SOUTHERN
SKY
MARTIN GEORGE describes the solar system’s changing landscape
as it appears in Earth’s southern sky.September 2018: Four-planet showcase
September opens with a fine
sight in the western evening
sky — brilliant Venus passes
within 2° of blue-white Spica.
At magnitude –4.6, the planet
shines nearly 200 times bright-
er than 1st-magnitude Spica,
the brightest star in the constel-
lation Virgo. And Venus bright-
ens even more as the month
progresses, reaching magni-
tude –4.8 at greatest brilliancy
September 21. The planet’s
eastward motion relative to the
background stars carries it near
the border with neighboring
Libra by month’s end.
This is probably the best
month to observe Venus
through a telescope. As its dis-
tance from Earth decreases, the
planet’s disk grows larger and
shows an increasingly obvious
crescent shape. On the 1st,
Venus appears 29" across and
40 percent lit. By the 30th, the
planet spans 46" and the Sun
illuminates 18 percent of its
Earth-facing hemisphere.
If you view Venus each clear
night, you’ll notice it edging
closer to Jupiter. The giant
planet lies in Libra the Scales,
creeping eastward from night
to night. Jupiter shines at mag-
nitude –1.9 in mid-September.
Although this is second only
to Venus among the planets, it
appears less than one-tenth as
bright as its sibling.
Jupiter stands high in the
west in early evening and pres-
ents a lovely sight in your tele-
scope. Notice that the gas
giant’s disk appears flattened,
measuring 33.7" across the
equator but just 31.5" through
the poles. The most distinctive
feature of the jovian cloud topsis an alternating series of bright
zones and darker belts. Small
scopes also reveal Jupiter’s four
brightest moons.
Saturn follows Jupiter in
a sweeping curve across the
northern evening sky. The
ringed planet appears as an
interloper among the stars of
Sagittarius. At magnitude 0.4,
however, it is more than a full
magnitude brighter than the
Archer’s brightest star.
With a declination of –23°,
Saturn continues to be placed
beautifully for observers in the
Southern Hemisphere. It lies
nearly overhead in early eve-
ning, so its light passes through
the least amount of Earth’s
image-distorting atmosphere.
Any telescope reveals the plan-
et’s golden disk, which appears
17" across at midmonth, sur-
rounded by a gorgeous ring
system that spans 38" and tilts
27° to our line of sight. Its
brightest moon, 8th-magnitude
Titan, shows up nicely through
small scopes, while several
fainter ones become visible
in 20-centimeter and larger
instruments.
Although the best days for
Mars are now behind us, it
remains a standout object from
evening twilight until well past
midnight. It trails about two
hours behind Saturn, and
passes nearly overhead in mid-
evening. It spends the month
traveling slowly eastward
against the backdrop of south-
western Capricornus.
As the Red Planet recedes
from Earth, it dims noticeably.
It starts September blazing at
magnitude –2.1, but it fades by
half, to magnitude –1.3, bymonth’s end. Mars’ apparent
diameter also shrinks during
September, from 21" to 16",
though that is still large enough
to reveal subtle surface details
during moments of good see-
ing. You also should notice the
planet’s gibbous phase, particu-
larly in late September when
the Sun lights 89 percent of its
visible hemisphere.
For all practical purposes,
Mercury can’t be seen this
month. The planet passes
behind the Sun from our per-
spective at superior conjunction
September 21. For those eager
to glimpse the elusive planet,
it will make a fine evening
appearance in early November.The starry sky
September evenings offer mag-
nificent views of the southern
Milky Way. Both Scorpius the
Scorpion and Sagittarius the
Archer — the constellations
closest to our galaxy’s center
— ride high in the western sky.
When I attend international
conferences, I often find that
my Northern Hemisphere col-
leagues envy me because this
region hangs so low in their sky.
As regular readers know,
this is one of my favorite parts
of the sky. This month, let’s
explore some of the interesting
star clusters in the vicinity
of the Scorpion’s Stinger.
Magnitude 1.6 Shaula (Lambda
[λ] Scorpii) and magnitude 2.7
Upsilon (υ) Sco mark the busi-
ness end of the Scorpion.
Although the spectacular
open clusters M6 and M7 lie
just a few degrees northeast of
Shaula, I want to highlight a
few lesser-known targets.Closest to the stinger are
Harvard 16 and Collinder 332.
The first lies about 0.5° north of
Shaula and Upsilon and forms
an equilateral triangle with the
two. Harvard 16 is quite scat-
tered and barely recognizable
as a cluster. Still, several of its
stars spread out roughly along
an east-west line and are quite
easy to see through a telescope.
Collinder 332 is more inter-
esting. It lies 0.2° due north of
Upsilon and also spreads out
along a line. To my eyes, it
looks like a dog with its gaze
fixed firmly in a northwesterly
direction as if about to run and
retrieve something.
Our next target is Collinder- You can locate it 1.1° south-
east of Shaula, roughly 40 per-
cent of the way from that star to
magnitude 2.4 Kappa (κ) Sco.
This cluster spans about 0.5°
and makes a nice target through
binoculars. In my 10x50s, I can
resolve several of its stars.
I have a tendency to save the
best until last, and this month
is no exception. Just 1.3° east
of Shaula lies the little cluster
NGC 6400. Although it is the
smallest of the four in our
September tour, it’s a beautiful
sight through a 20-cm telescope
and appears richer than the
other three. The 9th-magnitude
cluster spans about 8' and
stands magnification well.
If you do not have a go-to
scope, there’s an easy way to
find NGC 6400. Because it has
nearly the same declination as
Shaula, simply aim your tele-
scope at the bright star and
wait seven minutes for Earth’s
rotation to carry NGC 6400
into view.