One of the most recognisable of constellations is Leo, the Lion. The shape formed
by these stars almost irresistably describes the outline of a seated lion.
Eighty-eight officially defined regions of the sky, usually (especially in the northern
hemisphere) based on traditional groups of stars.
Traditionally, constellations simply described prominent groups of stars. For purposes of
nomenclature, internationally-agreed constellation boundaries were set up in 1930. This means
that every star, and indeed every point in the sky, falls within a specific constellation.
Many of the constellations we know today go back to ancient times: Ptolemy listed forty-eight of them in
the second century CE, and many are older still. Five hundred years before Ptolemy, for instance,
a Macedonian scientist named Aratos compiled a consistent list of constellations known as the
Phainomena. To the Greeks, constellations and groups of constellations held
great significance, and many have origins interweaved with mythology. In one part of the sky,
for example, we see Andromeda, with her mother and father Cassiopeia and Cepheus, being
rescued by the hero Perseus. Other star-groups (like Corona Borealis or Crater)
were said to represent items literally cast into the sky by the gods.
Supreme importance, though, was reserved for the twelve zodiacal constellations.
These are the constellations intersected by the ecliptic (the plane of the Solar System).
This means that, seen from the Earth, the Sun, Moon and all the planets but Pluto travel
through this group of constellations. Now we understand the workings of the Solar System,
we can see why this should be, but it was natural for the ancients to conclude that there
was something magical about the zodiacal constellations. Incidentally, gravitational effects
on the Earth since ancient times mean that the ecliptic now passes through thirteen constellations
(the new addition is Ophiuchus, the Serpent Holder). What's more, the planets can also sometimes
briefly enter other constellations, such as Cetus or Orion.
For all their inventiveness, there was one important limit on the Greek astronomers: there
were many stars in the southern hemisphere that they could never see. Not until the age
of discovery did Europeans see these stars, and the names of their constellations
are therefore much less ancient than those of the northern sky.
Compared with their ancient northern counterparts, the names of the southern constellations
seem rather arbitrary and uninspired. Many are named after scientific instruments,
such as the Telescope, Microscope, Reticule and Air-pump. The others seem to consist
mainly of a more or less random selection of birds and fish, with the occasional curiosity
such as the Chisel or the Indian.
The area of the sky isn't equally distributed among the constellations: some describe huge areas, while others are minute.
The largest, such as Hydra, Virgo and Ursa Major, have areas of around 1,300 square degrees,
and each describe about a fiftieth of the sky. By comparison, the smallest, like tiny Crux
or Equuleus the Foal, have areas of just seventy square degrees or so (about one nine
hundredth of the full sky).
Because 'constellations' are officially defined, some well-known star-groups can't,
technically speaking, be described with that term. Groups such as the Plough (Big Dipper),
the Square of Pegasus or the huge 'superconstellation' of Argo Navis are more properly
referred to as asterisms - traditionally accepted groups of stars that don't have
an official designation.
The Electronic Sky site contains entries for all
eighty-eight official constellations, and many of the better known asterisms. Visit the
Constellations Index for a full list.