The twelve of Plancius: Keyser, De Houtman and the southern sky

What the sources say

• Each of the twelve is among the 88 constellations of the International Astronomical Union, whose list dates from 1922. ^[4]
• The twelve new constellations were delineated in 1597 by Petrus Plancius, on the basis of positions of southern stars that the returning First Voyage (Eerste Schipvaart) had brought to Amsterdam. They were first shown on a globe made by Hondius. ^[1]
• On that globe the measurements of the stars were ascribed to Pieter Dirksz Keyser, who died during the voyage. Dekker adds that Plancius named only Keyser explicitly among all the observers. ^{[1], [2]}
• The original observations are lost. Verbunt and Van Gent place the catalogue that Kepler printed in 1627 in a ‘third class’ of 12 constellations with 136 stars, based, Kepler says, on Keyser through the intermediary of Jacobus Bartsch. ^[1]
• Frederick de Houtman published a star catalogue of his own in 1603, as an appendix to a dictionary. It has 304 entries in 21 constellations: the twelve new ones, eight from Ptolemy, and the Southern Cross split off from Centaurus. In the preface De Houtman says it rests on his own measurements during these voyages, improved at Atjeh in northern Sumatra. He gives right ascension and declination, so equatorial coordinates; the errors are spread over about 40 minutes of arc. ^[1]
• Because De Houtman does not mention Keyser, Knobel concluded in 1917 that he had merely copied Keyser. Verbunt and Van Gent call that conclusion not warranted. Dekker had already concluded that Keyser and De Houtman recorded their data separately; Verbunt and Van Gent suggest something else: independent reductions by Plancius and by Blaeu of one shared set of observations. ^[1]
• Who formed the figures? Dekker: the maker of Hondius’s globe of 1598 is the only serious candidate to whom the formation of the ‘twelve of Keyser and De Houtman’ can rightly be ascribed, and one should really call them ‘the twelve of Plancius’. Dekker adds that the observations of Keyser, who died on the voyage, can only have been worked up after the expedition returned. ^[2]

The twelve by name

• The twelve are Apus (Bird of Paradise), Chamaeleon, Dorado (Dolphinfish), Grus (Crane), Hydrus (Water Snake), Indus (the Indian), Musca (Fly), Pavo (Peacock), Phoenix, Triangulum Australe (Southern Triangle), Tucana (Toucan) and Volans (Flying Fish); these are the names on the International Astronomical Union’s list. ^[4]

Cellarius and the twelve

• Cellarius took the figures from the Dutch tradition. According to Van Gent the southern figures on his charts are those that Plancius formed in 1598 after Keyser and De Houtman, with eight more that Plancius added around 1612. ^[3]
• Van Gent counts eight figures that Plancius added around 1612: the Giraffe, the Jordan, the Euphrates and Tigris (counted as one figure), the Bee, the Unicorn, the Little Crab, the Southern Arrow and the Rooster. They stand beside the twelve on Cellarius’s southern charts. ^[3]
• On plate 27 of the Harmonia Macrocosmica (Haemisphaerium Stellatum Australe Antiquum) we can read, among others, Pavo, Indus, Grus, Toucan, Phoenix, Dorado, Musca, Chameleon and Triangulum Australe, the Southern Cross as ‘Cruzero’, and a figure labelled ‘Apis Indica’. We could not read Hydrus or Volans on it. Christiaan Huygens remarked, in his note on the atlas, that on the three southern hemispheres ‘Apus Indica’ should be put in place of ‘Apis Indica’. ^{[5], [6]}

• The plates of the Harmonia Macrocosmica

What we do not know

• How many stars Keyser measured: his original observations are lost, and the numbers we found (121, 135 or 136) are all second-hand.
• Whether Plancius or someone else decided the final names and figures of the twelve: Dekker thinks of Plancius, but there is no document.
• How plates 28 and 29 of the Harmonia Macrocosmica show the twelve: we have not checked them.
• What the rest of Dekker’s article (pages 70, 71 and 73 to 76) says: we read page 72 only.