The brightest star in the night sky visible to the naked eye is Sirius the Dog Star. Its proper astronomical name is 𝛂 Canis Majoris. Historically for navigators in the northern hemisphere the most important star was the pole star, currently Polaris (the star designated the pole star changes over time due to the precession of the equinox), whose proper astronomical name is 𝛂 Ursae Minoris. The astronomical name of Sirius means that it is a star in the constellation in Canis Major, the greater dog, whilst Polaris’ name means that it is a star in Ursus Minor, the little bar. But what does the alpha that precedes each of these names mean and where does it come from?
A constellation consists of quite a large number of stars and this means that we need some sort of system of labelling or naming them for star catalogues, star maps or celestial atlases. The system that is used is the letters of the Greek alphabet. These are however not simply attached at random to some star or other but applied according to a system. That system was determined by apparent brightness.
Anybody who looks up into the night sky, when it is cloud free and there is no light pollution, will quickly realise that the various stars vary quite substantially in brightness. The ancient Greek astronomers were very much aware of this and divide up the stars into six categories, or as they are known magnitudes, according to their perceived or apparent brightness. Our unaided perception of the stars does not take into account their differing distances, so a very bright star that is very far away will appear less bright than not so bright star that is much nearer to the Earth. The earliest record of this six-magnitude scheme (one is the brightest, six the dimmest) is in Ptolemaeus’ Mathēmatikē Syntaxis, but it was probably older. The attribution, by some, to Hipparchus is purely speculative. Ptolemaeus also indicates intermediate values by writing greater than or less than magnitude X.
Using this basic framework inherited from Ptolemaeus, the early modern German astronomer Johann Bayer (1572–1625) labelled each of the stars in his maps of the constellations in his Uranometria (first published Augsburg, 1603) with a letter of the Greek alphabet, starting with alpha, in descending order of brightness, creating what is now known as the Bayer designation for stars. In this system the Greek letter is followed by a three-letter abbreviation of the constellation name. So, Aldebaran in the constellation Taurus is designated 𝛂 Tauri, abbreviated 𝛂 Tau. Who was Johann Bayer and what is the Uranometria?
Johann Bayer was born in Rain, a small town in Bavaria about forty kilometres north of Augsburg. He attended the Latin school in Rain and then probably a higher school in Augsburg.
He entered the University of Ingolstadt in 1592, where, having completed the foundation course, he went on to study law, graduating with a master’s degree around sixteen hundred. Leaving the university, he settled in Augsburg, where he worked as a lawyer until his death in 1625. The University of Ingolstadt had a strong tradition of the mathematical science over the preceding century, home to notable mathematicians and astronomers such as Johannes Werner, Johannes Stabius and Andreas Stiborius at the end of the fifteenth century and father and son Peter and Phillip Apian in the middle of the sixteenth. It was certainly here that Bayer acquired his love for mathematics and astronomy. He also acquired an interest in archaeology and would later in life take part in excavation in the Via Nomentana during a visit to Rome.
In 1603 Bayer’s Uranometria was published in Augsburg by Christophorus Mangus, or to give it its full title the Uranometria: omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. (Uranometria, containing charts of all the constellations, drawn by a new method and engraved on copper plates), that is a star atlas. The name derives from Urania the muse of astronomy, which in turn derives from the Greek uranos (oυρανός) meaning sky or heavens, it translates as “measuring the heavens” in analogy to “geometria”, measuring the earth.
The Uranometria contains fifty-one star-maps engraved on copper plates by Alexander Mair (c. 1562–1617). The first forty-eight carts contain the northern-hemisphere constellations listed and described by Ptolemaeus. For the northern constellations Bayer used Tycho Brahe’s star catalogue, which hadn’t been published yet but was available through various sources. He, however, added one thousand more stars.
The forty ninth chart contains twelve southern-hemisphere constellations unknown to Ptolemaeus. Bayer took the star positions and constellation names for this southern-hemisphere chart from the 1597 celestial globe created by Petrus Plancius (1552–1622) of the observations collected for him by the Dutch pilot Pieter Dirkszoon Keyser (c. 1540–1596), which was printed by Jodocus Hondius (1563–1612).
The final two charts are planispheres labelled Synopsis coeli superioris borea (Synopsis of the northern hemisphere) and Synopsis coeli inferioris austrina (Synopsis of the southern hemisphere).
For each star chart there is a star catalogue. In the first column the stars are listed according to their Ptolemaic number and then in their second column Bayer gives them the Bayer designation. Because the Greek alphabet only has twenty-four letters and some constellations have more than twenty-four stars, Bayer continues his list with the Latin alphabet using lower case letter except for the twenty-fifth star, which is designated with a capital A to avoid confusing a small a with an alpha. The listing is not done strictly by order of brightness, listing the stars rather by the Ptolemaic magnitude classes. This means that by several constellations the star designated with an alpha is not actually the constellations brightest star.
Bayer was not the first astronomer to produce printed star maps in Europe (there are earlier printed Chinese star maps) that honour goes to the planispheres produced by Stabius, Dürer and Heinfogel in 1515.
His was also not the first printed star atlas that being the Sfera del mondo e De le stelle fisse (The sphere of the world and the fixed stars) of Alessandro Piccolomini (1508–1579), both published in 1540 and often together. Piccolomini was an Italian humanist, philosopher and astronomer best known for his popularisations of Greek and Latin scientific treatises, which he translated into the vernacular.
De le stelle fisse has charts of forty-seven of the Ptolemaic constellations, Equuleus (the little horse or foal) is missing. The book has a star catalogue organised by constellation, a series of woodblock plates of the constellations, tables indicating the stellar locations throughout the year and a section dealing with risings and settings of stars with reference to the constellations of the zodiac.
However, unlike the Dürer planispheres and Bayer’s Uranometria, Piccolomini’s De le stelle fisse doesn’t have constellation figures.
The book was very popular and went though, at least, fourteen editions during the sixteenth century. Piccolomini designated the stars in his catalogue with the letters of the Latin alphabet and there is the strong possibility that Bayer was inspired by Piccolomini in adopting his system of designation.
Bayer’s atlas was not free of problems. In the first edition the star catalogues were printed on the reverse of the constellation charts. This meant that it was not possible to consult the catalogue whilst viewing the chart. Also, the lettering of the catalogue showed through the page and spoiled the chart. To solve these problems the catalogue was printed separately in a smaller format under the title Explicatio charecterum aeneis Uranometrias in 1624, the year before Bayer’s death.
It was republished in 1640, 1654, 1697 and 1723. Unfortunately, the Explicatio was marred by printing errors from the start, which got progressively worse with each new edition.
The Uranometria was republished often, and editions are known from in 1624, 1639, 1641, 1648, 1655, 1661, 1666 and 1689. It set standards for star atlases and planispheres and continued to influence the work of other star cataloguers down into the eighteenth century.The next time that a popular science programme on the telly or a science fiction story starts on about Alpha Centauri, the next closest star to our solar system, then you will know that this is the Bayer designation for a magnitude one, possibly the brightest, star in the constellation Centaurus, a centaur being the half man half horse creature from Greek mythology. It’s actually slightly more complex than Bayer believed because Alpha Centauri is now known to be a triple star system and is now designated α Centauri A (officially Rigil Kentaurus), α Centauri B (officially Toliman), and α Centauri C (officially Proxima Centauri).