A Persian astrolabe made in AD 1712–13 by ‘Abd al-A’imma, one of the finest and most prolific astrolabe makers. For more information about this instrument and a larger image, click the picture.
(Museum of the History of Science, Oxford)
ASTROLABES are a prime source of star names from the days before printed star charts. At its simplest, an astrolabe consists of a flat base plate overlain by a rotating mask called the rete, Latin for net (or ‘ankabut in Arabic, meaning spider); pointers on the rete indicated the positions of prominent stars. Rotating the rete would show the positions of the stars at any given date or time, in the same way that a modern planisphere does. However, one significant difference from a modern planisphere is that an astrolabe bears a mirror-image of the sky, as on a celestial globe, rather than the sky as it appears to an observer on Earth. The pivot point of the rete is the celestial pole.
Anywhere from a handful of stars to dozens could be included on the rete. In many cases, the pointers or their supports were engraved with the names of the stars, which could be written in Arabic or Latin depending where and by whom the astrolabe was made. As there was no standard source of star names, different makers used different names and different spellings. While many of the names are familiar to us today, others are more puzzling.
For example, on this astrolabe from around 1500, thought to have been made in Italy, the names Vega and Aldebaran are easily recognized (click on the first of the small pictures in the second row of the linked page to see an enlargement). Alkair, Alfeta, Mirac, Razdalgeti, and Sceat are obvious enough variants of Altair, Alphecca (Alpha Coronae Borealis), Mirach (Beta Andromedae), Rasalgethi (Alpha Herculis), and Scheat (Beta Pegasi). But what are Alhabor at the bottom, Alhaiot just below centre, and Alramek and Azimech at the right? These are Arabic-derived names for the stars we know as Sirius, Capella, Arcturus, and Spica. Such Arabic names were commonly used on European astrolabes until about the 15th century, when the classical Greek and Roman names were reintroduced. In addition, Algomeisa at the bottom of this astrolabe is Procyon; the name Gomeisa has since been transferred to Procyon’s neighbouring star Beta Canis Minoris.
In some cases the pointers consisted of little pictorial icons called zoomorphs representing the star they pointed to. On this 14th-century example, thought to be English, the position of Sirius, the Dog Star in Canis Major, is indicated by a pointer on the rim in the shape of a dog’s head –presumably the dog’s tongue originally poked out at the star but has since been broken off. A dragon’s head on the rim points to Antares, the heart of the Scorpion, here given the Arabic-derived name Alacrab; while closer to the centre a small bird points out Vega, spelled Wega.
Looking at the extraordinary range of inscriptions on the retes of astrolabes, it’s easy to see how so many star names became corrupted. Mis-spellings and odd abbreviations were common.
One feature to note on the retes is the circle marking the ecliptic, engraved with the names of the zodiacal constellations.
For much, much more about the construction and use of astrolabes, see the late James Morrison’s highly informative site The Astrolabe (now archived at the Wayback Machine).