SOUTHERN SKY BY MARTIN GEORGE
July 2020
Jupiter and Saturn at their best
The solar system’s two
largest planets make a
spectacular pair this month.
Jupiter and Saturn appear low
in the east soon after darkness
falls in early July. And not only
do they climb higher hour by
hour, they also rise earlier as
the month progresses.
Jupiter shines at magnitude
–2.7 and stands out more than
its neighbor. The giant planet
reaches opposition and peak
visibility July 14, when it rises at
sunset and climbs highest in the
north around midnight local
time. You can find it among the
background stars of eastern
Sagittarius, though truth be
told, it’s easier to locate the con-
stellation using the planet as a
guide than vice versa.
July is the best time this
year to observe Jupiter through
a telescope. At opposition, its
disk spans 48" across the equa-
tor and 45" through the poles,
a difference that’s surprisingly
easy to see. Look for an alter-
nating series of bright zones
and darker belts that run paral-
lel to the giant planet’s equator,
as well as fine details that show
up during moments of good
seeing. Also watch for Jupiter’s
four bright Galilean moons as
they dance around the planet.
The best views come when the
planet lies high in the sky and
its light passes through less of
Earth’s atmosphere.
Saturn follows Jupiter across
the sky, reaching opposition just
six days after its companion,
on July 20. The ringed planet
spends the month’s first fewnights in western Capricornus
before crossing into Sagittarius.
Glowing at magnitude 0.1 at
opposition, it pales in compari-
son with Jupiter but easily out-
shines the background stars in
these constellations.
Few sights can match the
beauty of Saturn through a
telescope. As with Jupiter, plan
to wait until the ringed world
climbs high in the sky. The
planet spans 18" while the stun-
ning ring system measures 42"
across and tips 22° to our line
of sight. This large tilt affords a
nice view of the dark Cassini
Division that separates the
outer A ring from the brighter
B ring. Saturn’s brightest
moons, led by 8th-magnitude
Titan, are also a treat.
By late evening, a third
bright planet graces the sky.
Mars rises in the east before
midnight local time and climbs
highest in the north shortly
before twilight starts to paint
the sky. The Red Planet begins
and ends July among the back-
ground stars of Pisces the Fish,
but it spends the middle of the
month in Cetus the Whale.
Mars continues to brighten
rapidly. As July begins, it shines
at magnitude –0.5; it appears
75 percent brighter (magnitude
–1.1) by month’s end. Mars’
telescopic appearance improves
noticeably as well. Its disk
swells from 11.5" to 14.5"
across, big enough to show
some dusky surface markings.
As Jupiter and Saturn dip
low in the west before dawn,
Venus pokes above the easternhorizon. The brilliant planet
rises before twilight starts and
climbs high in the northeast as
the sky brightens. Venus shines
at magnitude –4.7, its brightest
for this morning apparition,
against the backdrop of Taurus.
It spends the first two weeks of
July passing in front of the
V-shaped Hyades star cluster.
It’s worth getting up early
to view Venus’ lovely crescent
through a telescope. On July 1,
the inner planet shows a
43"-diameter disk that is just
19 percent illuminated. By
month’s end, the world appears
28" across and 42 percent lit.
The second half of July also
features a brief appearance by
Mercury. For about a week on
either side of its July 22 greatest
western elongation, the small
planet lies low in the east-
northeast during morning twi-
light. At its peak on the 22nd,
the magnitude 0.2 world lies
6° high 45 minutes before
sunrise. A telescope reveals
Mercury’s 8"-diameter disk
and 38-percent-lit phase.The starry sky
Our view to the south early on
these winter evenings features
the spectacular Southern
Cross and two bright stars
nearby: Alpha (α) and Beta (β)
Centauri. Just southeast of
Alpha Cen, the night sky’s
third-brightest star, resides
the distinctive constellation
Tria ng u lu m Aust ra le t he
Southern Triangle. It seems at
first glance that Triangulum
Australe and Centaurus mustshare a border, but they do
not. Tucked between the two
is the tiny constellation
Circinus the Compasses.
Don’t confuse Circinus with
another constellation having a
similar common name: Pyxis
the Compass. The two repre-
sent entirely different instru-
ments. The compass Circinus
refers to is the type we all used
in school to draw circles. Pyxis,
on the other hand, represents a
mariner’s compass that seafar-
ers use to tell directions.
Circinus covers an area of
93.35 square degrees, making it
the fourth-smallest constella-
tion. (With an area of 68.45
square degrees, Crux the Cross
is the tiniest.)
French astronomer Nicolas
Louis de Lacaille introduced
Circinus in the 1750s. On his
star chart, he clearly marked
the constellation as le Compas
and described the device as the
one used in geometry class-
rooms. In the wonderful vol-
ume Histoire de l’Académie
royale des sciences, Lacaille
described it as le Compas du
Géomètre, not to be confused
with La Boussole ou le Compas
de mer, which was the mariner’s
compass we know as Pyxis.
Circinus holds a few objects
worth observing through a tele-
scope, though most of them are
best viewed with large aper-
tures. One of the nicest subjects
for small instruments is the
double star Alpha Circini. Its
two stars glow at magnitudes
3.2 and 8.6 and are easy to split
thanks to their 15" separation.