Astronomy

BOÖTES

Path of

Comet Catalina

Dec 1

6

11

16

21

26

31

VIRGO

LIBRA

Arcturus

j

k

/

c

d

o

g

109

+

`

o

c

f

Spica

E

N

5°

Comet Catalina (C/2013 US10)

Callisto’s shadow

Callisto

December 8, 5:30 A.M. EST

Jupiter

Io

Ganymede

S

W

30"

Shadow play on Jupiter

42 ASTRONOMY • DECEMBER 2015

COMETSEARCH

With bated breath, observers

across the Northern Hemisphere

have been waiting for Comet

Catalina (C/2013 US10) to pop

into the predawn sky. If all goes

according to plan, it could shine

as bright as 4th magnitude in

early December.

To many people, 4th magni-

tude hints at naked-eye visibility.

Unfortunately, other consider-

ations come into play. First, cit-

ies throw up a veil of light that

will hide all but the central area.

Second, magnitude predictions

assume all the light is com-

pressed into a star-like point. In

reality, a comet’s glow spreads

out into a fuzzy ball and the

average surface brightness is

lower. Finally, the Moon spills its

own light into the sky during

early December.

But don’t let that discourage

you. The comet should be a

pretty sight through binoculars

or a telescope at low power.

Imagers should be prepared the

morning of the 7th to capture

Catalina sharing an 8° field with

Venus and a thin crescent Moon.

After the 8th, two weeks of

Moon-free skies will let observ-

ers get detailed views of the

comet’s dust and gas tails. From

our perspective, the two tails

appear distinct. Although the

comet’s surface remains hidden

by a veil of dust, the surrounding

coma may show a greenish hue.

The Moon returns to the

morning sky in late December

as Catalina sets its sights on

brilliant Arcturus. By then, the

comet likely will have faded to

5th magnitude.

Tails of dust and gas at dawn

EVENING SKY MIDNIGHT MORNING SKY
Mercury (southwest) Jupiter (east) Venus (southeast)
Uranus (southeast) Uranus (west) Mars (southeast)
Neptune (south) Jupiter (south)
Saturn (southeast)

WHEN TO VIEW THE PLANETS

— Continued from page 37

though even a little optical aid
makes the task much simpler.
Uranus moves glacially (appro-
priate for cold December
nights) relative to the back-
ground stars in December,
remaining 2° due south of
magnitude 4.3 Epsilon (ε)
Piscium. To quickly find its
general location, run an imagi-
nary line diagonally across the
Great Square of Pegasus from
its northwestern corner (Beta
[β] Pegasi) to the southeast
(Ga mma [γ] Peg) and extend
it an equivalent distance.
If you swing your telescope
toward Uranus, you’ll see a
blue-green disk that spans 3.6".
If you’re observing the slightly
gibbous Moon on December
19, slew your scope about 1.5°

10 percent during the month

as Earth’s orbit brings the two

worlds closer together.

This large size means you

should get exquisite views of

jovian cloud features. Most

obvious are two relatively dark

belts, one on either side of a

bright zone that coincides with

the planet’s equator. The best

views come during moments

of steady seeing when the nor-

mally turbulent air f lowing

above your head (and tele-

scope) calms for a few seconds.

Take some time now to get

due north, and Uranus will

come into view.

Just as Uranus sets in the

west, the eastern sky lights up

with the arrival of brilliant

Jupiter. The giant planet

rises around 12:30 a.m. local

time in early December and

more than an hour earlier by

month’s end. Jupiter brightens

from magnitude –2.0 to –2.2

during December against the

backdrop of much fainter

stars in southeastern Leo.

Three hours after rising,

Jupiter has reached an altitude

of 30°. This provides three

solid hours of observing time

before twilight starts to paint

the sky. The planet’s disk

spans a healthy 36" in early

December and grows nearly

familiar with Jupiter and the

details visible in its massive

atmosphere. This practice

will prepare you to see the

most when the world reaches

opposition and peak visibility

in March.

Any telescope also will

show Jupiter’s four bright

moons. Io, Europa, Gany-

mede, and Callisto orbit above

the planet’s equator and thus

wander east and west of the

gas giant. When one of them

passes in front of Jupiter, you

can watch as its dark shadow

If this visitor from the Oort Cloud matches expectations, it could hit 4th

magnitude this month as it streaks north toward brilliant Arcturus.

On December 8, Callisto lies well east of Jupiter yet still manages to cast a

shadow onto the gas giant’s cloud tops.