The Andromeda Galaxy and the Double Cluster in al-Ṣūfi’s Book of the Fixed Stars
In his Book of the Fixed Stars, a revised and updated version of Ptolemy’s Almagest written c. AD 964, the Persian astronomer Ἁbd al-Raḥmān al-Ṣūfī (903–986) made the first recorded mention of what we now know as the Andromeda Galaxy, M31. At that time, of course, he did not know what it was and described it simply as al-latkhā al-sahābiya, the nebulous smear, in reference to its smudgy appearance to the naked eye. Surprisingly, it was not mentioned by Ptolemy or any of the other ancient Greek astronomers.
Unlike Ptolemy’s Almagest, al-Ṣūfī’s book contained illustrations of each constellation, and these varied from copy to copy. What is thought to be the oldest existing version of the book is the one known as manuscript Marsh 144 in the Bodleian Library, Oxford, dating from AD 1009-10. In addition to illustrating Andromeda as described by Ptolemy (albeit in Arabic clothing), the manuscript includes two other illustrations of Andromeda that incorporate Arabic constellation tradition. It is on the first of these illustrations that the Andromeda Galaxy appears.
The first fish and the Andromeda Galaxy
The ancient Arabs visualized two fish in this part of the sky, different from the pair of fish in Pisces but perhaps with a common root in forgotten times. The larger fish faces north, overlapping the body of the Greek Andromeda. In its mouth is a cluster of dots, indicating the Andromeda Galaxy. Most of the stars in this Arabic fish constellation are quite faint, and the Andromeda Galaxy is clearly meant to be part of the figure, so knowledge of this nebulous patch must go back long before al-Ṣūfī – he was simply the first to record it.
Even though ancient Greek astronomers such as Ptolemy mentioned several other nebulous objects including the Double Cluster in Perseus and the Beehive Cluster in Cancer, they never mentioned the Andromeda Galaxy. It should have been easily visible to the Greeks, as it was to the Arabs, so why they overlooked it is unexplained.
The second fish and the Double Cluster
The second, and smaller, of the two Arabic fish faces south, covering the feet of Andromeda, as shown in the illustration below (north is to the left). In the tail of this fish is another cluster of dots indicating a nebulous object, this time the Double Cluster in neighbouring Perseus, although shown as a single object rather than two. Ptolemy recorded this in the Almagest and termed it a ‘nebulous mass’. al-Ṣūfī referred to it as al-latkhā al-sahābiya, i.e. the nebulous smear or smudge, the same expression he used for the Andromeda Galaxy. Of course, he had no way of knowing that these two faint smudges were very different in nature, as this was still long before the invention of the telescope. The cluster was clearly intended to form part of the smaller fish, so the ancient Arabs must have known of it independently from the Greeks.
Incidentally, the Arabs called Andromeda al-mar’a al-musalsala, literally ‘the chained woman’, although there is no sign of chains on the illustrations.
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The larger of the two Arabic fish is seen overlapping the body of Andromeda on this illustration from al-Ṣūfī’s Book of the Fixed Stars. The cluster of black dots at the mouth of the fish represents the Andromeda Galaxy, the first known record of it. The brightest star in this fish is the modern-day Beta Andromedae (Mirach), of second magnitude. On this illustration, a second fish overlaps the first; this is presumably intended to be one of the fish of Pisces. North is to the left.
(Image: Bodleian Library, Oxford, manuscript Marsh 144)
The smaller of the two Arabic fish overlaps the feet of Andromeda on this illustration from al-Ṣūfī’s Book of the Fixed Stars. The cluster of black dots at the tail of the fish represents what we now know as the Double Cluster in Perseus. The brightest star in this fish is second-magnitude Gamma Andromedae (Almach).
(Image: Bodleian Library, Oxford, manuscript Marsh 144)
