Astronomy

SOUTHERN

SKY

MARTIN GEORGE describes the solar system’s changing landscape

as it appears in Earth’s southern sky.

August 2018: An evening extravaganza

Four bright planets stretch

across August’s evening sky.

Venus lies farthest west, shin-

ing brilliantly against the back-

ground stars of Virgo. On the

1st, it appears 30° below 1st-

magnitude Spica, the Virgin’s

luminary. The magnitude –4.3

planet outshines the star by

some 100 times.

Venus moves eastward

against the starry backdrop

during August. It reaches great-

est solar elongation on the 17th,

when it appears 46° east of our

star and stands more than 30°

high an hour after sundown.

The planet remains the sky’s

brightest point of light until it

sets shortly after 9 p.m. local

time. By month’s end, Venus

pulls within 1.5° of Spica.

The planet’s appearance

through a telescope changes

noticeably throughout August.

On the 1st, it shows a slightly

gibbous phase on a disk that

spans 20". At midmonth, the

Sun illuminates precisely half of

Venus’ 24"-diameter disk. And

by the time August closes, the

planet swells to 29" across while

its phase dwindles to a crescent.

Next in line is Jupiter. The

giant planet stands high in the

northwest as darkness falls,

residing among the relatively

dim stars of the constellation

Libra. Jupiter shines at magni-

tude –2.0, barely one-tenth as

bright as Venus but still notice-

ably brighter than any star.

The gas giant’s atmosphere

provides a visual treat through

any telescope. Look for two par-

allel dark belts, one on either

side of a much brighter zone

coinciding with the planet’s

36"-diameter equator. Smaller

features along the edges of these

belts show up during moments

of good seeing. You can also

track the movements of Jupiter’s

four largest moons over the

course of a few hours.

If you trace a line from

Venus to Jupiter and extend it

a bit farther than the distance

between those two, your eyes

will fall on magnitude 0.3

Saturn. The ringed planet

resides in Sagittarius and

appears high in the sky

throughout the evening hours.

(From a wide swath of the

Southern Hemisphere, it passes

nearly overhead at its peak.)

Saturn moves slowly westward,

or retrograde, relative to the

background stars as Earth

continues to outpace it in the

months following opposition.

Although Saturn is the dim-

mest of the four evening plan-

ets, it is also the most beautiful

through a telescope. And its

great altitude on August eve-

nings provides nearly perfect

viewing conditions. At mid-

month, the planet’s disk mea-

sures 18" across while the rings

span 40" and tilt 27° to our line

of sight. Any scope should

reveal the broad Cassini

Division that separates the

outer A Ring from the brighter

B ring. The narrow Encke Gap

near the A ring’s outer edge

shows up under excellent con-

ditions with a 20-centimeter or

larger instrument.

Head one constellation far-

ther east and you can’t miss

Mars. Only the Moon and

Ve nu s o u t s h i n e M a r s t h i s

month, and neither has the

Red Planet’s distinctive color.

Mars spends most of August in

southwestern Capricornus, but

its retrograde motion carries it

into far eastern Sagittarius in

the month’s final week.

The ruddy world reached

opposition and peak visibility

in late July, and it remains a

stunning sight throughout

August. Although it dims from

magnitude –2.8 to –2.1 and its

apparent diameter shrinks

from 24" to 21", these values

still exceed a typical martian

opposition. And its high eve-

ning altitude promises good

seeing conditions for viewing

fine detail through a telescope.

Mercury proves to be a dif-

ficult target during August.

The innermost planet passes

between the Sun and Earth

on August 9 and then slowly

climbs into view low in the

east-northeast before dawn.

Still, even at greatest western

elongation on the 26th, it

appears only 4° high a half-

hour before sunrise.

The starry sky

The southern sky contains

enough telescopic delights to

keep an observer busy for a life-

time. Even something as small

as a 20-cm instrument will let

you observe a huge number of

these deep-sky wonders.

In winter and early spring,

we reap the benefits of living

in the Southern Hemisphere.

In early evening, the spec-

tacular southern Milky Way

stretches across the sky while

the center of our galaxy passes

nearly overhead.

One of winter’s most famil-

iar star patterns is Scorpius the

Scorpion. The constellation

does resemble a scorpion,

though some see a reversed

question mark in its form. Its

most distinctive features are

1st-magnitude Antares, the star

that marks the Scorpion’s heart,

and the arachnid’s curved tail.

The arc of the tail encloses a

fascinating deep-sky object.

The Bug Nebula (NGC 6302)

lies near the center of the tail,

4° due west of magnitude 1.6

Shaula (Lambda [λ] Scorpii),

the bright star marking the

Scorpion’s Stinger. The Bug is a

planetary nebula, but the word

“planetary” is a misnomer —

such nebulae have nothing to

do with planets. They are the

glowing embers of material

ejected by aging Sun-like stars.

They got their name because

early observers saw a superficial

resemblance to the blue-green

glow of the planet Uranus.

The Bug Nebula looks spec-

tacular through a 20-cm tele-

scope, though smaller apertures

do show it. It is rather small,

spanning 50", but has a high

surface brightness and thus

shows up quite easily. The neb-

ula appears elongated and has a

bright center. You won’t see the

central star that puffed off

these wispy tendrils, however.

Astronomers finally detected

this ultrahot star, which has a

surface temperature of about

200,000 K, with the Hubble

Space Telescope in 2009.

Although photographs show

the Bug’s complexity, its overall

appearance is reminiscent of a

f lying insect with a large wing-

span. Every time I gaze at this

object, however, it reminds me

more of an unfortunate bug

that met an untimely end on

the windscreen of my car.