Sky & Telescope - September 2024 - 48

SEPTEMBER 2024 OBSERVING
Celestial Calendar by Bob King
Comet Tsuchinshan-ATLAS
on the Morning Stage
The dawn sky is briefly adorned with a low-hanging icy visitor.
H
ope is an essential ingredient in
amateur astronomy. We hope the
sky will clear for the next meteor shower
or eclipse, but until the moment arrives,
we survive on the quiet thrill of not
knowing exactly what to expect. And if
you're like me, this month you're hoping
that Comet Tsuchinshan-ATLAS (C/2023
A3) develops into a memorable sight.
The comet was discovered at Purple
Mountain Observatory in China on
January 9, 2023, and independently
on February 22nd by the Asteroid
Terrestrial-impact Last Alert System
(ATLAS). A new visitor from the Oort
Cloud, in spring the comet became an
easy telescopic object. Now it's poised to
potentially brighten to magnitude +0.5
by the end of September.
Provided the comet remains intact
as it dives sunward toward its September
27th perihelion, we can expect a
brief but bright appearance at dawn
in the final week of the month. For
observers at mid-northern latitudes,
Tsuchinshan-ATLAS will be at its highest
on the morning of the 29th, but
even then, it sits only about 5° above
the east-southeastern horizon 45 minutes
before sunrise.
Oppositions and Occultations
for Saturn and Neptune
SATURN AND NEPTUNE both come to
opposition in September, and both will
be occulted by the Moon. Saturn shines
at magnitude +0.6 opposite the Sun on
the 8th in eastern Aquarius.
The planet's famous rings give only
the slightest tip of the hat earthward,
their inclination varying from 3.5°
on the first of the month to 4.5° by
month's end. But the lack of ring glare
makes it easier to spot faint moons near
the planet. If you've already seen the
brighter ones - Titan (magnitude 8.3),
Rhea (9.7), Tethys (10.2), Dione (10.4),
Iapetus (variable, 10 to 12) - this season
is the time to hunt for closer-orbiting
Enceladus (11.7) and Mimas (12.9),
and distant Hyperion (14.2). And when
you run out of those, Saturn's got 138
more moons - just don't expect to
48 SEPTEMBER 2024 * SKY & TELESCOPE
make much of a dent in that number.
After Hyperion the next brightest satellite
is Janus at magnitude 14.5. Precious
few have made its visual acquaintance.
Iapetus is an interesting target.
When it's at eastern elongation on September
3rd, it presents its dark face and
glows dimly at around magnitude 12.
Exactly the opposite occurs at western
elongation - its icy hemisphere faces
our direction, and the moon brightens
to 10th magnitude. That occurs on
October 12th when Iapetus sits 9.4′ (or
13 ring widths) from the planet.
Saturn's near face-on presentation
at opposition also means its moons
will queue up in a straight line more
often this season instead of being
arrayed above and below the planet. For
that same reason, the visibility of the
pOn May 8, 2024, Comet Tsuchinshan-ATLAS
(C/2023 A3) exhibited a well-developed dust
tail, as shown in this photo by Dan Bartlett.
The comet glowed at 10th magnitude at the
time but could reach magnitude +0.5 later this
month in morning twilight.
You'll need a cloudless, unobstructed
horizon and a pair of binoculars
to catch sight of the comet in the
brightening twilight. The best mornings
will be those after a weather front
has moved through and whisked the
sky clear of haze and smoke. Given the
possibility that Tsuchinshan-ATLAS
may display a significant, westwarddiaphanous
crepe ring (also called ring
C), the innermost of the three major
rings, may be enhanced.
Where the crepe ring crosses in front
of the planet, it appears as a gray line
that can be tricky to distinguish from
the main rings and their shadows. But
when viewed near opposition, the particles
that form the ring stack up along
our line of sight, increasing the crepe
ring's visibility. Look for a filmy, grayish
arc between the inner edge of the ring
plane and Saturn's globe.
On the morning of September 17th,
observers in the western half of the
U.S. and Canada can watch the nearly
full Moon occult Saturn. From San
Francisco, California, the disappearance
begins at 4:06:38 a.m. PDT, and
the lunar disk takes about 60 seconds
to completely cover the planet and its
rings. Saturn returns to view an hour
later just as twilight gets underway. For a
list of cities and times, go to the International
Occultation Timing Association's
Sky & Telescope - September 2024

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