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TAURUS

PERSEUS

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Pleiades

Feb 1

Path of Comet PANSTARRS

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March 1

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Comet PANSTARRS (C/2016 R2)

30"

February 3, 4:00 A.M. EST

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Jupiter

Io

Io’s shadow

Callisto

Europa Ganymede

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Shadow play on Jupiter’s cloud tops

42 ASTRONOMY • FEBRUARY 2018

COMETSEARCH

The roller coaster from faint to

bright comets and back contin-

ues in 2018. The first months of

this year find us near the bottom

of the ride. February’s best entry

is Comet PANSTARRS (C/2016 R2),

which should glow at 10th or

11th magnitude. You’ll need a 5-

or 6-inch telescope to capture its

light under a dark sky, and you’ll

want to avoid the month’s first

few evenings and its final 10 days

when the Moon shares the stage.

The comet rides high in the

south as darkness settles in. It

appears against the backdrop

of Taurus, near the sparkling

Pleiades star cluster (M45). The

star-hop to the comet won’t be

easy because the background

here has few obvious patterns.

You’re looking for a small,

round, diffuse glow similar to a

companion galaxy in the Virgo

Cluster but without the bright

parent to guide you. PANSTARRS

may not be visible at low power.

If a quick scan doesn’t reveal it,

use medium power (around

100x) and search near the posi-

tion shown on the finder chart.

Once you spot it, bump up

the magnification to 150x or so.

This darkens the field and makes

the comet more noticeable. The

glow you see comes mostly from

sunlight reflecting off dust parti-

cles. PANSTARRS’ gas output

should be weak because the

comet lies nearly three times as

far from the Sun as Earth does.

The robotic Pan-STARRS

system picked up C/2016 R2 in

September 2016, nearly three

months after discovering

C/2016 M1, which soon will take

The Pleiades welcomes a guest

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

WHEN TO VIEW THE PLANETS

— Continued from page 37

a telescope. A quick view shows
two dark belts, one on either
side of a brighter zone that
coincides with the equator. A
cursory look also reveals up to
four moons aligned roughly
with the planet’s equator.
Patient viewers are
rewarded with remarkable
views of delicate wisps and
swirls in Jupiter’s turbulent
atmosphere. The gas giant’s
cloud tops resolve into a series
of alternating belts and zones
adorned with dark spots and
feathery festoons.
Jupiter’s four bright moons
provide a constantly changing
tableau visible through any
telescope. Innermost Io moves
fastest, completing an orbit

the geometry of the Sun’s illu-

mination relative to our view-

point. The other three moons

stretch out west of Jupiter in

reverse order of their orbital

distances.

East Coast observers can

see Ganymede eerily fade away

west of Jupiter on February 6

starting at 2:43 a.m. EST. As

the moon enters the planet’s

shadow, it takes about 15

minutes to disappear com-

pletely. If you return to Jupiter

at 4:24 a.m. EST, you can see it

gradually return to view as it

exits the shadow.

every 1.8 days, while Europa

takes 3.6 days, Ganymede

7.2 days, and Callisto 16.7

days. Sometimes a satellite

may disappear behind the

giant planet or get eclipsed by

its shadow. On the opposite

side of its orbit, a moon can

cast a distinct shadow onto

the planet’s cloud tops or hide

in plain sight as it passes in

front of the gaseous world.

In its quick orbit, Io expe-

riences more of these events.

A good example occurs

February 3. At 4:00 a.m. EST,

the satellite’s shadow falls near

the center of Jupiter’s disk

while Io itself lies just off the

planet’s eastern limb. The

scene dramatically reveals

Because the orbital plane

of the satellites tilts slightly to

our line of sight, outermost

Callisto does not pass in front

of or behind the planet’s disk.

You can spot this moon due

south of Jupiter just before

dawn February 11. That

morning brings two other

intriguing events: Europa dis-

appears into Jupiter’s shadow

starting at 5:08 a.m. EST, and

Io reappears from behind the

planet 10 minutes later.

Mars lies 12° east of

Jupiter on February 1 and

rises an hour after the giant

over R2’s spot at the top of our

observing list. Thankfully, the

comet roller coaster will return

us to much brighter subjects

this fall, when 46P/Wirtanen

could reach naked-eye visibility.

This visitor from the Oort Cloud should glow at 10th or 11th magnitude

as it heads north through Taurus not far from the Pleiades star cluster.

Although Io sits just off the planet’s eastern limb before dawn February 3,

it casts a dark shadow onto the center of the gas giant’s disk.