The group of stars has been known by the name Ophiuchus for two thousand years
or more. What is not known, though, is why the name 'Serpent Holder' was assigned to them.
Some suggest that the constellation represents Asclepius or Aesculapius, the
healer of Greek myth whose cult was closely associated with serpents. This identification
is by no means certain.
Ophiuchus is a large constellation (the eleventh largest in the sky), and contains a number
of bright stars. The star designated Alpha Ophiuchi is Rasalhague, a fairly nearby star
with a name that means 'Head of the Serpent Holder'. There are four other stars above
third magnitude: in descending order of brightness, these are Sabik, Zeta Ophiuchi, Yed Prior and Cebalrai.
More famous than any of these is Barnard's Star, which lies just under six light years
from Earth, and is one of the Sun's closest neighbours. It is a very small and faint
red star of the type known as subdwarfs, so that despite being so nearby, it is invisible
to the naked eye.
Ophiuchus abounds with Globular Clusters, and though none can be seen with the naked
eye, Messier 10 is quite easy to make out with even a small telescope. Other similar star clusters
in this constellation are M12, M19, M62 and M107.
These Globular Clusters are very distant objects, with the nearest (M10) being more
than 14,000 light years from Earth. Nearer to home, at just 1,000 light years,
a bright open cluster can be seen near Ophiuchus' border with Serpens Cauda. This
is NGC 6633.
The Thirteenth Sign of the Zodiac
When the zodiac was first delineated by the ancients, it consisted of twelve 'houses',
each associated with one of twelve constellations that lay along the Ecliptic: Aries,
Taurus, Gemini, Cancer, and so on. Since that time, gravitational effects on the
orbit of the Earth, known as 'precession', have caused the line of the Ecliptic to change slightly.
Specifically, it now passes through thirteen constellations, with Ophiuchus
being the new addition. Where the Sun once passed directly from Scorpius into
Sagittarius, it now spends nineteen days each year, from 30 November to 18 December, in the
new 'house' of the Serpent Holder.