Tycho Brahe’s great star catalogue
First true successor to the Almagest

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 Ptolemy’s 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.

Tycho’s observations came to a premature end in March 1597 when he was forced to leave his observatory on the Danish island of Hven, having lost his patronage from the Danish king. Tycho circulated his results, informally known as the ‘thousand-star’ catalogue, in handwritten form in January 1598 under the title Stellarum octavi orbis inerrantium accurata restitutio. An abridged version, containing a subset of 777 of the most accurately determined star positions, was printed in 1602, the year after his death; it formed part of a much larger book called Astronomiae Instauratae Progymnasmata. Tycho’s thousand-star catalogue was eventually edited and published in 1627 by his former assistant and eventual successor, the German mathematician Johannes Kepler. This catalogue formed part of the Rudolphine Tables, which were primarily tables for calculating planetary positions; they were named ‘Rudolphine’ after the Holy Roman Emperor Rudolf II. For more discussion and detailed information, see Three Editions of the Star Catalogue of Tycho Brahe by Frank Verbunt and Robert van Gent (2010).

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 gave different coordinates for them.

Tycho’s 777-star catalogue was divided into 45 constellations, as listed in the table below. Because he worked at latitude 55°.9, some 25° farther north 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, 1992 April, p. 37).

Tycho’s catalogue was a landmark in positional astronomy, widely admired and gratefully received by fellow astronomers and celestial map makers. 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 compiled with the aid of a telescope, starting with John Flamsteed’s Catalogus Britannicus of 1725.

Tycho Brahe’s observatory called Stjerneborg on the Danish island of Hven was partially underground to protect his sighting instruments from wind and weather. His largest instrument, a wall-mounted quadrant, was not kept here but was in his adjacent castle called Uraniborg. This illustration of Stjerneborg comes from Tycho’s book Astronomiae Instauratae Mechanica of 1598.