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During September, Mercury will steadily swing away from the Sun the western evening sky. Due to the shallow angle of the evening ecliptic, Mercury will remain very close to the horizon for observers at mid-northern latitudes. Those viewing the swift planet from the southern United States will see Mercury more easily — higher and in a darker sky, especially after the first week of the month. Meanwhile, observers located near the Equator and in the Southern Hemisphere will get their best look at Mercury for 2020. Viewed in a telescope during September, Mercury will wane in phase from 91 per cent to 77 per cent illuminated, and the planet’s apparent disk size will increase by about 20 per cent. After sunset on September 18, the very slim crescent Moon will sit a slim palm’s width to the upper right (or five degrees to the celestial north) of Mercury.
During September, Venus will continue to rise during the wee hours and shine very brightly in the eastern pre-dawn sky. Meanwhile it will be slowly moving sunward, shifting from Gemini into Cancer on September 4, and then through western Leo for the final week of September. Venus will end the month near Leo’s brightest star Regulus. (Venus will pass very close to Regulus on October 2-3.) During September, Venus will diminish slightly in visual brightness. Viewed in a telescope, the planet will exhibit a waning gibbous phase, and an apparent disk size that shrinks from 19.5 to 15.6 arc-seconds. On the mornings surrounding September 13, Venus will pass just to the lower right (or two degrees to the celestial south) of the large open stars cluster Messier 44 in Cancer. The old crescent Moon will join them on September 14. On the mornings surrounding September 22, Venus will overtake and pass within two degrees of the slower main belt asteroid Vesta.
Mars will become a prime target for skywatchers during September — a dress rehearsal for its big opposition event in mid-October, which will the best in the 2020s. As the month opens, the Red Planet will rise shortly before 10 p.m. local time and cross the overnight sky as a -1.83 object among the stars of southern Pisces. On September 9, Mars will cease its regular eastward motion and commence a retrograde loop that will last until mid-November. That reversal will keep the planet within the V of Pisces all month long. By month-end, much brighter (magnitude -2.5) Mars will rise during dusk and invite telescope views all night long, especially after midnight, when it will climb higher in the sky for mid-northern observers. During September, Mars’ apparent disk size will increase from 19 to 22.5 arc-seconds and its phase will grow to almost fully-illuminated. Since Mars’ solstice occurs on September 3, the planet’s bright southern polar cap will be tipped towards Earth, shrinking as it warms but still prominent in telescopes. On September 5-6, the bright, waning gibbous Moon will pass only one degree to the southwest of Mars. Observers in central and northeastern South America, Cape Verde Islands, northern Africa, and southern Europe will see the moon occult Mars around 05:00 GMT on September 6. Since the pair will not set in the west until mid-morning on September 6, skywatchers have a chance to see Mars in the morning daytime sky using binoculars and backyard telescopes by using the Moon as a reference.
During September, Jupiter will continue to be well-placed for evening observing in the lower part of the southwestern sky. The earlier sunsets of autumn will keep Jupiter in sight, even as it slides farther into the west every night. In early September, Jupiter will already be shining in the lower part of the southern sky as dusk begins – with nearby, dimmer Saturn appearing soon afterward. On September 13, Jupiter will complete a retrograde loop that began in mid-May, and resume its regular eastward motion through the stars of eastern Sagittarius. From that point on, Jupiter’s faster orbit will cause it to diminish its angular separation from slower Saturn. During September, Jupiter will drop slightly in brightness, from magnitude -2.55 to -2.35. Its apparent disk diameter will shrink from 44 to 40 arc-seconds. On September 14 at 06:57 GMT, Ganymede’s larger shadow and the Great Red Spot will join Io’s smaller shadow already progressing across Jupiter’s disk. The trio will remain visible until Io’s shadow moves off Jupiter at about 08:30 GMT. On September 24 the waxing gibbous moon will sit to the celestial southwest of Jupiter and Saturn – a grouping that will make a beautiful wide field image.
Like nearby Jupiter, Saturn will be well-positioned for evening observing during September, although they will remain rather low in the sky for mid-Northern observers. The ringed planet will move retrograde (westward) through the stars of northeastern Sagittarius until September 29, and then resume regular prograde motion, allowing faster Jupiter to move closer to it during autumn. The rings, and many of Saturn’s moons, are easily visible in backyard telescopes. During September, Saturn will diminish slightly in apparent size, and dim from magnitude 0.3 to 0.4. On September 25, the waxing gibbous Moon will sit 3.6 degrees to the south of Saturn. With Jupiter positioned nearby, the scene will make a beautiful wide-field image.
During September, blue-green Uranus (magnitude 5.7) will transition from a late-night object to an evening object, eventually rising at 8 p.m. local time by month-end. The planet will be traveling slowly westward in southwestern Aries. Find it 11 degrees south of Aries’ brightest star Hamal, or five degrees north of the stars that form the top of Cetus’ head. On September 6, the waning gibbous Moon will be positioned four degrees south of Uranus.
Neptune will be visible all night during September — moving retrograde (westward) among the stars of eastern Aquarius. On September 11, Neptune will reach opposition. At that time, the blue-tinted planet will be four light-hours or 28.9 Astronomical Units from Earth, it will shine at a slightly brighter magnitude 7.8, and will be visible all night long in good binoculars and backyard telescopes in a dark sky. Around opposition, Neptune’s disk size will grow to 2.4 arc-seconds. Throughout the month, the planet will be creeping toward that constellation’s naked-eye star Phi (φ) Aquarii, located about two degrees to Neptune’s west.
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.