A small southern counterpart of the great water-snake, Hydra, with which it is
not to be confused. This is one of several examples of the repetition of
constellation figures in the sky, as in the Great and Little Bear, the Great
and Little Dog, the two lions, the horses Pegasus and Equuleus, the Northern
and Southern Crown and the Northern and Southern Triangle.
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Hydrus shown by Johann Bode in his Uranographia (1801). “Nubecula Minor”, at centre, is the Small Magellanic Cloud. Part of the Large Magellanic Cloud, Nubecula Major, is visible in the bottom right corner.
Hydrus was one of the 12 southern constellations introduced at the end of the
16th century by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de
Houtman and it first appeared on Petrus Plancius’ globe of 1598. It represents the sea snakes that the Dutch explorers would have
seen on their voyages. Hydrus is a male water snake, whereas the much larger
Hydra is a female. To emphasize the difference in gender, Nicolas Louis de
Lacaille termed it l’Hydre Mâle on his planisphere of the southern skies published in 1756.
Hydrus was originally visualized as wriggling beneath the feet of Tucana and
Pavo and was thus shown on Bayer’s Uranometria atlas of 1603. During Lacaille’s reorganization of the southern skies he rerouted Hydrus to pass between the
two Magellanic Clouds, transferring some of its stars to Tucana in the process.
In addition, Lacaille docked the snake’s tail to make way for Octans, one of his own inventions. He also commandeered a couple of stars from Hydrus
for Horologium and Reticulum, another two of his new figures.
Lacaille’s truncated version of Hydrus terminated at Beta Hydri. In de Houtman’s catalogue published in 1603, the tip of the tail had extended to the star we now know as
Nu Octantis. However, Bayer’s Uranometria showed the tail of Hydrus curling even further south, to the modern Lambda
Octantis; this depiction was based on the earlier and now-lost star list of
Keyser, who seems to have recorded a number of different stars in this area
from de Houtman. Differences between Keyser and de Houtman’s versions of Hydrus are well shown on two globes by the Dutchman Willem
Janszoon Blaeu. The first, made in 1602, was based on Keyser’s observations, as was the chart of the southern stars in Bayer’s Uranometria. The second globe, made the following year, plotted the stars in de Houtman’s catalogue which had just been published.
The brightest stars of Hydrus are of third magnitude, but none are named.
© Ian Ridpath. All rights reserved
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