Al-Jauza

Betelgeuse is one of the best known star names in the sky and it is a mistake. The Arabic is yad al-jauza, the hand of al-Jauza; medieval transmission misread the initial ya as a ba, and the corrupted form entered European catalogues and stayed. Rigel is cleaner: rijl al-jauza, her foot.

Al-Jauza is a female figure of the pre-Islamic Arabian sky occupying the stars Greece made a hunter. The name is generally connected with a root meaning the middle, and she was understood as standing across the middle of the sky or as the central one among her people. There is a second figure, al-jauza of the sky and al-jauza the sister, and the accounts vary.

What survives of her is the two star names, and they survive because Arabic astronomical writing was the channel through which Ptolemy reached medieval Europe. Translators took the coordinates and the Greek figures and, star by star, the Arabic descriptive names, and the descriptive names stuck even where the figure they described was discarded.

The sky is full of this. Deneb is the tail, Fomalhaut the mouth of the fish, Altair the flying eagle, and in each case the name is a body part of a figure the modern list either kept or lost. Al-Jauza is the clearest case of a figure entirely lost and thoroughly named.

Abd al-Rahman al-Sufi finished his Book of the Fixed Stars around 964, and it is one of the most important astronomical works ever written for a reason that is easy to miss. It is not a translation of Ptolemy, it is a critical revision: al-Sufi checked the magnitudes and positions against his own observation, corrected them, and illustrated every constellation twice, once as seen in the sky and once as seen on a celestial globe, that is, mirrored.

It also does something no Greek source does. For each Ptolemaic constellation al-Sufi records the indigenous Arabian names of the stars within it and the native figures they belonged to, setting the two systems side by side without collapsing one into the other. That is why the older sky is recoverable at all.

He is also the earliest known observer to record the Andromeda galaxy, which he describes as a little cloud, and to note the Large Magellanic Cloud from reports of southern observation.

The book was copied and illustrated for centuries, and the star names it carried entered European use through Latin and Castilian translations. When a modern astronomer says Aldebaran, Algol, Rigel or Betelgeuse, the chain of transmission runs through this book, and roughly two thirds of the proper names of bright stars in current use are Arabic.