Mercury will not be observable until after it passes inferior solar conjunction on October 9. Several days later, the upright morning ecliptic will allow Mercury to quickly return to visibility low in the eastern pre-dawn sky. This, the best morning apparition for the year for mid-Northern latitude observers, will continue until November. At Mercury’s greatest western elongation on October 25, the planet will extend to a modest 18.4 degrees from the Sun. Viewed in a telescope during October, Mercury will wax from a slim crescent to 78-per-cent-illuminated, while its apparent disk diameter shrinks from 10 to 6 arc-seconds. Meanwhile the planet will dramatically increase in apparent brightness.
During October, extremely bright Venus’ position below a shallow evening ecliptic will continue to prevent the planet from climbing very high — or from shining in a dark sky — for observers in the Northern Hemisphere. But the nearly vertical ecliptic available in the Southern Hemisphere will allow the planet to sit relatively high in a dark sky there. For mid-northern latitude observers, Venus will set shortly before 11 p.m. local time on October 1 and an hour later on October 31. Venus’ eastward prograde motion will see it depart Libra, cross Scorpius, and enter Ophiuchus during the month, all the while brightening steadily from magnitude -4.26 to -4.56. Viewed through a telescope during October, our sister planet will show a gradually waning, half-illuminated phase and an apparent disk diameter that swells dramatically from 19 to 25.6 arc-seconds. On October 9, the young crescent Moon will shine just above Venus amidst the claw stars of Scorpius. Venus will pass a thumb’s width above (or 1.25 degrees north of) Antares on October 15-16.
Mars will pass solar conjunction on October 8 — so the Red Planet will be too close to the Sun to be observed from mid-northern latitudes during October. Observers in the tropics might glimpse the magnitude 1.7 planet sitting very low in the eastern sky before sunrise at the end of the month.
The earlier sunsets of autumn will extend our evening Jupiter-viewing time during October. The magnitude -2.6 planet will shine as a very bright, white dot sitting a third of the way up the southeastern sky after dusk, and then set in the west-southwest in the hours after midnight. Unfortunately, the low ecliptic will prevent Jupiter from climbing very high in the sky. As Jupiter approaches the end of its retrograde period on October 18, its motion across the stars of eastern Capricornus will slow, and then cease. By month’s end it will resume travelling prograde eastward above the Sea-Goat’s tail, with fainter, yellow-hued Saturn shining 16 degrees to its right (or celestial east). Views of Jupiter in amateur telescopes will show equatorial bands across its 44 arc-seconds-wide disk. The Great Red Spot will appear for a few hours every second or third night. One or two round, black shadows of Jupiter’s Galilean satellites will cross Jupiter’s disk on October 3, 11, 14, 18, and 27. The waxing gibbous Moon will pass less than a palm’s width below (or 5 degrees to the celestial south of) Jupiter on October 14-15.
During October, the ringed planet Saturn will be observable all evening. It will first appear in the lower part of the southern sky after dusk, and then descend in the southwest towards midnight. Much brighter Jupiter, shining 16 degrees to Saturn’s left (east), will catch your attention earlier. Unfortunately, the low ecliptic will prevent Saturn from climbing very high in the southern sky for mid-northern latitude observers. Saturn’s westerly motion through western Capricornus will slow and then cease as it ends its retrograde loop on October 11, and then it will return to regular prograde motion for the rest of the year. Saturn will decrease slightly in brightness during the month, from magnitude 0.47 to 0.61. Viewed in a telescope, the planet will display a mean apparent disk diameter of 17 arc-seconds, and its rings will subtend 40 arc-seconds, and several moons will be arrayed around the planet. The 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. On October 13, the bright, waxing gibbous Moon will shine a palm’s width below (or 5 degrees to the celestial southwest) of Saturn.
During October, blue-green Uranus will rise in early evening and be best observed in the hours after midnight, when it will climb two-thirds of the way up the southern sky. The magnitude 5.7 planet will be traveling retrograde westward in southern Aries, surrounded by a palm-sized ring of similarly-bright stars. Start your search about midway between the stars Hamal (Alpha Arietis) and Omicron Tauri. In a telescope, Uranus will exhibit a 3.7 arc-seconds-wide disk. The bright, waning gibbous Moon will sit two finger widths below (or 3 degrees to the celestial southeast of) Uranus on October 21.
Recently past opposition, the distant, blue planet Neptune will be climbing the southeastern sky after dusk in October, and will remain observable nearly all night long. Neptune is visible in good binoculars if the sky is very dark. The magnitude 7.8 planet will be traveling retrograde westward among the stars of northeastern Aquarius. To locate Neptune, find the up-down grouping of five medium-bright stars Psi, Chi, and Phi Aquarii (or ψ, X, and φ Aqr) in binoculars. Neptune’s little blue dot will sit several finger widths to the left (or 3.5 degrees to the celestial NNE) of the top star, Phi. Viewed in a telescope, Neptune’s apparent disk size will be 2.4 arc-seconds. Your best views will come around 11 p.m. local time, when the blue planet is highest. As a bonus, the minor planet designated (2) Pallas will be positioned about a palm’s width to the right (or 6 degrees to the celestial west) of the same star grouping.
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.