1° CYGNUSCEPHEUS¡i+hc
bPath of Comet 21PJuly 811141720 NEComet 21P/Giacobini-Zinner30"July 1, 11:30 P.M. EDTSaturnTitanRheaTethys
Dione EnceladusIapetusSWGlimpse Saturn’s two-toned moon42 ASTRONOMY • JULY 2018
COMETSEARCH
It’s been more than a year since
a short-period comet put on a
decent show. But this autumn
promises no fewer than four
that should crest brighter than
10th magnitude, and one of
these — 46P/Wirtanen — could
reach naked-eye visibility.
The first up is 21P/Giacobini-
Zinner. French astronomer
Michel Giacobini first spotted
this comet in 1900, and German
astronomer Ernst Zinner recov-
ered it two orbits later, in 1913.
In September 1985, it became
the first comet to be visited
by a spacecraft when the
International Cometary Explorer
flew past.Giacobini-Zinner could reach
6th magnitude at its September
peak. This month, it should glow
around 10th magnitude. Plan to
look for it under Moon-free skies
during July’s second and third
weeks. It then appears against
the photogenic northern Milky
Way, passing from northern
Cygnus into southern Cepheus.
Although this region remains vis-
ible all night, it climbs highest
before dawn. The comet likely
won’t be visible at low power
through a 4-inch scope, however.
Use an eyepiece that provides a
magnification of around 100x,
dark adapt, and try averted vision
to pick up the faint cotton ball.The return of a periodic visitor
EVENING SKY MIDNIGHT MORNING SKY
Mercury (west) Mars (southeast) Mars (southwest)
Venus (west) Jupiter (southwest) Uranus (southeast)
Mars (southeast) Saturn (south) Neptune (south)
Jupiter (south) Neptune (east)
Saturn (southeast)
WHEN TO VIEW THE PLANETS
— Continued from page 37
but the view changes little in
July. The gas giant sports a
disk measuring 18" across sur-
rounded by a spectacular ring
system that spans 41". Structure
in the rings shows up well
because they tilt 26° to our
line of sight.
Saturn also hosts a large
family of moons. Any tele-
scope reveals 8th-magnitude
Titan, the planet’s brightest
satellite. It circles the giant
world once every 16 days.
A 4-inch scope brings in
four more moons. Tethys,
Dione, and Rhea all glow at
10th magnitude and orbit
closer to Saturn than Titan.
Iapetus is trickier to locate. It
orbits well beyond Titan and
shines five times brighter
(magnitude 10.2) when farthest
And its elliptical orbit brings it
closest to Earth (35.8 million
miles away) four nights later.
This is the month to view the
Red Planet — it won’t be as
good again until 2035.
Mars spends July among
the background stars of
Capricornus, but it shines
more than 100 times brighter
than any of the Sea Goat’swest of the planet than when
it’s farthest east (magnitude
11.9). Your best chance to spot
Iapetus comes July 1, when it
glows at 11th magnitude some
1.7' due north of Saturn.
You can find Pluto 15° east
of Saturn. Everyone’s favorite
dwarf planet reaches opposi-
tion July 12, but its appear-
ance doesn’t change during
the month. It glows at 14th
magnitude, so you’ll need an
8-inch or larger telescope to
see it. Pluto passes a mere 35"
west of the 6th-magnitude star
50 Sagittarii on the evening of
July 3. The finder chart (oppo-
site page, top) will help you
find Pluto on other nights.
On the night of July 26/27,
Mars reaches its most favor-
able opposition since 2003.stars. In early July, the Red
Planet rises around 10:30 p.m.
local daylight time and climbs
highest around 3 a.m. It
shines brilliantly at magni-
tude –2.2. But Mars doesn’t
stand pat. It brightens to mag-
nitude –2.8 in late July, far
brighter than any other night-
time object except for the
Moon and Venus (which setsThis periodic comet returns to the inner solar system every 6.6 years.
You can find it in mid-July as it treks from Cygnus into Cepheus.Iapetus’ brightness varies by a factor of five as it orbits the ringed planet.
A good time to catch it is when it passes due north of the gas giant July 1.