Astronomy

SOUTHERN

SKY

MARTIN GEORGE describes the solar system’s changing landscape

as it appears in Earth’s southern sky.

July 2018: Red Planet renaissance

July offers planet observers

quite a feast. As twilight deep-

ens in early July, four naked-eye

planets span the sky roughly

from east to west. But it is the

fifth bright planet, which rises

shortly after darkness falls, that

undoubtedly will command the

most attention.

Mars puts on its finest

appearance in 15 years this

month. The ruddy world rises

around 7 p.m. local time July 1.

It then shines at magnitude

–2.2, but it grows noticeably

brighter as it approaches a

splendid opposition on the

27th. Mars gleams at magni-

tude –2.8 during July’s entire

final week. That’s more than

100 times brighter than any of

the stars in its host constella-

tion, Capricornus.

Although the Red Planet

comes to opposition roughly

every 780 days, some of these

close approaches are consider-

ably better than others. This

one is particularly favorable

because the distance to Mars

shrinks to just 57.6 million

kilometers (on July 31), which

is about as close as it can get.

To make this appearance

even better, the planet lies at

a declination of –26°. From

mid-southern latitudes, Mars

passes nearly overhead around

midnight local time and

remains at an altitude above

45° from 9 p.m. to 3 a.m. Its

lofty position means observers

should get sharp views through

their telescopes.

At its closest approach in

late July, Mars’ disk spans 24.3",

and it remains above 20" all

month. Consider using eye-

piece filters to bring out surface

details. Red and orange filters

reveal dark markings well,

while a blue filter shows the

polar regions and clouds more

clearly. Make the most of this

apparition — Mars won’t be as

close again until 2035!

Although Mars rules the

night, don’t overlook July’s

other bright planets. Let’s

tackle them one by one, mov-

ing from west to east across the

early evening sky. First up is

Mercury, which appears low

in the northwest after the Sun

sets. The innermost planet

reaches greatest elongation

July 12, when it lies 26° east

of the Sun and stands 15° high

45 minutes after sundown.

A telescope reveals an

8"-diameter disk that’s slightly

less than half-lit. Over the fol-

lowing 10 days, the planet’s

disk grows while its phase

wanes to a pleasing crescent.

Look to Mercury’s upper

right and you can’t miss Venus.

The brilliant world brightens

from magnitude –4.1 to –4.3 in

July and dominates the evening

sky. Venus spends all month in

Leo the Lion, passing 1.1° north

of 1st-magnitude Regulus on

July 9. When viewed through

a telescope, the planet shows a

disk that grows from 16" to 20"

across during July and a gib-

bous phase that wanes from

70 percent to 57 percent lit.

The solar system’s two larg-

est planets also put on great

shows this month. Jupiter

rides high in the north after

darkness falls. Shining at mag-

nitude –2.2 at midmonth, it

overshadows the background

stars of Libra. Even a small tele-

scope resolves the gas giant’s

equatorial region into two par-

allel dark belts that sandwich

a brighter zone. Larger aper-

tures reveal more atmospheric

details on a disk that measures

40" across.

Last but not least, Saturn

stands as the brightest object

between Jupiter and Mars. The

ringed planet resides in north-

western Sagittarius, just north

of the Archer’s conspicuous

Teapot asterism, and climbs

highest in the north in late eve-

ning. Saturn shines at magni-

tude 0.1 in mid-July, when a

telescope reveals the planet’s

18"-diameter disk surrounded

by a lovely ring system that

spans 41" and tilts 26° to our

line of sight.

A slight partial solar

eclipse occurs July 13 for resi-

dents of southeastern Australia.

From Hobart, Tasmania, the

eclipse lasts from 2h52m

to 3h56m UT, and the Moon

covers 10 percent of the Sun’s

diameter at maximum.

Half a lunar month later, on

July 27, the Moon dips deeply

into Earth’s shadow. People in

Africa and Australia can wit-

ness a total lunar eclipse with

brilliant Mars at opposition just

7° south of the Moon. The

eclipse’s umbral phase begins at

18h24m and ends at 22h19m

UT, while totality runs from

19h30m to 21h13m UT. (In

Australia, the event takes place

before dawn July 28.) The 103

minutes of totality makes this

the longest total lunar eclipse

since 2000.

The starry sky

Southern Hemisphere observ-

ers enjoy a splendid view of the

Milky Way splayed across the

southern sky on July evenings.

Our galaxy’s disk stretches

from the eastern to southwest-

ern horizon, with Crux the

Cross near its center. Although

our eyes tend to focus on the

bright objects, the Aboriginal

people of Australia formed

patterns from the Milky Way’s

dark dust lanes.

Perhaps the best known of

these is the Emu. The most

famous dark nebula in the sky,

the Coal Sack, represents the

Emu’s head. The Coal Sack

tucks up against the southeast-

ern side of the Cross and is easy

to see under a dark sky. It

stands out well with averted

vision, which brightens the

Milky Way and thus increases

contrast, making the Coal Sack

appear darker.

At first sight, the Coal Sack

seems devoid of stars — indeed,

it blocks our view of most

objects beyond. It is far from

empty, however. Pay particular

attention to the open star clus-

ter NGC 4609. It lies 1.8° east of

1st-magnitude Acrux, Crux’s

brightest star, in the direction

of Alpha (α) and Beta (β)

Centauri. The 5th-magnitude

star BZ Crucis dominates this

group, with the remaining stars

spreading out to its northwest.

Based on its estimated dis-

tance of more than 4,000 light-

years, NGC 4609 lies behind the

Coal Sack and shows through

its dust. Although binoculars

show the 7th-magnitude cluster,

it appears most pleasing

through 10-centimeter and

larger telescopes. A magnified

view reveals quite a few stars,

many of them forming pairs.