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Tycho Brahe’s great star catalogue
First true successor to the Almagest
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During the last two decades of the 16th century the Danish astronomer Tycho
Brahe (1546–1601), usually known simply as Tycho, exercised his exceptional ingenuity and
energy to produce the first major star catalogue since the Almagest over 1,400 years earlier. Tycho’s achievement has been described by science historian Dennis Rawlins as “one of history’s outstanding attempts at mass-perfection in the search for knowledge” (DIO, vol. 3, p.3, 1993 October).
Tycho’s obsessive attention to detail resulted in a tenfold improvement in positional
accuracy over his predecessors. Working solely with naked-eye instruments – the telescope had not yet been invented – he set new standards in celestial surveying and provided reliable data to
constellation chartmakers, for whom accurate measurements of star positions and
brightness are the essential raw material. Tycho’s star catalogue formed the basis of the first great celestial atlas, Johann
Bayer’s
Uranometria, published in 1603, two years after Tycho’s death.
Johann Bayer used Tycho’s data when plotting the stars for his Uranometria atlas of 1603. The German astrometrist F. G. W. Argelander examined the Uranometria in 1842 and concluded that Bayer had worked from Tycho’s 777-star catalogue of 1602, rather than the manuscript of the full
thousand-star catalogue, since the full version of the catalogue contained
stars either not in the Uranometria or for which different coordinates were given.
Tycho’s 777-star catalogue was divided into 45 constellations, as listed in the table
below. Because he lived about 25° farther north in latitude than Ptolemy, Tycho was unable to observe the more
southerly stars in the Almagest, but he observed additional stars in most of the other constellations. Notable
examples are Cassiopeia, in which Tycho’s full catalogue lists 46 stars versus Ptolemy’s 13; Orion, for which Tycho lists 62 stars against Ptolemy’s 38; and Ursa Major, with 56 stars against 35. He also made two amendments to
the constellations recognized by Ptolemy, listing Antinous and Coma Berenices
separately whereas Ptolemy had regarded them as parts of Aquila and Leo
respectively. Antinous was eventually reabsorbed into Aquila, but Coma
Berenices continues its independent existence.
How many stars were in Tycho’s full catalogue? It depends how you count them. The number of entries was 1,004
but that is not the whole story. After taking into account duplicates, errors
and what Dennis Rawlins has termed ‘fakes’ (i.e. stars not properly observed but inserted to make up the numbers), the
total of separate stars genuinely recorded by Tycho and his assistants comes
down to 965 (DIO, vol. 2.1, p.37, 1992 April).
Tycho’s catalogue was a landmark in positional astronomy, widely admired and
gratefully received. But as astronomy advanced it, too, was inevitably
superseded, initially by the 1,500-star catalogue of Johannes Hevelius (1690), also made with the naked eye, and later by the arrival of catalogues
made with the aid of a telescope, starting with John Flamsteed’s Catalogus Britannicus of 1725.
Notes:
1. Cynosura is an alternative name given by Aratus. It comes from the Greek Κυνόσουρα, literally “dog’s tail”.
2. Helice is an alternative name given by Aratus. It comes from the Greek Ελίκη, meaning ‘twister’ – hence our word helical.
3. Arctophylax is an alternative name given by Aratus, referring to the man
driving the bear around the pole.
4. “Gnossia” was an epithet (slightly mis-transliterated from Greek) applied to Ariadne by
the mythologists because she was born in Cnossus on Crete, better known to us
today as Knossos. Kepler dropped this alternative name from the Rudolphine Tables.
5. The name Engonasi comes from the Greek εη γόνασι meaning “the kneeler”. In the Rudolphine Tables the order of names was reversed, i.e. Engonasi came before Hercules.
6. In the Rudolphine Tables, the alternative name Vultur Cadens (“falling vulture”) was added.
7. In the Rudolphine Tables, the name was given as “Olor, Cygnus”. The word “olor” is a Latin alternative for swan.
8. In the Rudolphine Tables, the name was given as “Auriga, Heniochus, Erichthonius”. Heniochus is a Latin transliteration of the Greek word Ηνιοχος, meaning charioteer. Erichthonius is the mythological character the constellation
supposedly represented.
9. In the Rudolphine Tables, the alternative name Serpentarius is added.
10. In the Rudolphine Tables, the name was given as “Serpens Ophiuchi”.
11. In the Rudolphine Tables, the name was given as “Sagitta sive Telum”. The word Telum is a Latin alternative for arrow.
12. In the Rudolphine Tables, the name was given as “Aquila seu Vultur Volans”. The name Vultur Volans means “flying vulture”.
13. In the Rudolphine Tables, the alternative name Equi Sectio is added, literally meaning “section of the horse”.
14. In the Rudolphine Tables, the alternative name Equus Alatus is added meaning “winged horse”.
15. In the Rudolphine Tables, the alternative name Deltoton is added.
16. In the Rudolphine Tables, the alternative name Procyon is added.
17. In the Rudolphine Tables, the name was changed to Argo Navis.
The order in which the constellations are presented in Tycho’s 1602 catalogue differs from that in the Almagest, by putting the zodiac first. The traditional order, starting at the north pole
with Ursa Minor, was restored by Kepler in the Rudolphine Tables of 1627. Also, in the Rudolphine Tables, “Centaurus, Chiron” with 4 stars was tacked on at the end, bringing the total number of
constellations to 46. Ptolemy’s Lupus, Ara, Corona Australis and Piscis Austrinus were not included in either
version of Tycho’s catalogue because they were too far south for him to observe from Hven, nearly
latitude 56° north.
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