1°CANCER
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abedM44 cPath of Comet 67P
Sept 10
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16
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25NEComet 67P/Churyumov-Gerasimenko10°September 21, 1 hour before sunrise
Looking eastGEMINILEOCastor
PolluxProcyonVenusMarsJupiterRegulus
AlphardVenus dazzles before dawn (^)
42 ASTRONOMY • SEPTEMBER 2015
COMETSEARCH
Let’s piggyback on the excite-
ment stemming from the
Rosetta spacecraft’s ongoing
encounter with Comet 67P/
Churyumov-Gerasimenko by
looking at the icy visitor with
our own eyes. City and suburban
lights will overwhelm the com-
et’s fuzzy glow, so you’ll need to
observe under a dark sky. The
object climbs highest in the east
shortly before dawn. The best
observing window opens as the
waning Moon exits the morning
sky around September 10 and
lasts about two weeks.
Comets are notorious trick-
sters, even those like 67P that
have returned to the inner
solar system several times.
Astronomers offer a wide range
of predictions on how bright it
will be, from a dim magnitude
10.5 to a feeble magnitude 13.5.
If we’re on the lucky side of
these estimates, an 8-inch scope
will pick it up nicely; if not, you’ll
need a 12-inch or larger instru-
ment. In a final, ironic twist,
we view the comet’s dusty
tail through the veil of debris
left behind by countless other
comets. Known as the zodiacal
band, this soft glow envelops
the path of the planets where
67P resides.
On September 10, you can
find the comet 1° south of 6th-
magnitude Lambda (λ) Cancri.
The pair lies almost directly
above the stunning conjunc-
tion of the crescent Moon
and Venus. A week later, 67P
passes 2° north of the Beehive
star cluster (M44). Closest
approach occurs the mornings
of September 16 and 17. The
5th-magnitude star Gamma
(γ) Cancri lies 0.4° south of the
comet on the 17th.
There’s a slim possibility
that 67P could get upstaged
by Comet 141P/Machholz if the
latter ends its life with a major
flare-up. Coincidentally, this
dissolving ball of ice and dust
is traveling in the same area of
sky. The two comets cross paths
September 1 when less than 1°
separates them. Unfortunately,
the Moon is just past Full phase
then, making any visual observa-
tion a challenge even with Luna
on the opposite side of the sky.
Rosetta’s target comes into view
EVENING SKY MIDNIGHT MORNING SKY
Mercury (west) Uranus (southeast) Venus (east)
Saturn (southwest) Neptune (south) Mars (east)
Neptune (southeast) Jupiter (east)
Uranus (southwest)
WHEN TO VIEW THE PLANETS
— Continued from page 37
due north of the planet Sep-
tember 7 and 23 and due south
on the 15th. A trio of 10th-
magnitude satellites — Tethys,
Dione, and Rhea — orbit
closer in and show up through
4-inch and larger scopes.
Remote and mysterious
since its discovery 85 years
ago, Pluto remains distant but
is far less enigmatic thanks to
the eagle-eyed sensors aboard
NASA’s New Horizons space-
craft. Experienced observers
can spy the magnitude 14.2
dwarf planet north of the
Teapot asterism in Sagittarius
through an 8-inch or larger
telescope. Fortunately, magni-
tude 3.5 Xi^2 (ξ^2 ) Sagittarii lies
The ice giant world lies
roughly midway between
4th-magnitude Lambda (λ)
and 5th-magnitude Sigma (σ)
Aquarii. Don’t confuse Nep-
tune with a nearby magnitude
6.9 star in early September.
On the 4th, the planet slides
3.6' due north of this star. A
nearby. Pluto spends Septem-
ber between 0.6° and 0.7° west
of this star. Sketch or photo-
graph the area, and return to
it a few nights later. The “star”
that moved is Pluto.
Neptune reached opposi-
tion and peak visibility on the
final evening of August, and
its appearance in September
barely suffers. The planet
appears in the southeast as
darkness falls and climbs
nearly halfway to the zenith in
the southern sky as midnight
approaches. You’ll need bin-
oculars or a telescope to spot
its magnitude 7.8 glow among
the background stars of cen-
tral Aquarius.
telescope will help confirm
Neptune’s identity. Under
moderate magnification and
steady seeing, you’ll see its
2.4"-diameter disk and blue-
gray color.
As Neptune climbs higher
in the southeastern sky,
Uranus appears low in the
September provides a nice opportunity to spy this periodic comet from
afar while the Rosetta spacecraft examines it from up close.
The morning “star” gleams at its brightest of the year September 21,
though it remains a beacon all month.