This is a very watery part of the heavens, for the zodiacal pair
of Fishes follows the Water-carrier (their only bright star in the
northern hemisphere), and there is also a solitary Fish, Piscis
Australis, swimming in the water poured out; while a little further
east is the great Sea-Monster, which belongs to the northern group of
Perseus and Andromeda and her parents Cepheus and Cassiopeia. Only the
head of Cetus, the Sea-Monster, is in the northern hemisphere, but
that contains its brightest star. Between the dim Sea-Monster and the
bright Orion flows the winding River Eridanus: it rises near Orion’s
foot, and now ends very far south in the bright star Achernar; but this
star was not visible to Ptolemy when he drew up his star-catalogue in
Alexandria, and the original Achernar, or Last-of-the-River, seems to
have been what we now call Theta Eridani, which was much brighter some
centuries ago than it is now. Al-Sufi, an Arab writer of the tenth
century A.D., calls it a star of first magnitude.
To complete our survey of the constellations south of the equator we
must add the tip of the Eagle’s wing, the legs of the Unicorn, and
part of Ophiuchus with the Serpent he is strangling as he treads the
Scorpion under foot (a gallant hero, to contend with both these enemies
at once).
It is worth noting that just as the three stars of Orion’s belt mark
the celestial equator in one part of the sky, so the three bright stars
of the Eagle mark it in the opposite part (Altair, with β and γ on
either side); but they are just north of it, and Orion’s belt is just
south. As it is often interesting to know where the ecliptic lies, we
may point out that the following southern stars lie near it: Spica, α
Librae, Antares, α and β Capricorni (and in the north the Pleiades,
Aldebaran, Regulus). It is also convenient to remember the positions
of a few constellations as a guide to right ascension. Thus, Achernar
is in the Ist hour, Canopus and Sirius are in the VIth, the Cross and
Corvus in the XIIth, and the Bow of Sagittarius is in the XVIIIth.[4]
[4] Right ascension in the skies corresponds with longitude on earth,
but is more often reckoned in time than in degrees. For instance, R.A.
1 hour 35 minutes, the right ascension of Achernar, means that this
star will be on the meridian 1 hour 35 minutes later than the “first
point of Aries”—that is, the point at which the equator cuts the
ecliptic at the spring equinox, the fundamental point corresponding
with Greenwich in earthly longitude.
There are more bright stars in the southern hemisphere than in the
north, for a count of all those above fourth magnitude shows that
there are 228 south of the equator against only 164 north. But whereas
the stars in the north are fairly evenly distributed, there is a more
strongly marked tendency in the south to congregate on the Milky
Way, so that, while the tract through Argo and the Cross to Scorpio
and Sagittarius is extraordinarily rich, the part between Orion and
Fomalhaut is comparatively dark and bare, and the regions round the
south pole and north of Argo are also very barren of bright stars. Some
regions, however, which look dull to the eye abound in marvels for the
telescope and camera.
The brilliance and the complex structure of the Milky Way is
undoubtedly what most strikes the northerner travelling south. In
England we have a glimpse of it in Cygnus and Aquila, where this rich
and bright part begins; but there is nothing to equal the brightness or
the mingling of dark and light which we see in Argo and near the Cross,
in Scorpio and in Sagittarius. When this part is about to rise, there
is often a glow on the horizon as if it were dawn.