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 September 13-14. Visually, the planet will decrease in brightness by a factor of four, from magnitude -0.05 to +1.49. In a telescope, Mercury will exhibit an illuminated phase that wanes from 73 per cent to a mere 17.4 per cent, and its apparent disk size will swell from 5.93 to 9.54 arcseconds. On September 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 September 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 relatively high in the sky there. For mid-northern latitude observers, Venus will set at about 9 p.m. local time on September 1, and 30 minutes earlier on September 31. After a close pass, only a thumb’s width above (or 1.5 degrees to the celestial north of) Spica on September 5, Venus’ eastward prograde motion will see it depart Virgo for Libra on September 18. The main belt asteroid designated (4) Vesta will be travelling on a parallel track about 5 degrees north of Venus. They’ll begin the month at almost the same right ascension, but the planet’s faster motion soon outruns 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 arcseconds. On September 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 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, with fainter, yellowish Saturn shining 16 degrees to Jupiter’s right (or celestial east). The planet will catch the eye in the southeastern sky before the end of evening twilight. Unfortunately, the low summertime ecliptic will prevent those two planets from climbing very high in the southern sky. Views of Jupiter in amateur telescopes will show its equatorial bands, and the Great Red Spot will appear every second or third night. Jupiter’s four large Galilean satellites will occasionally cast their round, black shadows on the planet. The nearly full Moon will pass less than five degrees below (celestial south of) Jupiter on September 17-18.
Yellowish Saturn will be shining in the lower part of the southeastern sky, and ready for observing, right after dusk in September. But its early August opposition means it will be descending the western half of the sky after late evening. During the month, Saturn will be travelling retrograde westward across the stars of Capricornus – with much brighter Jupiter positioned 16 degrees to its left (east). Unfortunately, a low ecliptic will prevent either planet from climbing very high up in the southern sky. Saturn will decrease slightly in brightness during the month. In a telescope, Saturn will show a mean apparent disk diameter of 18 arcseconds, and its rings will subtend 42 arcseconds. Saturn’s rings will be tilting more edge-on to us every year until the spring of 2025. This year they are already closed enough for Saturn’s southern polar region to extend beyond them. September will still be a good time to view Saturn’s moons with a backyard telescope in a dark sky. On September 23, the bright, waning gibbous Moon will shine a generous palm’s width to the right (or 7.5 degrees to the celestial southwest) of Uranus.
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. local time. The blue-green, magnitude 5.7 planet will be travelling retrograde westward in a part of southern Aries that is poor in bright stars. Search for the planet about midway between Hamal (Alpha Arietis) and Omicron Tauri. In a telescope, it will exhibit a blue-green, 3.7 arcseconds wide disk. It will also be surrounded by the magnitude 5 stars Sigma, Omicron, and Pi Arietis — creating a distinctive asterism for anyone viewing Uranus in binoculars. The waning gibbous Moon will hop past Uranus on September 24-25.
During September the distant, blue planet Neptune will be an all-night target and already climbing in the southeastern sky after dusk. On September 14, it will reach opposition. At that time Neptune will be closest to Earth for this year — a distance of 2.69 billion miles, 4.33 billion kilometres, four light-hours, or 28.9 Astronomical Units. The blue planet will shine with a slightly brighter magnitude 7.8. Since it’s directly opposite the Sun in the sky, it will be visible all night long in good binoculars if your sky is very dark, and backyard telescopes. Your best views will come after 9 p.m. local time, when the blue planet has risen higher. Around opposition, Neptune’s apparent disk size will grow to 2.4 arcseconds and its large moon Triton will be most visible. Throughout September, Neptune will be located among the stars of northeastern Aquarius, about 4 degrees to the left (or celestial east) of the naked-eye star Phi Aquarii (φ Aqr).
Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.