Sky & Telescope - November 2023 - 45

Evenings with the Stars by Fred Schaaf
Seeking
Sights in
Cetus
Whale or Sea Monster, the
enormous constellation is a
naked-eye delight.
T
he great Greek myth of Perseus, the
Hero, is represented by no fewer
than six autumn constellations. Five of
them - Perseus, Andromeda, Pegasus,
Cassiopeia, and Cepheus - occupy a
huge, unbroken swath of the heavens.
The sixth is Cetus, the Whale (or Sea
Monster), which is separated from the
others by Pisces, the Fish.
During long November nights, Cetus
seems to swim tail-first across the
southern horizon. The tail is marked
by what is usually the constellation's
brightest star, 2.0-magnitude Beta (β)
Ceti, also known as Deneb Kaitos, or
Diphda. Deneb Kaitos translates as
" the tail of the whale, " while Diphda
means " frog " and is derived from an
early Arabic imagining in which the star
is one of two frogs. The other frog is
Fomalhaut, a much brighter luminary
in Piscis Austrinus, the Southern Fish.
On autumn evenings, you should
have little trouble identifying both
frogs so long as your view toward the
southeast and the south isn't blocked
by buildings or trees. Fomalhaut is
more than 10° farther south than
Diphda, which means it's visible for
fewer hours each night. For observers
at mid-northern latitudes, there is a
period in the early evenings in November
during which the two stars are
roughly at the same altitude - Fomalhaut
is nearly at its highest while
Diphda is still rising in the southeast.
Coincidentally, a similar positional
relationship exists between Cetus's
tail and its head, a flattened pentagon
with Alpha (α) Ceti at its easternmost
pWHALE OF A MONSTER In mythology, Cetus is either a sea monster or, as depicted in this
chart from Alexander Jamieson's 1822 Celestial Atlas, a rather menacing-looking whale.
point. A couple of hours after the head
rises it's at roughly the same altitude as
the tail. In reality, the head is actually
farther north than the tail and sticks
up just above the celestial equator like
a whale rising to the water's surface to
spout. As it happens, the right ascension
(west to east) difference between
Fomalhaut and Diphda is a bit less than
2 hours, while the span from Diphda to
the head of Cetus is only slightly more.
Let's look more closely at the head of
Cetus. On our center star chart (pages
42 and 43), it's depicted as a rather
compact polygon connected to the rest
of the constellation by a single line,
which gives the impression of a very unwhalelike
long, skinny neck. But perhaps
that's not an unreasonable depiction if
we consider the Perseus myth in which
the creature attacking Andromeda, the
Chained Maiden, was an unspecified
" sea monster " rather than a whale.
The brightest star in the head of
Cetus is Alpha Ceti, also called Menkar.
Although Menkar is the constellation's
Alpha star, at magnitude 2.5
it's actually half a magnitude dimmer
than Beta Ceti. Nearly 7° southeast
of Alpha is 4.1-magnitude Delta (δ)
Ceti. Delta is notable for lying just 1
/3
°
north of the celestial equator. Roughly
midway along Cetus's width we arrive
at the belly of the beast, 3.7-magnitude
Zeta (ζ) Ceti. The star is also known as
Baten Kaitos, which translates as " the
Sea Monster's Belly. "
You might have noticed that earlier
in this column I said that Diphda was
usually the brightest star in Cetus.
That cautious wording was in deference
to Omicron (ο) Ceti, better
known as Mira, the most famous and
brightest long-period variable star.
It usually ranges in brightness from
magnitude 2.0 to 10.1 over a 332day
period. Unfortunately, Mira was
last at maximum (when it peaked at
magnitude 3) in June this year, and by
November it surely will have dimmed
to below naked-eye visibility. But
it's worth keeping an eye on because
sometimes it surprises. In 1779, Mira
not only outshined Diphda, it rivaled
1st-magnitude Aldebaran, in Taurus!
¢ FRED SCHAAF loves observing all
celestial creatures great and small.
sk yand tele scope .o r g * NOVEMBER 2023 45
ALEXANDER JAMIESON / CELESTIAL ATLAS (1822) / UNITED STATES
NAVAL OBSERVATORY LIBRARY / PUBLIC DOMAIN
http://skyandtelescope.com
Sky & Telescope - November 2023

Table of Contents for the Digital Edition of Sky & Telescope - November 2023
