Why Seven Pleiades?

In another thread, Chris Peterson asked why people
think that the Pleiades have seven stars. This is
a question much discussed among ethnoastronomers,
and there's an answer that seems very compelling.
Much of the material below comes from E.C. Krupp's
"Beyond the Blue Horizon," but I'm sure he didn't
invent the theory -- in fact, it had occurred to me
independently long before I read his book.

Most people will agree that the Pleiades are basically
a 6-star group, regardless of whether they see a few
fainter stars flitting around the fundamental pattern.
The 6 brightest stars range quite evenly from mag 2.8
to mag 4.3, and they're arranged in a strikingly
symmetric pattern. The 7th-brightest star, 28 Tau
a.k.a. Pleione, is mag 5.1 -- a huge jump -- and its
additionally hard to see because it's so close to
27 Tau (Atlas). The 8th and 9th are mag 5.5, which
doesn't count as prominent even in pristine skies.

Nonetheless, the idea that there are 7 Pleiades is
found all around the world. Among the main civilizations
of Europe and Asia, one might attribute this to common
roots in ancient Mesopotamia, but this can't explain
why the idea is widespread among native tribes of
North America and Australia. This must be a case
of convergent evolution.

The key is the striking resemblance between the
Pleiades and the Big Dipper. Indeed, many people,
when I first show them the Pleiades, ask "isn't
that the Dipper?" Seeing the resemblance seems
to be hardwired into the human brain. The Big
Dipper, of course, has 7 stars; therefore, the
Pleiades "ought" to have 7 stars too. The
missing star, by the way, shouldn't be where
any of the 5th-magnitude stars actually is; it
should be well to the left of Atlas, forming a
long handle like the Big Dipper's.

And indeed, both the ancient Greeks and several
North American tribes had a legend that the "lost
Pleiad" is Alcor, which was stolen from the handle
of the Pleiades and hidden in the Big Dipper's
handle. No doubt the idea is buttressed by the
fact that at mag 4.0, Alcor is much too faint
to belong to the Big Dipper, but would blend
right in among the Pleiades.

- Tony Flanders

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