Astronomy – September 2019

1°

AQUARIUS

PISCES

q

r h

s

s

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Neptune

N

E

The giants of the

solar system take

center stage this month. While

Jupiter and Saturn stand out

every September evening and

deserve the lion’s share of your

attention, Neptune ranks a

close third as it reaches peak

visibility. The distant world

skims past a relatively bright

star, making it easier to find

than usual. Uranus lies farther

east and makes a worthy morn-

ing object. Finally, Mercury and

Venus appear in evening twi-

light at the end of the month.

Let’s start our tour in the

evening sky with the toughest

targets. Both Mercury and

Venus make fleeting appear-

ances in bright twilight as

September draws to a close. On

the 30th, Venus stands 1° above

the western horizon 30 minutes

after sunset. Gleaming at mag-

nitude –3.9, it shows up only

because it shines so brightly.

You might see magnitude –0.2

Mercury one binocular field to

Venus’ upper left.

The inner planets fare poorly

because the ecliptic — the path

of the Sun and planets across

the sky — makes a shallow

angle to the western horizon

after sunset from mid-northern

latitudes. Thus, the angular

separation of a planet from the

Sun translates mostly into dis-

tance along the horizon and not

into altitude.

You won’t have to strain at

all to see Jupiter. The giant

planet shines brilliantly in

southern Ophiuchus. It stands

25° high in the south-southwest

an hour after sunset in early

September and 20° high in the

southwest at the same time as

the month closes. During the

same period, Jupiter fades from

magnitude –2.2 to –2.0. This

makes the planet some 20 times

brighter than Antares, the red

supergiant star that lies 8° to

Jupiter’s lower right. The First

Quarter Moon stands 4° to

Jupiter’s right and 7° above

Antares on September 5.

Jupiter appears best through

a telescope starting in late

twilight, when the planet is

high enough to avoid the poor

seeing conditions that typically

interfere with objects near the

horizon. This gives you a cou-

ple of hours of prime observing

in early September and at least

an hour of good views as the

month winds down.

Jupiter reveals a wealth of

detail through any scope. Even

an untrained eye will notice

two dark equatorial belts, one

on either side of a brighter zone

coinciding with the equator.

The gas giant’s disk measures

37.4" across the equator and

35.0" through the poles at mid-

month, an obvious difference

once you know to look for it.

Lower-contrast details pop

into view along the edges of

the equatorial belts during

moments of steady seeing.

Jupiter’s fast spin drives these

features. The giant planet’s

equatorial regions rotate once

every 9 hours 50 minutes, tak-

ing five minutes less than at

higher latitudes.

Any telescope also reveals

Jupiter’s four bright moons. All

four typically show up against

the blackness of space near the

planet, but occasionally one or

more hides in plain sight as it

crosses in front of the planet.

Watching these transits, and

the far easier shadow transits

36 ASTRONOMY • SEPTEMBER 2019

A distant world’s close encounter (^)
September 2019
Neptune peaks
in Aquarius
SKY THIS MONTH
THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY.
Visible to the naked eye
Visible with binoculars
Visible with a telescope
BY MARTIN RATCLIFFE AND ALISTER LING
Neptune’s
distinct color
shows up in
amateur scopes,
though the
cloud features
that Voyager 2
saw in 1989 are
out of reach.
NASA/JPL
Neptune lies in eastern Aquarius at opposition the night of September 9/10,
just 6' from 4th-magnitude Phi (φ) Aquarii. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY