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

In 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-but-still-brilliant Mars is in the south-southeast. 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 progresses becomes quite poorly placed for viewing at mid-northern latitudes. The sunset altitude of the planet drops from about 15[degrees] to 7[degrees] during September for observers around latitude 40[degrees] north. The interval between sunset and Venus-set 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 1 1/3[degrees] lower left of Spica (binoculars help to see the star). Venus is more than 23[degrees] 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 evenings of September 27-29, Venus and Jupiter will be at a minimum separation of less than 14[degrees].

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[degrees] upper left of Alpha (a) 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[degrees] above or upper left of M8 (the Lagoon Nebula) and east of M20 (the Trifid Nebula), and slowly begins to move eastward in northern Sagittarius. During the month, the gap between Saturn and Jupiter closes from about 45[degrees] to 41[degrees]. On the other side--eastward--Mars increases its separation from Saturn from 27[degrees] to 33[degrees] during September. Saturn's rings look even more three-dimensional than usual this month with Saturn passing through eastern quadrature (90[degrees] east of the Sun) on September 25th. Saturn sets a little after 1 a.m. local time on September 1st and a little after 11 p.m. on September 30th.

Mars begins the month at a bracing magnitude -2.1, still a bit brighter than Jupiter. The imposing orange-yellow beacon loses half of its brightness during September, however, fading to magnitude -1.3. Its apparent diameter also dwindles from 21" to 16" this month --but even this is wider than Mars has been in any of the past 13 years, bar one. Furthermore, Mars reaches its highest in the south conveniently early in the night this month, passing the meridian around 10:20 p.m. on September 1st and a bit before 9 p.m. on September 30th.

In space, Mars reaches perihelion, 1.38 a.u. from the Sun, on September 16th. In the heavens, Mars is trekking back from Sagittarius into Capricornus. The Red Planet sets not long before 3 a.m. as September begins and a little after 1:30 a.m. as the month ends.

ALL NIGHT

Neptune reaches opposition, visible all night in Aquarius, on September 7th. It shines at magnitude 7.8 and is 2.4" wide in telescopes. Uranus glows in southwestern Aries this month, two magnitudes brighter than Neptune and appearing considerably larger at 3.7". Neptune transits the meridian roughly around midnight, Uranus about three hours later. Finder charts for these two planets are on pages 48-49.

DAWN

Mercury appears low in the east-northeast 30 to 45 minutes before sunrise in the first week of September. On September 5th and 6th, 1st-magnitude Regulus may be glimpsed with optical aid a bit more than 1[degrees] from Mercury, which shines at magnitude -1. Mercury is lost to view around September 11th and reaches superior conjunction with the Sun on September 20th.

SUN AND MOON

The Sun passes through the September equinox at 9:54 p.m. EDT on September 22nd, marking the start of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

The Moon is a thin waning crescent 13A[degrees] above Regulus (use binoculars) at dawn on September 8th. The waxing lunar crescent is 9[degrees] upper right of Venus at dusk on September 12th, but only some 4[degrees] upper right of Jupiter the next night. On September 17th a slightly gibbous Moon is 4[degrees] left of Saturn. A fatter Moon is almost 4 1/2[degrees] above Mars on the evening of September 19th. On the night of September 29-30, the waning gibbous Moon passes less than 1[degrees] from Aldebaran for North America.

Contributing Editor FRED SCHAAF teaches astronomy at Rowan University and Rowan College in Gloucester County, both in southern New Jersey.

To find out what's visible in the sky from your location, go to skypub.com/ almanac.

Caption: These scenes are drawn for near the middle of North America (latitude 40[degrees] north, longitude 90[degrees] 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[degrees] scale bar is about the width of your fist at arm's length.

Caption: ORBITS 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.
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Title Annotation:SEPTEMBER 2018 OBSERVING: Sun, Moon & Planets
Author:schaaf, Fred
Publication:Sky & Telescope
Date:Sep 1, 2018
Words:1010
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