30"IoJupiterGanymedeCallistoJan. 12, 8 P.M. ESTSW4°EPath of
Comet BorrellyJan. 1510152025(^30) PISCES
CETUS
Diphda
Alrescha
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θ
ζ ι
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IC 1613
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38 ASTRONOMY • JANUARY 2022
Dueling moons
COMET SEARCH I Bowling across the stars
COMET C/2021 A1 (LEONARD)
claimed the title of comet of
the year in 2021, but it plum-
mets to 12th magnitude by
January’s end.
Next up: Nicely placed on the
evening stage well to the left of
Jupiter is the periodic Comet
19P/Borrelly, floating near
Diphda, the 2nd-magnitude
nose star of Cetus. Though it’s
not a contender for this year’s
title, Borrelly should sport a
broad, short fan extending to
the south. Even from darker
country skies, a 4-inch scope
won’t show much more than an
out-of-round pale gray cotton
ball glowing at 10th magnitude.
Try magnifications above 100x
with a 6-inch to note the well-
defined northern flank.
During the bright Moon
period from the 5th to the 21st,
the geometry barely changes,
but a surprise flare-up can never
be ruled out. Its close approach
to the Sun on Feb. 1 is closer to
Mars’ orbit than Earth’s.
Discovered from France by
Comet 19P/Borrelly
pair of dark equatorial belts
in any telescope. The four
Galilean moons are on ready
display — catch them this
month before Jupiter leaves the
evening sky in mid-February.
It’s your last chance before
solar conjunction to watch the
changing configuration of the
four moons.
Although the observing
window for seeing satellite
transits is narrow, don’t miss
the Jan. 5 transit of Ganymede’s
large shadow — the event’s long
duration (3.5 hours) means
most observers across the U.S.
will see part of the transit as
darkness falls, although you’ll
need to catch it early if you’re in
the Pacific time zone. It begins
at 6:20 P. M. EST and ends at
9:50 P. M. EST — that’s 6:50 P. M.
PST for those in the western
U.S. As its shadow creeps across
Jupiter’s face, Ganymede itself
stands west of the planet.
Another event not to miss
is the Jan. 12 dual transit of
Ganymede and Callisto across
Jupiter. It’s quite rare to catch
these two moons transiting
together. Callisto’s transit
begins at 5:22 P. M. E S T, fol-
lowed by Ganymede at 6:50 P. M.
EST. Watch as long as you can
to see Ganymede catching up
with Callisto — the latter’s
smaller orbit results in faster
movement across Jupiter’s face.
Also don’t miss Io creeping up
on the western limb of the
planet, disappearing behind
Jupiter at 8:36 P. M. E S T, a n
event visible from the western
half of the country. In the
Pacific time zone, Callisto’s
transit ends at 6:45 P. M. PST
and Ganymede exits the disk
at 7:24 P. M. PST.
By Jan. 31, Jupiter stands
only 11° high an hour after
sunset; such a low altitude
makes viewing any planetary
details very difficult.
Neptune is a binocular
object shining at magnitude 7.8
and located in Aquarius the
Water-bearer. It stands halfway
up in the southwestern sky
Jan. 1 as soon as it’s dark. Since
Neptune sets mid-evening, try
to catch it early. Binoculars are
a good first step to finding the
planet. Try on Jan. 6, when the
crescent moon stands 12° east
of Jupiter and Neptune stands
8° northeast of the Moon.
Neptune lies 3.3° northeast
of the 4th-magnitude star
Phi (φ) Aquarii, an easy guide
star with two 6th-magnitude
stars to its northeast. Neptune
moves east along the ecliptic
SKY THIS MONTH
Alphonse Borrelly in 1904, this
comet was the third to be visited
by a spacecraft. In 2001, Deep
Space 1 unveiled its 5-mile-long
bowling pin shape, similar in size
to Halley’s Comet. Jupiter tugs
on Borrelly every 12 years or so,
morphing its orbit with each
apparition and wreaking havoc
on its arrival times into the inner
solar system.
By midnight, the 10th- to
11th-magnitude Comet 67P/
Churyumov-Gerasimenko has
risen in the east not far from
M44, the Beehive star cluster.
— Continued from page 33
Skywatchers this month will be treated to a rare double transit of Ganymede
and Callisto. Watch the latter start to catch up with the former as they race
across Jupiter’s face. Europa, not pictured, lies farther west.
Comet Borrelly spends January in Cetus the Whale. It passes by bright
Diphda early in the month and the 10th-magnitude dwarf galaxy IC 1613
toward month’s end.
EVENING SKY
Mercury (west)
Venus (southwest)
Saturn (west)
Jupiter (southwest)
Uranus (south)
Neptune (southwest)
MIDNIGHT
Uranus (west)
MORNING SKY
Venus (southeast)
Mars (southeast)
WHEN TO
VIEW THE
PLANETS