Sky and Telescope - September 2018 - 46

SEPTEMBER 2018 OBSERVING
Sun, Moon & Planets by Fred Schaaf
To find out what's
visible in the sky
from your location,
go to skypub.com/
almanac.

Autumn Arrives
As summer gives way to fall, lengthening nights offer planet-spotting from dusk to dawn.

n these September dusks, Venus
ﬂames up to maximum brightness but
moves deeper into the Sun's afterglow
with each passing week. Jupiter is upper
left of Venus, and the gap between
them shrinks until late in the month.
Around nightfall, Saturn is at its highest in the south and the fading-butstill-brilliant Mars is in the southsoutheast. At dawn, Mercury is visible
only early in September.

DUSK AND EARLY EVENING
Venus was at greatest eastern elongation on August 17th but as September
q These scenes are drawn for near the middle
of North America (latitude 40° north, longitude
90° west); European observers should move
each Moon symbol a quarter of the way toward
the one for the previous date. In the Far East,
move the Moon halfway. The blue 10° scale bar
is about the width of your fist at arm's length.

progresses becomes quite poorly placed
for viewing at mid-northern latitudes.
The sunset altitude of the planet drops
from about 15° to 7° during September
for observers around latitude 40° north.
The interval between sunset and Venusset dwindles from roughly 85 minutes
to 45 minutes. But at least the resplendent planet brightens from -4.6 to an
even more dazzling -4.8. The phase of
Venus narrows from 40% to 17% in
September while its apparent size grows
from about 29″ to 46″.
On September 1st Venus is only
about 11/3° lower left of Spica (binoculars help to see the star). Venus is more
than 23° lower right of Jupiter but
closes the gap in the next few weeks.
Late in the month, however, the eastward motion of Venus slows drastically
as it starts to come "around the corner"
of the near side of its orbit. On the

Dawn, Sept 7- 8

Dusk, Sept 14-17

30 minutes before sunrise

1 hour after sunset
Moon
Sept 17
Moon
Sept 7

10°

evenings of September 27-29, Venus and
Jupiter will be at a minimum separation
of less than 14°.
Jupiter is very much dimmer than
Venus this month, and its radiance
diminishes from magnitude -1.9 to
-1.8. Jupiter begins September a little
more than 2° upper left of Alpha (α)
Librae (Zubenelgenubi) but is moving
eastward, away from the double star.
Jupiter's disk appears less than 33″
wide by late September. The planet sets
around 10:15 p.m. as September opens
and a bit less than 2 hours earlier as the
month closes.

DUSK TO AFTER MIDNIGHT
Saturn shines at magnitude +0.4 to
+0.5 this month. It's at its highest in
mid-twilight in early September. On
September 6th, Saturn halts its retrograde (westward) motion 2° above or

Moon
Sept 16
Moon
Sept 15

Saturn

β
Antares

γ Leo

Moon
Sept 8

C a t 's
Eyes

SCORPIUS

Mercury

S E P TE M B E R 2 018 * SK Y & TELESCOPE

δ
Jupiter

S A G I T TA R I U S

Regulus

Looking East

Moon
Sept 14

Looking South-Southwest

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Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018