The Milky Way in Johann Bayer's Uranometria, 1603

Abstract

Johann Bayer's monumental star adas, Uranometria, is commonly regarded as the first modem celestial atlas, and it is one of the most famous. For a whole era, its design and aesthetical appeal set a standard for celestial cartography. But even more than their artistic value, the representations of the starry sky contained in the Uranometria were the first to be regarded as scientifically usable in astrometric and photometric terms, even if its accuracy was qualified by later research.'On its 50 plates, the Uranometria displays every of the 48 Ptolemaic constellations on separate star maps, while the twelve new southern constellations, which had been introduced only years before its publication, are shown on a single planispheric map of the southern celestial polar cap. These plates truly contain a wealth of information, and not least, no fewer than 31 of them show the course of the Milky Way among the stars. However, although this largest of all celestial structures is rather prominently engraved among the constellation figures, no major attempt to establish the data Bayer had used to accurately plot the Milky Way has previously been made, a fact that is surprising, considering the extensive research that has been undertaken to uncover the sources of the celestial data Bayer used to plot the stars in his maps. This paper intends to shed light on Bayer's sources for the Milky Way representations in the Uranometria.Doubtless a celestial structure as large as the Milky Way, and displayed in such large dimensions at is the case in the Uranometria, needed a substantial data basis. A source candidate for such a data basis must therefore contain enough positional details to explain as many of the peculiarities of Bayer's illustrations as possible. As we will show, there is no source, written or pictorial, that completely matches the Uranometria Milky Way, which leaves it not improbable that Bayer gathered the appropriate positional data on the Milky Way path through visual observations made by himself - a hypothesis recently advanced by Jurgen Hamel.2 But even a cursory comparison of Bayer's Milky Way and the real picture leaves Hamel's thesis unsustainable. The southern parts of the Milky Way (declinations south of about -40°) are invisible to observers from locations around 50° latitude, which would mean that Bayer would have had to have obtained the data during a journey to far southern latitudes - but no such journey is known, and is very improbable.3 Thus, at least for the southern Milky Way, Bayer must have used external data sources. On the other hand, systematical observations of the northern Milky Way can be excluded as well, because of the existence of quite a number of obvious differences - not to say errors - from the actual visual appearance. The most striking examples of such spurious structures can be found in the constellations Cassiopeia ('hole'; void in the Milky Way), Cygnus (missing bright parts east of a Cyg), Ophiuchus (missing gap in the western Milky Way branch); and the merging of the two Milky Way branches in 1southern Scorpius / Ara (see Figures 6-8 for representations of these peculiarities). For a complete analysis, however, these deviations need to be analysed and explained.True enough, the exclusion of systematic observations by Bayer does not imply that he made no use whatever of his own observational data. To the contrary, some small-scale Milky Way features may very well have been altered as a result of his own observations, and there are unambiguous indications of corrections of the pathway made only after a first draft of the star map plates had already been engraved. Examples for such corrections can be found in Cassiopeia, Cygnus, and especially Ophiuchus.The General PictureThe Milky Way can be traced as a great circle across the whole celestial sphere as depicted by Bayer, along 31 of the 50 plates of the Uranometria.4 This makes it a rather conspicuous structure, dominating much of the respective constellation illustrations. …