Hydrus

star chart,CC BY IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

One of the 88 IAU constellations. The constellation Hydrus, known as the Lesser Water Snake, is located in the southern celestial hemisphere, positioned between the Large and Small Magellanic Clouds. It is notable for being near the celestial south pole.

Etymology and History

Bayer (1603): Constellation Hydrus. His Hydrus has 15 stars as in de Houtman's catalogue.

Hydrus from Bayer (1603) mapped to Stellarium, WGSN 2025.

The often-cited analogy to the great water snake is not convincing because the constellation looks completely different. A ‘snake line’ is also unimaginative as any collection of points can ultimately be connected to form a snake line.

Hydrus begins next to Achernar, the brightest star in the constellation River. The star we call Achernar today was only added to the river later; on European maps it is only found in early modern times. The Arab Bedouins may have used it for orientation a few centuries earlier. However, the Arabic name Achernar for ‘end of the river’ was transferred from ϑ Eridani to this star in early modern times and refers to the Ptolemaic constellation.

screenshot of de Houtman's 1603 first catalogue of stars in Hydrus; notethe typesetting of the "W" featured as 2 "V"s.

It seems that Pieter Keyser and Frederick de Houtman, who navigated the Eerste Schipvaart from Europe via Madagasscar to the East Indies, continued the line of the ancient river southwards to the star Achernar and evern further south to find the position of the Celestial South Pole by star hopping. At least that's how it looks in the Uranometria (1603), because the tip of the water snake's tail is next to the South Pole. This view seems to be shared by Paulus Merula in his Cosmographiae generalis libri tres (1605) as one of the name alternatives he gives as "Hydrus Polaris". However, the celestial globe by Blaeu (who directly used de Houtman's original data) the image is not that clear. So perhaps, the curly watery line pointing due south is a creation by Plancius, Hondius or Bayer.

Yet another problem occured with the star hopping from Orion's foot to the south pole: after the extension of the river, the brightest star was no longer at the end of the river as it is written in the Almagest (137 CE). Ptolemy's Almagest reports the last star in Eridanus ‘the brightest’. The brightest star in the ancient chain is β Eridani (2.75 mag) at the northern end of the river, but the coordinates of the "last" and "brightest" are given at the southern end of Ptolemy's river. The star at the southern endpoint is ϑ Eridani (3.2 mag). The stars of Hydrus also lie in this brightness range, but Achernar is significantly brighter at 0.45 mag. It is therefore possible that one of the Dutch cartographers made the decision to place the brightest star (then Achernar) traditionally at the end of the river. Calling the rest of the snake line ‘water snake’ then seems like a pun or a play on words (‘snake of water’?). However, Hydrus was spread across the globe by Plancius in 1598 as a constellation. The Uranometria only recorded a few animals, Indus and this elongated river in the southern starry sky. As a result of further regroupings, in particular the introduction of the constellation ‘Octant’ by Lacaille in 1756, contemporary Hydrus no longer marks the pole of the sky with few stars.

Terminology

author	year		term	English
de Houtman	1603	Star Catalog	de VVaterslang	Water Snake
Bayer	1603	Uranometria (Maps)	Hydrus
Merula	1605	Cosmographiae generalis libri tres Part I, Book II, pp. 105-108 no. IX	Hydrus Polaris Hydrus Pelaru(?) Water-slanghe
Lacaille	1756		l'hydre male	Male Water Snake

The first book of Merula's Cosmographiae was written mostly in 1597 and in a letter, dated 30 August 1600, to Marcus Welser in Augsburg, Merula claims that the first part of his work was ready to be printed. It might have influenced Bayer's work, too (e.g. be the reason for Bayer to draw Hydrus to the pole). Note that Hydrus's tail was reworked on Plancius's globe; the earlier (erased) version was closer to the pole.

We suspect that Merula's description is based on a list which he received from Plancius around 1597/98 when was he still busy forming his constellations. For a few constellations, he had not yet decided which figures and names would finally be shown on his celestial globe; e.g. the constellations Volans, Grus and Apus also were given with alternative names.

de Houtman's Star Catalogue

	de Houtman (1603)	Knobel (1914) after: Uranometria Argentina
1	The head of the water-snake	alf Hyi	α Hyi
2	One star in the neck	bet Hor	β Hor
3	The second following	zet Hyi	ζ Hyi
4	The third "	eps Hyi	ε Hyi
5	The fourth "	del Hyi	δ Hyi
6	The fifth is above the small nebula (Nubecula minor)	lam2 Tuc	λ2 Tuc
7	The sixth following	pi Tuc	π Tuc
8	the seventh "	_ Tuc	_ Tuc
9	A star above the preceeding one. The small nebula	_ Tuc SMC	_ Tuc SMC
10	The ninth following	bet Hyi	β Hyi
11	The tenth "	tet Oct	θ Oct
12	The eleventh situated in the tail	bet Oct	β Oct
13	The end of the tail	nu Oct	ν Oct
14	One before the breast of the water-snake	bet Ret	β Ret
15	One situated below the last	gam Hyi	γ Hyi

Transfer and Transformation of the Constellation

Originally an extension of Eridanus, creating a meandering chain of stars which flows close to the celestial south pole.

• Bayer (1603) draws the tail tip of Hydrus as a marker of the south pole.

• On Blaeu’s globe of 1602, copied from Plancius and Hondius, it follows much the same path as on Bayer’s atlas, but without Bayer's artistic coils.
• Blaeu made a second globe the following year, this time from de Houtman’s catalogue. On this globe the tail of Hydrus ends in quite a different position, under the feet of Pavo and pointing away from Apus.
• In de Houtman’s catalogue the tip of the tail ended at the star we now know as ν Octantis, which is actually the brightest star in Octans.
• Lacaille’s version passes between the two Magellanic Clouds, transferring some of its stars to Tucana and chopping off its tail to make way for Octans.

• 

  Hydrus on the celestial globe by Plancius/ Hondius (1597/8)

• 

  Hydrus in Plancius (1598). Note that the end was reworked: the first (erased) version went towards the pole, just like Bayer's (1603) version.

• 

  Hydrus on Blaeu's Globe (Willem Jansz.Blaeu, 1602).

• 

  Bayer (1603): Constellation Hydrus (CC0)

• 

  Hydrus on the van der Keere & Plancius (1612) globe, it does not deviate from Plancius' 1598 version.

• 

  In Lacaille's map of the celestial south (1756), the tail of Hydrus is modified to place Octans (a newly invented nautical instrument) at the celestial south pole

• 

  Hydrus in Fortin's Atlas Céleste, 3rd edition (1795).

• 

  Eridanus and Hydrus in Stellarium (modern map, 2024)

Greek Mythology

No ancient mythology.

IAU WGSN Names

In 2023, it was proposed to use the original name of the constellation (in any of its forms) as a star name. The first depiction on the Hondius/Plancius globe shows the form "Waterslange". It might be an idea to use this term for the brightest star in Hydrus or for a star close to the south pole in constellation Octans in order to create public visibility of the fact that constellations are human-made and were changed over time (without the motion of stars). alf Hyi is an F-type (yellow) main sequence-star (V=2.84 mag, SIMBAD), tau Oct was closest to the pole in 1600 and the southernmost star in de Houtman’s 1603 catalogue is the 11^th star in the tail of the “Waterslanghe” at declination −83° 40′, or β Octantis (mag 4.13), a high proper motion star of white A-type (SIMBAD).

Weblinks

• Ridpath, Ian, "Star Tales: online edition".

References

• References (general)
• References (Babylonian)
• References (ancient Greco-Roman)
• References (medieval)