December 1, 5 A.M.
Looking southeastArcturusDenebola
JupiterMarsVenus SpicaAlphard10°CORVUSBOÖTESLEOVIRGOHYDRAA planetary lineup (^)
36 ASTRONOMY • DECEMBER 2015
Visible to the naked eye
Visible with binoculars
Visible with a telescope
MARTIN RATCLIFFE and ALISTER LING describe the
solar system’s changing landscape as it appears in Earth’s sky.
December 2015: A gemlike shower
SKYTHIS
MONTH
T
he morning sky sees
most of the planetary
action in December,
with Venus, Mars, and
Jupiter all prominently
displayed. A highlight of the
month occurs during daylight
on the 7th when the Moon
passes directly in front of
Venus and hides it from view.
The evening sky possesses
more subtle attractions. Ura-
nus and Neptune remain bin-
ocular targets all month, while
Mercury climbs into view
during twilight after mid-
December. But perhaps the
month’s biggest event comes
on the 14th when the annual
Geminid meteor shower peaks
under a Moon-free sky.
disk that measures 7" across
and appears close to half-lit.
Keep your binoculars
handy as the sky darkens, and
you can put them to work
tracking down the outer two
major planets. Neptune lies
farther west than Uranus and
Let’s begin our tour of the
solar system shortly after the
Sun goes down in the latter
half of December. Mercury
appears above the southwest-
ern horizon starting about
30 minutes after sunset. On
the 15th, it lies only 4° high
but should show up because
it shines so brightly, at mag-
nitude –0.6. (Binoculars will
help you to spot it in the twi-
light glow.)
The innermost planet
remains within 0.1 magnitude
of this benchmark for the rest
of December but becomes
significantly easier to see as
it climbs away from the hori-
zon. By the time it reaches
greatest elongation on the
28th, when it lies 20° east of
the Sun, Mercury stands 9°
high a half-hour after sun-
down and doesn’t set until
an hour later. When viewed
through a telescope in late
December, the world shows a
thus is closer to setting, so
target it first. The ice giant
planet lies in central Aquar-
ius, a large and relatively
inconspicuous constellation
that appears nearly halfway
from the southern horizon to
the zenith as night falls. At
midmonth, it sets after 10 p.m.
local time.
The best guide star for
finding Neptune is magnitude
4.8 Sigma (σ) Aquarii. If your
sky is less than perfect, first
locate magnitude 3.8 Lambda
(λ) Aqr; Sigma lies 6° south-
west of Lambda. Neptune
appears in the same binocular
field as Sigma all month. On
the 1st, the planet resides 1.5°
northeast of the star. The gap
grows to 2.0° by month’s end.
Neptune glows at magni-
tude 7.9, nearly 20 times dim-
mer than Sigma. You may
have a hard time seeing this
faint with hand-held binocu-
lars. Mount them on a tripod,
however, and you can easily
view objects a full magnitude
fainter. Although binoculars
certainly can reveal Neptune,
A Geminid meteor shoots past Orion (lower center) shortly before dawn at the peak of the 2013 shower. Similarly
Moon-free conditions bode well for this year’s shower. TONY ROWELL
Venus, Mars, and Jupiter stretch across the predawn sky on chilly
December mornings. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY
Martin Ratcliffe provides plane-
tarium development for Sky-Skan,
Inc., from his home in Wichita,
Kansas. Meteorologist Alister
Ling works for Environment
Canada in Edmonton, Alberta.