Sky & Telescope - April 2021 - 45

Evenings with the Stars by Fred Schaaf

W O GEMINI! The celestial twins, Castor and
Pollux, are prominent in the upper portion of
this image of the constellation Gemini.

Pollux and Castor's
April Prominence
The most famous two-star pattern in the heavens
deserves a second look.

A K IR A FUJII

L

ast month I discussed the visual
basics and physical nature of the
Gemini twins, Pollux and Castor. There
is, however, a lot more to say as they
adorn the April evening sky.
Twins at the top of spring's arch.
When Gemini rises on December evenings, the constellation lies on its side
to the left of the even more spectacular
constellation Orion. But in April, Pollux and Castor assume a marvelously
prominent position as they descend in
the hours after nightfall. As our star
map on page 42 shows, the two stand
at the top of a giant arch of bright stars
that includes Sirius in the southwest,
along with Procyon and Capella in
the northwest. At the end of April (or
during nightfall later in spring), Sirius,
Orion, and Taurus have disappeared,
leaving only the arc of Procyon, Pollux,
Castor, and Capella. Sky & Telescope
Senior Editor Alan MacRobert popularized the name Arch of Spring for this
striking ﬁgure.

Three bright pairings. Pollux ranks
as the 17th brightest star in the night
sky and Castor is 23rd. But in the farsouth constellations of Centaurus and
Crux, there are two similarly close pairs
of even brighter stars. Alpha and Beta
Centauri are the 3rd and 11th brightest stars, while Alpha and Beta Crucis
rank 13th and 19th. (Unfortunately,
these stars are too far south to be seen
at mid-northern latitudes.) The amazing thing is that the separations of all
three pairs is so similar. Alpha and Beta
Crucis are 4.2° apart, Alpha and Beta
Centauri 4.4°, and Pollux and Castor
4.5°. Remarkably, Alpha Centauri's great
proper motion (its detectable motion in
the night sky) will bring it closer to Beta
Centauri than Alpha and Beta Crucis
are to each other. You might expect that
such a shift would take millennia, yet it
will occur in just 150 years.
The brightest zodiacal pair. Pollux and Castor are near the ecliptic,
which means they're often visited by

the Moon and planets. However, the list
of 1st-magnitude stars that the Moon
can occult presently only includes
Aldebaran, Regulus, Spica, and Antares.
But there was a time when Pollux was
on that list, too. Remarkably, the last
time the Moon occulted Pollux was not
tens of thousands of years ago, but as
recently as 117 BC. Thanks to the combined effects of Earth's precession and
Pollux's own motion in space, the star
no longer lies in the Moon's path.
Pollux, Castor, and St. Elmo's
ﬁre. Castor and Pollux are named for
the famed twins of Greek mythology,
though Pollux is actually the Roman
name for the Greek " Polydeuces. " (For
Guy Ottewell's outstanding discussion
of the myths of the twins, visit universalworkshop.com/the-shining-twins.)
According to certain versions of
the story, immortal Pollux and mortal
Castor hatched from eggs, and the fair
Helen of Troy was their sister. Pollux
and Castor were on the ship Argo with
Jason and the Argonauts when the ship
was beset by a terrible storm. Fortunately, the turbulence was tamed by
Orpheus playing a lyre (Lyra, bejeweled
by the brilliant star Vega, rises in the
northeast on April evenings). As the
seas calmed, everyone aboard was awed
by the mysterious light that appeared
around the heads of Castor and Pollux, who therefore received some of the
credit for saving the ship.
This is how Castor and Pollux
became the ﬁgures Roman sailors
prayed to for safety in storms. Their cry
of " O Gemini " eventually became " by
Jiminy " in modern times. And what
were the mysterious glows above the
heads of the twins? Perhaps it was St.
Elmo's ﬁre, a kind of slow electrical
discharge that can appear above pointed
objects in the waning stages of a storm.
 FRED SCHAAF is the author of 13
books, including The Brightest Stars:
Discovering the Universe through the
Sky's Most Brilliant Stars.
sk yandtelescope.org * APRIL 2 021

45
http://www.universalworkshop.com/the-shining-twins http://www.universalworkshop.com/the-shining-twins http://www.skyandtelescope.org
Sky & Telescope - April 2021

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