1°AQUARIUSPISCESqr hsssNeptuneNEThe 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 latetwilight, 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 transits36 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