During September, Mercury will put on its best showing of the year for observers in the Southern Hemisphere, where the near-vertical ecliptic at southerly latitudes will allow the speedy planet to shine in a darkened sky after sunset all month long. For those at mid-northern latitudes, the canted-over ecliptic will force Mercury to set very soon after the sun every evening. The best views in either hemisphere arrive with Mercury's greatest elongation, 27 degrees east of the sun, on Sept. 13-14. Visually, the planet will decrease in brightness by a factor of four over the month, from magnitude -0.05 to +1.49. In a telescope, Mercury will exhibit an illuminated phase that wanes from 73% to a mere 17.4%, and its apparent disk size will swell from 5.93 to 9.54 arc-seconds. On Sept. 8, the very young crescent moon will be positioned a slim palm's width to the upper right (or 5 degrees to the celestial north) of Mercury. On Sept. 20-21, Mercury will pass only a thumb's width below (or 1.4 degrees to the celestial south of) Spica.
Extremely bright Venus' position close to a steeply tilted evening ecliptic will prevent the planet from climbing very high - or from shining in a dark sky - for observers in the Northern Hemisphere during September. But the near-vertical ecliptic available in the Southern Hemisphere will allow the planet to shine in total darkness and sit relatively high in the sky there. For mid-northern latitude observers, Venus will set at about 9 p.m. local time on Aug. 1 and 30 minutes earlier on Aug. 31. After a close pass, only a thumb's width above (or 1.5 degrees to the celestial north of) Spica on Sept. 5, Venus' eastward Prograde motion will see it depart Virgo for Libra on Sept. 18. The main belt asteroid designated (4) Vesta will be traveling on a parallel track about 5 degrees north of Venus. They'll begin the month at almost the same Right Ascension coordinate, but Venus' faster motion soon outpaces the more distant asteroid. Viewed through a telescope during September, our sister planet will show a gradually waning, barely gibbous phase and an apparent disk diameter that grows from 15.2 to 18.8 arc-seconds. On Sept. 9, the young crescent moon will shine several finger-widths to the upper right (or 4 degrees to the celestial northwest) of Venus.
Mars will be too close to the sun to be observed from mid-northern latitudes during September, but Southern Hemisphere observers might glimpse the magnitude 1.8 planet sitting very low in the western post-sunset sky early in the month.
Recently past opposition, bright, white, magnitude -2.8 Jupiter will be well-placed for observing nearly all night long during September while the planet travels retrograde across the stars of eastern Capricornus. Fainter, yellowish Saturn will be shining 16 degrees to Jupiter's right (or celestial east). Jupiter will catch your eye in the southeastern sky before the end of evening twilight. Unfortunately, the low summertime ecliptic will prevent it from climbing very high in the southern sky. Views of Jupiter in amateur telescopes will show its equatorial bands. The Great Red Spot will appear every 2nd or 3rd night. Occasionally, the round, black shadows of Jupiter's four large Galilean satellites will make several-hour traverses of the planet. The nearly full moon will pass less than a palm's width below (or 5 degrees to the celestial south of) Jupiter on Sept. 17-18.
Immediately after dusk in September, yellowish Saturn will be shining in the lower part of the southeastern sky - ready for many hours of observing. But Saturn's early August opposition means it will now be descending the western half of the sky after late evening. During the month, Saturn will be traveling retrograde westward across the stars of Capricornus - with much brighter Jupiter positioned 16 degrees to its left (east). Unfortunately, the low ecliptic will prevent Saturn from climbing very high in the southern sky. Initially at magnitude 0.3, Saturn will decrease slightly in brightness during the month. In a telescope Saturn will show a mean apparent disk diameter of 18 arc-seconds, and its rings will subtend 42 arc-seconds. Saturn's rings will be tilting more edge-on to Earth every year until the spring of 2025. This year they are already closed enough for Saturn's southern polar region to peek out beyond them. September will also be a good time to view Saturn's moons with a backyard telescope in a dark sky. On Sept. 16, the bright, waxing gibbous moon will shine several finger widths below (or 4.5 degrees to the celestial south) of Saturn.
During early September, Uranus will rise in late evening and be best observed during the second half of the night. At month's end Uranus will be rising at about 8 p.m. local time and will culminate over the southern horizon, two-thirds of the way up the sky, at around 3 a.m.. The magnitude 5.7 planet will be traveling retrograde westward in a part of southern Aries that lacks bright stars, but you can locate the planet about midway between Hamal (Alpha Arietis) and Omicron Tauri. In a telescope, Uranus will exhibit a blue-green, 3.7 arc-seconds wide disk. It will be surrounded by the 5th magnitude stars Sigma, Omicron, and Pi Arietis — creating a distinctive "flux capacitor" asterism for anyone viewing Uranus in binoculars. The bright, waning gibbous moon will hop past Uranus on Sept. 23-24.
During September, the distant, blue planet Neptune will be an all-night target that is already climbing the southeastern sky after dusk. It will be traveling retrograde westward among the stars of northeastern Aquarius, a few finger-widths to the left (or 4 degrees to the celestial east) of the medium-bright star Phi Aquarii (φ Aqr). When Neptune reaches opposition on Sept. 14, it will be closest to Earth for this year — a distance of 2.69 billion miles, 4.33 billion km, 4 light-hours, or 28.9 Astronomical Units. The blue planet will then shine at a slightly brighter magnitude of 7.8 and Neptune's apparent disk size will grow to 2.4 arc-seconds. Since it's directly opposite the sun in the sky, Neptune will be visible all night long in good binoculars if your sky is very dark, and in backyard telescopes from almost any site. Your best views will come after 9 p.m. local time, when the blue planet has climbed higher. Neptune's large moon Triton can be seen more easily around opposition, too. The very bright, nearly full moon will hop past Neptune on Sept. 19-20, and Neptune will pass only 1.5 arc-minutes south of the magnitude 6.25 star designated HIP115953 on Sept. 23.
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Monthly skywatching information is provided to Space.com by Chris Vaughan of Starry Night Education, the leader in space science curriculum solutions. Follow Starry Night on Twitter @StarryNightEdu and Chris at @Astrogeoguy.