Sky & Telescope - June 2024 - 22

Seeing Double
Alcor
80
α
ζ
Mizar
η
γ
β
Talitha
ι
URSA MAJOR
Tania Borealis
λ
μ
Tania Australis
ν
ξ
A CASCADE
OF COUPLES
Ursa Major, the
twin-ness to form an entire
constellation seems to have
originated in Mesopotamia. I
assume the fact that the line
from Xi to Mu Geminorum
is more or less parallel to the
Pollux-Castor line, plus the fact
that the Xi-Mu line contains a
second pair (Tejat and Propus), is
what first prompted some unknown
Mesopotamian astronomer to create Gemini,
the Twins, in the first place. The Greeks then inherited
the constellation and associated it with various sets of brothers
- most often the heroes Castor and Pollux, but also quite
a number of others.
According to Claudius Ptolemy's 2nd-century Almagest,
Great Bear, is replete
with stellar duos. A good test
of the acuity of your eyesight is
whether you can split Mizar and Alcor
without optical aid. Celestial cartographer
Johannes Bayer assigned Greek
letters to Ursa Major not in order of
brightness but on geometric principles:
The stars of the Big Dipper are lettered
clockwise, as are the stars
of the pairs that mark the
Great Bear's paws.
tions - is fairly arbitrary. The probability that a
person with no preconceptions would independently
invent the S&T stick figure for Gemini is
vanishingly small.
I find that dazzling Procyon (Alpha Canis
Minoris) bonds strongly to Pollux and Castor.
Procyon is only a little farther from Pollux than
Alhena is, and it's much brighter. Moreover, Procyon
forms a weak, unevenly bright pair with Beta Canis
Minoris (Gomeisa). Procyon and Gomeisa have almost
the foundation of Western astronomy, Alpha and Beta represent
the Twins' heads; Xi, Gamma, and Nu represent three of
their feet; and the Mu-Eta pair marks the heel and toe of the
foremost foot, just as shown in the Sky & Telescope stick figure.
As for the rest of the Twins, if you're determined to see
all the stars between the heads and feet as their bodies and
arms, it's easy enough to concoct ways to do so. Nonetheless,
the constellation as a whole - like almost all our constella22
JUNE 2024 * SKY & TELESCOPE
exactly the same separation as Castor and Pollux, but to my
eyes, this coupling is far less impressive because 2.9-magnitude
Gomeisa is only one-tenth as bright as +0.4-magnitude
Procyon. The Pollux-Castor duo is much more than the
sum of its parts, but as far as I'm concerned, Procyon would
be more impressive shining in solitary splendor without
Gomeisa. No doubt others will disagree. That's the beauty of
naked-eye stargazing: We all live under the same night sky,
but each of us sees it differently.
The Great Bear's Twosomes
Uneven star pairs may seem unconvincing when the components
are far apart, but most people find them striking when
they appear very close together. The classic example is Mizar
Alula Borealis
Alula Australis
κ
Alkaphrah
ε
δ
Sky & Telescope - June 2024

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