SOUTHERN SKY BY MARTIN GEORGE
May 2020
Jupiter and Saturn on the prowl
Although Venus has
dominated our early
evening sky for the past several
months, its days are numbered.
The brilliant planet disappears
in twilight by late May, but not
before putting on a grand dis-
play. In early May, you can
find Venus hanging low in the
northwest as darkness falls.
Gleaming at magnitude –4.7,
the planet stands out despite
lying just 10° high 45 minutes
after sundown. Its altitude dips
to 6° at the same time relative
to sunset in mid-May.
Telescopes of all sizes offer
stunning views. Because the
Sun illuminates Venus from
behind, it shows a lovely cres-
cent shape. On May 1, the plan-
et’s disk spans 39" but appears
only one-quarter lit. Two weeks
later, on the evening of the
15th, Venus’ diameter has
swollen to 49" while the Sun
illuminates just 12 percent of its
Earth-facing hemisphere.
The solar system’s two larg-
est planets come on the scene
later in the evening. Jupiter
pokes above the eastern hori-
zon shortly after 10 p.m. local
time in early May and some
two hours earlier by month’s
end. Saturn follows its bigger
brother closely all month, ris-
ing less than a half-hour later.
Jupiter resides in eastern
Sagittarius, not far from
the Archer’s border with
Capricornus the Sea Goat.
Shining at magnitude –2.4 —
a magnitude brighter than the
night sky’s brightest star, Sirius
— the gas giant rules the lateevening sky. It looks just as
impressive through a telescope.
The planet’s disk measures 43"
across in mid-May and should
show a wealth of detail in its
dynamic atmosphere. Look in
particular for two parallel dark
belts, one on either side of a
brighter zone that coincides
with the jovian equator. And
it’s always fun to follow the
changing positions of Jupiter’s
four bright moons: Io, Europa,
Ganymede and Callisto.
The ringed planet graces far
western Capricornus. Glowing
at magnitude 0.5, it appears less
than 10 percent as bright as its
planetary companion. Still, that
makes it brighter than all but a
handful of stars visible in May’s
late evening sky.
The proximity of Jupiter and
Saturn makes it easy to swing
your telescope between the two.
Although you won’t be disap-
pointed with your view of
Jupiter, Saturn steals the show.
The more distant world boasts
a 17"-diameter disk surrounded
by a glorious ring system that
spans 39" and tilts 21° to our
line of sight. Modest apertures
also bring four or five satur-
nian satellites into view.
Don’t go to bed after you’ve
satisfied your giant planet crav-
ings, because Mars will join
them on the celestial stage
within a couple of hours. The
Red Planet heads eastward
from Capricornus into
Aquarius during May. The
ruddy world brightens from
magnitude 0.4 to 0.0 this
month, far outshining thedim stars that call these con-
stellations home.
Mars will put on a great
show later this winter and
spring, but it is still worth
viewing through a telescope
now. By the end of May, the
diminutive planet’s disk spans
9". Its southern hemisphere
continues to tip toward Earth,
so the white south polar cap
should show up well during
moments of good seeing.
After Mercury passes on
the far side of the Sun from
our viewpoint May 4, it slowly
climbs into view after sunset.
Those with sharp eyesight
might glimpse it low in the
northwest by month’s end. On
the 31st, the magnitude 0.0
planet stands 6° high 45 min-
utes after sundown.The starry sky
The southern half of the celes-
tial sphere holds several out-
standing open star clusters.
Although several of my favor-
ites reside in Carina the Keel,
this month I want to spend
some time with the showpiece
of Crux the Cross: the Jewel
Box Cluster (NGC 4755).
During May, this distinctive
constellation climbs highest in
the south in midevening.
The Jewel Box lies just 1°
southeast of the magnitude 1.3
star Beta (β) Crucis. Although
the cluster looks like a fuzzy
star to the naked eye, binocu-
lars reveal its true nature. The
view through a telescope, how-
ever, is the one you won’t soon
forget. NGC 4755 looks a bitlike an arrowhead, but I find
it more closely resembles an
uppercase letter A. The cluster’s
brightest star is magnitude 5.8
SAO 252069, which marks the
top of the A. Kappa (κ) Cru is
0.1 magnitude fainter and lies
at the A’s bottom right.
But perhaps the cluster’s
most outstanding feature is the
presence of a distinctly orange-
red star in the A’s crossbar. This
magnitude 7.5 sun, cataloged as
SAO 252073, is a red supergiant
swimming in a sea of mostly
blue giants and supergiants.
The Jewel Box got its name
from English astronomer John
Herschel — son of William,
who discovered Uranus —
when he observed the cluster
from South Africa in the 19th
century. He wrote that the
colors of its stars “give it the
effect of a superb piece of
jewellery.” And he called
SAO 252073, “The central star
(extremely red) of a most vivid
and beautiful cluster of from
50 to 100 stars.”
Astronomers estimate
NGC 4755 lies between
6,500 and 7,800 light-years
from Earth. (Its distance is
difficult to pinpoint because
the Coalsack Nebula lies nearby
and obscures some of its light.)
Compare this to the Southern
Pleiades (IC 2602) in Carina,
which lies only 550 light-years
away. If the Jewel Box were at
this distance, the combined
light of its stars would match
Sirius, and the cluster would
spread across more than 2°
of sky!