rather than catalogued, it has no fixed position among the stars and instead sweeps around the entire zodiac roughly once a day as the Earth turns, the geometry that makes it, unlike a fixed star, different for every birth time even at the identical location.