Astronomy

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Neptune

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Neptune’s Labor Day special

36 ASTRONOMY • SEPTEMBER 2017

Visible to the naked eye

Visible with binoculars

Visible with a telescope

MARTIN RATCLIFFE and ALISTER LING describe the

solar system’s changing landscape as it appears in Earth’s sky.

September 2017: An ice giant pinnacle

SKYTHIS

MONTH

S

cientists divide the

planets into two distinct

groups: the smaller,

rocky inner planets and

the larger, gaseous outer

planets. This month’s night

sky splits these objects along

the same lines. September eve-

nings feature excellent views

of the four remote worlds.

Highlights include seeing

Neptune at its annual peak

and the opportunity to observe

Saturn during the final days of

the Cassini mission.

Predawn observers can

feast their eyes on Earth’s

neighbors in the inner solar

system. Mercury makes its

finest morning appearance of

the year in mid-September,

coincidentally around the

same time that it skims within

1° of Mars. Meanwhile, bril-

liant Venus rules over its two

of this star on the 5th. At mag-

nitude –1.7, the planet appears

12 times brighter than the star.

If you plan to target Jupiter

through a telescope, do so early

in the month when it lies about

10° high 45 minutes after sun-

set. Because of its low altitude,

Jupiter won’t show much detail

on its 32"-diameter disk. Still,

you should see the planet’s two

dark equatorial belts and up to

four bright moons.

After a quick view of

Jupiter, turn your attention

toward Saturn. The magni-

tude 0.5 ringed planet stands

nearly 30° above the southern

horizon soon after sunset. As

the sky darkens, look for 1st-

magnitude Antares 13° west-

southwest of Saturn. The

planet’s yellow color, which

comes from ref lected sunlight,

contrasts nicely with the

intrinsic orange glow of the

distant red supergiant star.

Saturn resides in southern

Ophiuchus near 4th-magnitude

Xi (ξ) Ophiuchi. The planet

begins September 0.9° due

south of Xi and moves to a

siblings from higher in the

eastern sky. But the highlight

of this predawn gathering

comes during September’s

third week, when the waning

crescent Moon slides past the

three planets.

The first object you’ll want

to target these autumn nights

is Jupiter. Although the giant

planet has been spectacular

all year, its days are num-

bered. The world hangs low

in the west-southwest during

evening twilight all month. It

sets within two hours of the

Sun in early September and

less than an hour after sun-

down late in the month.

Jupiter lies among the

background stars of Virgo.

It spends most of September

within 4° of the Maiden’s

luminary, 1st-magnitude

Spica, passing 3° due north

position 1.4° southeast of the

star by month’s end. The pair

sets just before 12:30 a.m.

local daylight time on the 1st

and about a half-hour earlier

with each passing week.

Swing a telescope toward

Saturn, and you’ll immedi-

ately see the planet’s spectacu-

lar rings. They span 38" and

tip 26.9° to our line of sight in

mid-September. This wide tilt

affords earthbound observers

with superb views of ring

structure. The dark Cassini

Division that separates the

outer A ring from the brighter

B ring shows up clearly

through any instrument.

Modest scopes also reveal

several of the ringed world’s

moons. The brightest is 8th-

magnitude Titan, an easy

target through any scope.

It approaches within 1.3' of

Saturn when it passes north

(September 11 and 27) or

south (the 3rd and 19th) of

the planet. The moon strays

up to 2.9' from the gas giant

at greatest eastern and west-

ern elongation.

Neptune’s atmosphere displayed wispy clouds and a raging dark storm

when Voyager 2 flew past in August 1989. In typical amateur scopes, the

distant world shows a small, blue-gray disk. NASA/JPL

Neptune shines brightest at opposition the night of September 4/5, when it

appears against the backdrop of Aquarius. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY