ORIONPERSEUS
AURIGATAURUS PleiadesAlgol`¡aaAldebaranPath of
Comet 46P/WirtanenDec 111315171921NE5°Comet 46P/Wirtanen (^)
Decemb er 3, 1 hour before sunrise
Looking southeast
CORVUS
BOÖTES
VIRGO
Arcturus
Spica
Moon
Venus
10°
Brilliant Venus meets the Moon (^)
42 ASTRONOMY • DECEMBER 2018
COMETSEARCH
Comet observers haven’t had it
this good for a couple of years.
Comet 4 6 P/Wirtanen is a rela-
tively active comet that makes
its closest approach to the Sun
this month just outside Earth’s
orbit. And to top things off, we
hit the timing almost perfectly:
On December 1 6 , Wirtanen
swoops within 7. 2 million miles
of Earth, just 30 times the
Moon’s average distance.
The comet’s peak brightness
remains the key unknown.
Conservatively, it will glow
around 7th magnitude and be a
decent binocular object. But
some astronomers estimate it
might reach 4th magnitude,
which would make it visible to
the naked eye under a dark sky.
Either way, you should be able
to follow changes to the dusty
inner coma through a 4-inch
telescope from the suburbs.
The comet also rides high in
December’s sky. It resides
among the background stars
of Taurus at closest approach,
between the magnificent
Pleiades star cluster (M45) and
the 1st-magnitude star
Aldebaran. Although this area
remains visible nearly all night, it
climbs highest in late evening.
Leading up to 1 6 th, we are
looking slightly down on the
comet’s stubby, fan-shaped dust
tail. If it also sports a tail of ion-
ized gas, it should appear as a
short spike of green or blue
angled to the northeast as the
solar wind carries the ions
directly away from the Sun.
Significant changes occur in
short order. Earth passes through
Wirtanen’s orbit on the night of
December 15/1 6. That means we
see the tails edge-on and con-
fined to a short, straight line. The
best views come after 1 2 :30 A.M.
local time when the Moon sets.
Comet observers may also
want to track 9th-magnitude
The year’s brightest comet stays up all night
EVENING SKY MIDNIGHT MORNING SKY
Mars (south) Uranus (west) Mercury (southeast)
Saturn (southwest) Venus (southeast)
Uranus (southeast) Jupiter (southeast)
Neptune (south)
WHEN TO VIEW THE PLANETS
— Continued from page 37
the latter constellation’s Circlet
asterism. The Red Planet then
glows at magnitude 0.5 and
sets shortly before midnight
local time.
Slow-moving Neptune
remains in Aquarius through
the end of the year. On
Christmas Eve, it passes 15'
due south of 81 Aqr. This star
serves as a useful guide in the
second half of December after
Mars has moved on.
You can find Uranus on
the opposite side of Pisces.
This world actually begins
December in the southwestern
corner of Aries the Ram, but it
stands 1.6° north-northeast of
this star in early December and
moves to a point 1.3° almost
due north by month’s end.
A telescope reveals the
planet’s 3.7"-diameter disk and
striking blue-green color. You
crosses into the Fish on the
3rd. Uranus glows at magni-
tude 5.7, so technically it’s vis-
ible with the naked eye from a
dark-sky site, but your best bet
is to search for it through bin-
oculars or a telescope.
The challenge comes in
finding the right area —
there are no bright stars
nearby to guide you. Start
at 3rd-magnitude Beta (β)
Arietis, which lies near the
center of the Star Dome map
on p. 38–39. Then move 12°
due south and slightly west
to locate 4th-magnitude
Omicron (ο) Piscium. Uranus
can be forgiven for thinking
Uranus has gained a moon
the night of December 24/25
when a 9th-magnitude field
star slides 1' to its south. Try
to view the ice giant during
the early evening hours when
Comet 38P/Stephan-Oterma.
Our best views of this periodic
visitor come in the morning sky
during December’s first half,
when it lies roughly 10° from
Gemini’s twin stars, Castor and
Pollux. It makes its closest
approach to Earth on the 17th.
This periodic visitor might reach naked-eye visibility near the Pleiades
star cluster when it passes closest to Earth in mid-December.
Earth’s planetary “twin” shines brightest before dawn in early December,
but it looks most spectacular when the crescent Moon passes by on the 3rd.