skyandtelescope.com • SEPTEMBER 2019 47tquThese 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 fi st
at arm’s length. For clarity, the Moon is shown
three times its actual apparent size.
10 p.m. daylight-saving time.
Saturn follows about two hours
behind Jupiter. It culminates in the
south for our best telescopic views of
it not long after 9 p.m. as September
opens, but only a little more than half
an hour after sunset as the month
closes. Saturn fades from magnitude
+0.3 to +0.5 this month. It halts its
retrograde motion and resumes direct
motion (eastward relative to the stars)
on September 18th, so we see it dawdle
near the handle of the Teapot of Sagit-
tarius all month. The apparent equato-
rial diameter of Saturn decreases to less
than 17′′ late this month, but the gor-
geous rings remain nicely tilted at 25.1°.
NIGHT
Neptune comes to opposition on Sep-
tember 10th and is therefore visible all
night long. It’s at its highest — for best
telescopic observations — in the middle
of the night. Neptune is the most
distant of the major planets, so even
when closest to Earth in this month
of opposition, its magnitude-7.8 light
takes 4 hours (indeed almost exactly 4
hours) to reach us.
Uranus doesn’t climb to its highest
until about 3:30 a.m. daylight-saving
time in mid-September. But the mag-
nitude-5.7 world will arrive at opposi-
tion — and therefore more convenient
visibility — late next month. You can
access a chart for fi nding Neptune in
Aquarius and Uranus in Aries by going
to https://is.gd/urnep.DAWN
Mars is at conjunction with the Sun
on September 2nd, a day and a half
before Mercury. Since Mars’s orbital
speed isn’t much less than Earth’s, theJupiter
SaturnUranusNeptune
MercuryVenusEarthMars
March
equinoxJune
solsticeDecember
solsticeSept.
equinox
SunORBITS OF THE PLANETS
The curved arrows show each planet’s movement during September. The outer planets don’t
change position enough in a month to notice at this scale.skyandtelescope.com • SEPTEMBER 2019 47Sept 19 –21
AldebaranTAURUSMoon
Sept 19Moon
Sept 20Moon
Sept 21PleiadesAround 6 amLooking South, high in the skyDawn, Sept 25 –27
1 hour before sunriseRegulusDenebolaLEOMoon
Sept 25Moon
Sept 26Moon
Sept 27Looking EastSickleRed Planet usually moves rather swiftly
eastward relative to the background
stars, and therefore it takes many
weeks to emerge from morning twi-
light after its conjunction. Mars won’t
return to visibility until about the third
week of October.SUN AND MOON
The Sun arrives at the September
equinox at 3:50 a.m. EDT on September
23rd. This event inaugurates the start
of autumn in the Northern Hemisphere
and spring in the Southern Hemisphere.
The Moon reaches its fi rst-quarter
phase on the American evening of Sep-
tember 5th. That’s also the evening it
forms a beautiful triangle with Antares
7° or less below it and Jupiter much
closer to its left (the gap between the
Moon and Jupiter shrinks all evening
as they sink towards their setting). The
waxing gibbous Moon is 5–6° right of
Saturn at nightfall on September 7th,
and a similar distance left or lower left
of the planet on September 8th. The
distant full Moon (it’s at apogee) on the
night of September 13–14 is the Harvest
Moon. The waning gibbous Moon is in
the Hyades and near Aldebaran high in
the south at dawn on September 20th.
A very thin waning lunar crescent is
just a few degrees left of Regulus at
dawn on September 26th.¢FRED SCHAAF has penned this col-
umn since 1993.