SOUTHERN
SKY
MARTIN GEORGE describes the solar system’s changing landscape
as it appears in Earth’s southern sky.April 2018: Mars slides past Saturn
Although early evening planet
hunters won’t have much to
look at when April begins, con-
ditions improve steadily. Venus
sets an hour after the Sun on
the 1st and hangs so low in
bright twilight that you’ll have
trouble seeing it. But the bril-
liant planet slowly pulls away
from the Sun and becomes
easier to see. By the 30th, Venus
appears some 10° high in the
northwest a half-hour after
sundown and grows more con-
spicuous as the sky darkens.
The inner planet then
stands just below the Hyades
star cluster in Taurus. Venus
shines at magnitude –3.9 and
appears 100 times brighter than
1st-magnitude Aldebaran, the
Bull’s luminary. A telescope
shows the planet’s 11"-diameter
disk, which appears nearly full.
On the sky’s opposite side,
Jupiter dominates the scene.
The giant world rises after twi-
light ends in early April but
stands clear of the horizon as
night falls late in the month.
Jupiter lies in Libra, where it
is moving slowly westward in
advance of its May opposition.
Gleaming at magnitude –2.4 in
mid-April, the solar system’s
largest planet outshines Libra’s
brightest star by 100 times.
Jupiter climbs high in the
east by late evening and passes
nearly overhead after midnight.
Plan to spend some quality time
viewing the planet through a
telescope. Even the smallest
instruments show details in
Jupiter’s f lattened disk, which
spans 44" at midmonth. You
can expect to see at least two
dark atmospheric belts sand-
wiched around a brighter zonecoinciding with the planet’s
equator. Small scopes also
reveal up to four bright moons.
The next two planets arrive
in late evening. On April 1,
Mars and Saturn rise shortly
before 11:30 p.m. local time and
within 10 minutes of each other.
Ruddy Mars glows a bit brighter
and stands less than 2° to the
upper right of golden Saturn.
The Red Planet slides 1.3° south
of its neighbor on the 2nd.
Both worlds reside in
Sagittarius the Archer. Saturn
barely budges from its spot 2°
north of the 5th-magnitude
globular star cluster M22.
Because the planet remains
essentially stationary relative to
this backdrop, it rises four min-
utes earlier each day just as the
stars do, and comes up by 9:30
p.m. in late April. The ringed
planet brightens slightly, from
magnitude 0.5 to 0.4, during
the course of the month.
For many people, Saturn is
their favorite sight through a
telescope. And the views don’t
get much better than what
you’ll find this month when the
planet climbs nearly overhead
shortly before twilight begins.
Any scope shows Saturn’s
17"-diameter disk surrounded
by a ring system that spans 39"
and tips 25° to our line of sight.
The wide tilt affords spectacu-
lar views of ring structure,
including the Cassini Division
that separates the outer A ring
from the brighter B ring.
Unlike Saturn, Mars moves
quickly and grows noticeably
brighter during April. The Red
Planet crosses most of northern
Sagittarius, ending the month
in the eastern part of this con-stellation. It also brightens by 75
percent during April as its mag-
nitude rises from 0.3 to –0.3.
If you turn a telescope
toward Mars, you’ll see its
apparent diameter grow along
with its brightness. The planet’s
disk spans 8" in early April and
11" by month’s end, big enough
to show detail through modest
scopes. The martian south pole
tilts 12° in our direction in late
April, affording us nice views
of the south polar cap. Also
keep an eye out for subtle dark
markings. These will become
increasingly apparent as Mars
approaches a fine opposition
in late July, when it will appear
24" across.
Our final planet bursts on
the scene with the approach of
dawn. Mercury is spectacular
in April’s second half as it
enjoys its finest morning
appearance of 2018. The inner
planet reaches greatest elonga-
tion April 29, when it lies 27°
west of the Sun and appears
nearly 15° above the eastern
horizon an hour before sunrise.
Watch as a thin crescent Moon
passes near the magnitude 1.7
world April 14 and 15.
The best views through a
telescope also come on these
mid-April mornings, when
Mercury appears 10" across and
the Sun illuminates less than
20 percent of its disk. At great-
est elongation, the magnitude
0.3 planet spans 8" and shows
a nearly half-lit phase.The starry sky
In the southern sky, the
region encompassing Crux
the Cross and the two pointer
stars — Alpha (α) and Beta (β)Centauri — is undoubtedly the
best known. You can find this
area in the southeast on April
evenings and high in the south
around midnight.
However, appearances can
be deceiving. These stars lie at
different distances from Earth,
and the striking pattern they
create owes a great deal to how
those distances combine with
the stars’ intrinsic luminosities.
The scene would look quite
different if all the stars were at
the same distance.
Astronomers compare stel-
lar luminosities — the total
amount of light a star radiates
into space — by quoting the
magnitude a star would appear
to have if it were at a standard
distance of 10 parsecs (32.6
light-years). Let’s imagine what
Crux and the pointer stars
would look like to the naked
eye if they were all at this stan-
dard distance.
Perhaps the biggest surprise
would be Alpha Centauri,
which would glow rather dimly
at magnitude 4.4. It would be
difficult to see from a light-
polluted city. Beta Cen, on
the other hand, would be the
brightest in the entire group.
Shining at magnitude –4.8,
it would match Venus at its
most brilliant.
Among the stars of Crux,
Alpha, Beta, and Delta (δ)
Crucis would all be extremely
bright, shining at magnitudes
–3.7, –3.4, and –2.3 respec-
tively. Gamma (γ) Cru, at the
top of the Cross, would be a
respectable magnitude –0.6.
Surprisingly, it would shine
at the same magnitude as
Epsilon (ε) Cru.