SOUTHERN SKY BY MARTIN GEORGE
June 2020
Bright planets rule all night
Our first taste of plan-
etary viewing in early
June comes during evening
twilight. That’s when you’ll
find Mercury hanging low in
the northwest. The innermost
planet reaches greatest elonga-
tion June 4, when it lies 24°
east of the Sun and stands 8°
high 45 minutes after sunset.
Mercury appears against the
backdrop of Gemini the Twins,
some 15° to the west (left) of
the constellation’s two bright-
est stars, Castor and Pollux.
Mercury remains on display
through mid-June, though it
becomes harder to see as it
dims and sinks closer to the
horizon. A telescope reveals the
planet’s crescent-shaped disk.
At greatest elongation, Mercury
spans 8" and appears 38 percent
lit. The disk grows larger and
the phase wanes as the month
progresses.
Later in the evening, the two
largest planets steal the show.
Jupiter and Saturn appear in
the east, gaining about 12° of
altitude with each passing hour.
The two behave like dancing
partners, carrying out their
retrograde (westward) motions
relative to the background stars
near the border between
Sagittarius and Capricornus.
Jupiter lies just west of this
border and rises about a half-
hour before Saturn. At magni-
tude –2.7, the giant planet
shines brighter than any other
star or planet in the evening
sky. For those eager to target
Jupiter through a telescope,
wait until late evening when itappears higher and its light
passes through less of Earth’s
image-distorting atmosphere.
The gas giant’s disk spans 46" at
midmonth and should show a
wealth of cloud-top detail. Also
keep your eyes out for Jupiter’s
four bright moons, which show
up through any instrument.
Saturn resides in western
Capricornus and trails Jupiter
by about 5°. Glowing at magni-
tude 0.3, the ringed world
appears some 15 times fainter
than its companion. But in
terms of its visual impact
through a telescope, Saturn is
the winner. In mid-June, the
planet’s globe measures 18" in
diameter while the rings span
41" and tilt 21° to our line of
sight. Saturn also boasts several
moons, led by 8th-magnitude
Titan, within reach of amateur
instruments.
You’ll have to wait until near
midnight local time for the next
planet to appear. Mars rises in
the east between 11 p.m. and
midnight all month. The Red
Planet drifts eastward relative
to the background stars, mov-
ing through Aquarius before
entering Pisces in June’s final
week. The ruddy world stands
out against this dim backdrop,
brightening from magnitude
0.0 to –0.5 during June.
Mars’ telescopic appearance
continues to improve as its dis-
tance shrinks. The planet’s disk
swells from 9" to 11" across this
month and should show some
of the dusky surface markings
that have intrigued observers
for more than a century.Although Venus has been a
prominent evening object since
early 2020, it disappeared in the
Sun’s glow in late May. It passes
between the Sun and Earth on
June 3 and then emerges in the
predawn sky by midmonth. On
the 15th, it rises more than an
hour before the Sun and stands
out in the east-northeast as twi-
light brightens. Gleaming at
magnitude –4.2, you won’t mis-
take it for any other object. By
month’s end, Venus rises two-
and-a-half hours before our
star and shines brilliantly at
magnitude –4.7. Throughout
this period, the planet lies in
Taurus the Bull, not far from
the Hyades star cluster.
Early risers will want to tar-
get Venus through a telescope
to see its large, crescent-shaped
disk evolving rapidly. On the
15th, the planet spans 54" and
shows a 4-percent-lit phase. By
the 30th, Venus’ diameter has
shrunk to 44" while the Sun
illuminates 18 percent of its
Earth-facing hemisphere.
The June 21 solar eclipse
is annular along a narrow path
in the Northern Hemisphere,
but parts of southern Africa,
Madagascar, Indonesia, and
northern Australia will experi-
ence a partial eclipse. From
Darwin, Australia, the event
begins at 8h03m UT and
reaches maximum at 8h35m,
when the Moon obscures 11
percent of the Sun’s diameter.The starry sky
If you look high in the south
after darkness falls in June, youcan’t miss the distinctive shape
of Crux the Cross. Although
the Southern Cross appears
nearly upright, inquiring minds
might want to know precisely
when this orientation occurs.
First, it depends on what
you mean by the word
“upright.” The two axes of the
Cross intersect at an angle of
82°, not 90°. However, most
people would view the Cross as
upright when the line joining
Acrux (Alpha [α] Crucis) and
Gacrux (Gamma [γ] Cru) —
the stars that form the bottom
and top of the asterism, respec-
tively — appears vertical.
It’s a fortunate coincidence
that the Cross looks upright
when it lies highest in the sky, a
position astronomers call
“upper culmination.” That’s
because Acrux and Gacrux
have nearly the same right
ascension, differing by only
4.6 minutes.
This slight difference means
that the two stars have the
same azimuth when they are
slightly past culmination, at an
azimuth of 183°, just 3° west
of due south. From Sydney,
Australia, at a latitude of 34°
south, this happens at a sidereal
time of 12h40m, which occurs
at 19h56m local time June 1.
Surprisingly, the azimuth
and sidereal time necessary to
see the Cross as upright change
slightly with latitude, with the
sidereal time increasing the
closer you are to the equator.
From 20° south latitude, for
example, the sidereal time
works out to 12h45m.