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Constellation of the Month: Delphinus

Delphinus snip  

Image credit: By IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) | CC BY 3.0


This is Delphinus – the Dolphin. It’s an ancient constellation, one of Ptolemy’s first 48. Despite its small size and non-descript squarish shape, this is a fairly easy constellation to find. By chance, everything else around this constellation is less impressive; it’s a bright little diamond off by itself.



Image credit: Gerard Mercator (1512–1594) |The Mercator Globes at Harvard Map Collection | Public domain



Image credit: ‘Delphinus, Sagitta, Aquila, and Antinous’, plate 13 in Urania's Mirror, a set of celestial cards accompanied by A familiar treatise on astronomy... | Jehoshaphat Aspin, 1825 | Public domain


Delphinus is part of the constellation family called the ‘Heavenly Waters’. I love constellation families because they make remembering them all much easier. Orion has a family, Hercules has a family (mostly consisting of things he killed to prove his worth), Perseus has a family, and on and on it goes. So the Heavenly Waters is all things watery, and includes Columba (the dove), Eridanus (the River), and Carina, Vela and Puppis (making up the Argo – see May’s Constellation of the Month: Carina). The Heavenly Waters is one of the most broken-up families, with little pockets of constellations, rather than a big block.

One of my favourite things about this tiny constellation is the names of the two first stars. Normally these stars would be named Alpha Delphini and Beta Delphini, but in some constellations the main stars also have common names, for example, Alpha Orion is also known as Rigel, and Alpha Coma Berenices is Diadem. Mostly these names are Greek or Arabic, from ancient roots. However, Delphinus breaks this mould in an extraordinary way.

Its first two stars are called Sualocin and Rotanev. These names don’t come from a language – they are in code. So, let’s do some cryptography!

                Step one: Reverse the letters:Sualocin = Nicolaus and Rotanev = Venator

                Step two: Translate from Latin to Italian: Nicolaus Venator = Niccolò Cacciatore

                Step three: Celebrate!

Niccolò Cacciatore was an Italian astronomer who headed the Palermo Astronomical Observatory. In 1814, this observatory published a catalogue of stars including the unfamiliar Sualocin and Rotanev. It was almost fifty years later when astronomer Reverend Thomas William Webb figured out, and published, this explanation. It’s unorthodox to name stars after yourself and wouldn’t have been approved by any astronomical organisations, so this was very cheeky! It could have been Cacciatore himself, in a stroke of self-aggrandising genius, or possibly his colleague and observatory director, Piazzi, who came up with the joke. Whoever it was, I’m pretty sure they would have been laughing for the rest of their lives. 


Image credit: Niccolò Cacciatore (1770–1841) | Public domain


While Delphinus is in a section of the Milky Way rich in deep space objects, none of them are particularly bright or exciting. Still, one cool one is a globular cluster enigmatically named NGC 6934, or Caldwell 47.



Image credit: NGC 6934 globular cluster | Hubble Space Telescope, NASA | Public domain 


A globular cluster is a dense sphere of stars which are often some of the oldest clusters in the universe. The best known (and easiest to see) include Omega Centauri and 47 Tucanae. NGC 6934 is about 50,000 light years away from us here on Earth, and although invisible to the naked eye, it makes a good object to try for with a wee telescope.

And with that, you and Delphinus are well acquainted – a small constellation, but one that brings me immeasurable joy. May it too light up your life with its sneaky star names and distant clusters.

Happy sky watching!