The Pleiades prove that, several thousand years ago even as now,
stars of the seventh magnitude were invisible to the naked eye of
average visual power. The group consists of seven stars, of which six
only, of the third, fourth, and fifth magnitudes, could be readily
distinguished. Of these Ovid says (_Fast._ iv. 170):
“Quæ septem dici, sex tamen esse solent.”
Aratus states there were only six stars visible in the Pleiades.
One of the daughters of Atlas, Merope, the only one who was wedded to
a mortal, was said to have veiled herself for very shame and to have
disappeared. This is probably the star of the seventh magnitude, which
we call Celæne; for Hipparchus, in his commentary on Aratus, observes
that on clear moonless nights _seven stars_ may actually be seen.
The Pleiades were doubtless known to the rudest nations from the
earliest times; they are also called the _mariner’s stars_. The name is
from πλεῖν (_plein_), ‘to sail.’ The navigation of the Mediterranean
lasted from May to the beginning of November, from the early rising
to the early setting of the Pleiades. In how many beautiful effusions
of poetry and sentiment has “the Lost Pleiad” been deplored!--and, to
descend to more familiar illustration of this group, the “Seven Stars,”
the sailors’ favourites, and a frequent river-side public-house sign,
may be traced to the Pleiades.