10°December 15, 45 minutes before sunrise
Looking southeastVIRGOLIBRA
OPHIUCHUSSCORPIUSSpicaVenusMercuryJupiterPath of JunoDec 1 6 1116212631ERIDANUS17242221NE0.5°Catch Juno near its historic best
Mercury puts on a predawn show (^)
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Minor planet 3 Juno continues
its brief reign as the brightest
asteroid. Glowing at magni-
tude 7. 6 in early December, the
170-mile-wide, potato-shaped
space rock dims to magnitude
- 2 at month’s end. That makes
it bright enough to see with
binoculars from the suburbs
and an easy target through
even the smallest telescope.
Juno reached opposition
and peak visibility in mid-
November. As Earth sprints
ahead of it on the solar system’s
orbital race track, Juno climbs
higher in the southeastern sky
after darkness falls. The major-
ity of main belt asteroids orbit
the Sun in roughly the same
plane as the planets, known as
the ecliptic, but Juno tilts 13° to
this plane. That places it well
south of the ecliptic this month,
plying the relatively star-poor
waters of the great celestial
river Eridanus, located to the
west of Orion.
Use the star chart below to
get to the right vicinity. (Or, if
you use an equatorial mount,
simply slew 2 8° south of the
Pleiades star cluster.) Once you
arrive, Juno will be one of the
brightest points of light in the
field of view. Unlike the
crowded, uniform star fields
you find closer to the Milky Way,
here the uneven background
with its wide range of star
brightnesses proves useful. You
should be able to track Juno’s
night-to-night motion with a
modest magnification of 50x.LOCATINGASTEROIDS
Juno fords a cosmic stream
it peaks some 60° above the
southern horizon.
Although the evening sky
has its charms, the morning
sky offers December’s three
brightest planets. Even in this
dazzling company, Venus
stands out. The inner planet
peaks at magnitude –4.9
December 1 and fades only
slightly (to magnitude –4.6) by
month’s end. It dominates the
southeastern sky from the time
it rises before 4 a.m. until
shortly before sunrise more
than three hours later.
Be sure to catch this bea-
con when a slender crescent
Moon hangs 5° above it the
morning of December 3. First-
magnitude Spica — Virgo the
Maiden’s brightest star —
appears 7° to the right of this
pair. Venus crosses from Virgo
into Libra on December 13
and remains there until the
end of 2018.
Viewing Venus through a
telescope against a dark sky
can be dazzling, so experi-
enced observers often wait
for the onset of twilight. The
planet won’t disappoint. Its
disk spans 40" and appears
one-quarter lit December 1;
by month’s end, the planet’s
diameter has shrunk to 26"
while its phase has fattened to
nearly half-lit.
Mercury joins Venus in
the morning sky. The inner-
most planet reaches greatest
elongation December 15,
when it lies 21° west of the
Sun and climbs 10° above the
southeastern horizon 45 min-
utes before sunrise. Mercury
then shines at magnitude –0.4
some 24° to Venus’ lower left.
Mercury sinks deeper into
twilight during December’s
second half. As it descends,
Jupiter rises to meet it. On
December 21, they stand 0.9°
apart. At magnitude –1.8,
Jupiter appears more obvious
than magnitude –0.5 Mercury.
Enjoy this pretty conjunction
starting about 45 minutes
before sunup, when the pair
lies 8° above the horizon.
Those with eagle eyes, or bin-
oculars, may notice the 1st-
magnitude star Antares 5° to
Jupiter’s lower right.
Like Venus, Mercury tran-
sitions through a variety of
phases — it just does so more
quickly. On December 5,
Mercury spans 9" and is one-
quarter lit. It appears exactly
half-lit on the 11th, although
its diameter has shrunk to 7".
Its phase waxes to gibbous
after this, reaching 63 percent
illumination at greatestelongation. And by the time
Mercury passes Jupiter on the
21st, it sports a 6"-diameter
disk that’s 77 percent lit.
Jupiter shows none of the
dramatic changes the two
inner planets do. Its disk spans
31" at the time of its conjunc-
tion with Mercury and grows
barely 1 percent by month’s
end. Although the giantMartin 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.planet’s low altitude washes
out atmospheric detail,
conditions will improve
dramatically in January.The innermost planet rises nearly two hours before the Sun at its peak
December 15 and stands out in the twilight glow. Six days later, the small
world has a spectacular conjunction with giant Jupiter.
December’s brightest asteroid takes a sharp turn northward in
December as it navigates the shoals of northern Eridanus the River.