Comet look-alike NGC 1931
30"April 12, 4:20 A.M. EDTJupiter IoEuropaGanymedeSWGanymede returns to view
42 ASTRONOMY • APRIL 2019
COMETSEARCH
When it comes to observing
comets, luck plays a role. We
never know when the solar sys-
tem will send us a surprise from
the distant Oort Cloud, or when
a periodic comet might brighten
dramatically. That’s one of the
lessons we can draw as we move
from last autumn’s cosmic boun-
ty to this spring’s bread crumbs.
Unless Comet C/2017 M4
(ATLAS) brightens unexpectedly,
it will glow at 13th magnitude
this month — a target for
observers with large scopes. For
the rest of us, let’s hone our skills
by viewing the sky’s finest comet
imposter: NGC 1931 in Auriga.
The nebula and embedded star
cluster lies 1° west of the bright
open cluster M36.This masquerader is bright
enough to observe through a
2.4-inch telescope at 40x from
under a dark sky or with an
8-inch instrument from a subur-
ban backyard. At low power,
NGC 1931 looks remarkably like
a comet — it sports a relatively
bright “head” and a fainter, fan-
shaped “tail.” In bigger scopes,
boost the power to 200x to get
a closer look. The comet’s head
turns out to be four closely
spaced stars embedded in a
cocoon of pale light.
NGC 1931 is a reflection neb-
ula, a cloud of dust and gas that
scatters starlight. Perhaps not
too surprisingly, that’s pretty
much what comet dust does
to sunlight.Hop in the comet simulator
EVENING SKY MIDNIGHT MORNING SKY
Mars (west) Mercury (east)
Uranus (west) Venus (east)
Jupiter (south)
Saturn (southeast)
Neptune (east)
WHEN TO VIEW THE PLANETS
— Continued from page 37
Even the smallest telescope
also shows Jupiter’s four bright
moons. You’ll typically see all
of them, but occasionally one
or more will be passing in
front of the planet or will be
hidden behind the massive
world or lost in its shadow.
Tracking these dramatic events
opens a new window onto the
jovian system.
As the innermost moon, Io
orbits fastest and thus experi-
ences the most events. Each
time Io transits Jupiter’s face, it
also casts a pitch-black shadow
onto the planet’s cloud tops.
This month, the shadow
crosses about an hour before
the moon itself, and each takes
about two hours to traverse
the disk. For North American
for it to return to full sunlight.
Seeing the moon slowly reap-
pear is like watching a celestial
magic act. The brightening dot
appears 22" west-southwest of
Jupiter’s limb and 26" south of
Europa. Ganymede itself dis-
appears behind Jupiter’s south-
western limb at 6:34 a.m. EDT,
an event best seen from west-
ern North America.
All four moons orbit Jupiter
in the planet’s equatorial plane.
That plane currently tilts 3° to
our line of sight, so the more
distant moons cross the plan-
et’s disk at higher latitudes.
Callisto is so far out that it
misses the giant worldobservers, the best shadow
transits begin at 3:25 a.m. EDT
on April 2, 5:19 a.m. EDT on
April 9, 1:41 a.m. EDT on
April 18, and 3:34 a.m. EDT
on April 25.
The next moon out from
Jupiter, Europa, has only
one well-timed event during
April. You can see its shadow
first touch Jupiter’s clouds at
3:33 a.m. EDT on the 26th.
The moon itself follows nearly
two hours later.
Perhaps this month’s most
exciting event involves giant
Ganymede. On the morning
of April 12, the solar system’s
largest moon starts to emerge
from Jupiter’s shadow at 4:02
a.m. EDT. But Ganymede is
so big that it takes 15 minutescompletely. You can see it
near the planet’s south pole as
Jupiter rises April 6, and near
the north pole around dawn
on the 14th.
Saturn rises shortly after
3 a.m. local daylight time
April 1 and climbs some 20°
high in the southeast as twi-
light starts to paint the sky
two hours later. By the 30th,
the planet clears the horizon
by 1:15 a.m. and stands 25°
high in the south-southeast
as twilight commences.
Saturn shines at magnitude
0.5 among the much fainter
background stars of eastern
Sagittarius. Its only realTry a range of magnifica-
tions. Although the field grows
darker at higher powers, be
patient and give your eye timeto adjust to the reduced light-
ing. That’s when you can pick
up finer structure that can’t be
seen in a wider, brighter field.Viewing the bright “head” and fainter “tail” of NGC 1931 will improve
your comet-observing skills. AL AND ANDY FERAYORNI/ADAM BLOCK/NOAO/AURA/NSFJupiter’s largest moon starts to exit the planet’s shadow at 4:02 A.M. EDT
on April 12. By 4:20 A.M., it has returned to full brightness.