Listen to the story
On October 1, Mercury will be at its widest separation, 26 degrees east of the Sun, low in the post-sunset sky. The planet will then swing sunward, towards inferior conjunction on October 25. In the meantime, due to the shallow angle of the evening ecliptic, Mercury will remain too close to the southwestern horizon for observation by mid-northern latitude dwellers. Those viewing the swift planet from the southern USA and farther south will see Mercury more easily — higher, and in a darker sky, especially before mid-month. Viewed in a telescope during October, Mercury will wane in illuminated phase, and the planet’s apparent disk size will increase somewhat. After sunset on October 17, the very slim crescent Moon will sit a slim palm’s width above (or 5.5 degrees to the celestial north) of Mercury.
During October, Venus will rise a few hours before dawn and shine at an extremely bright magnitude -4 in the eastern sky. Meanwhile, our sister planet will be slowly moving sunward — starting the month in southern Leo with a very close pass of Regulus on October 2-3. Their minimum separation of 5 arc-minutes will be seen only by observers at western Asian longitudes. On October 23, Venus will cross into Virgo. Viewed in a telescope during October, the planet will exhibit a waning gibbous phase, and an apparent disk size that shrinks from 15.5 to 13 arc-seconds. On October 14, the delicate crescent Moon will sits a palm’s width to the lower left (or 5.75 degrees to the celestial northeast) of Venus.
As October begins, Mars will already be brighter than Jupiter. The Red Planet will be visible all night as it crosses the sky, while moving retrograde among the stars of Pisces. Earth’s minimum distance from Mars — closer than any approach until 2035 — will occur overnight on October 5-6. That night, Mars will shine at magnitude –2.6 and will show an apparent disk diameter of 22.6 arc-seconds in telescopes. It will be 38.57 million miles, 62.07 million kilometres, 0.515 Astronomical Units, or 3.45 light-minutes distant from Earth. Due to Mars’ elliptical orbit, opposition will be delayed until October 13. On that night, Mars will reach a maximum visual magnitude of -2.62. October’s close approach offers a fine opportunity to view Mars’ bright southern polar cap in telescopes. During the week, between closest approach and opposition, Mars’ Earth-facing hemisphere will display the dark Syrtis Major Planum, Tyrrhena Terra, Cimmeria Terra and Sirenum Terra regions, and the lighter toned Hellas Planitia region. Owners of larger telescopes should try to see additional surface details and Mars’ two small moons, Phobos and Deimos. After opposition, Mars will steadily diminish in both brightness and apparent disk size, but it will become better placed for evening observing. At mid-northern latitudes, the planet’s climb to a healthy 50 degrees above the southern horizon will occur several hours earlier than in early October — and much higher than at its previous opposition in 2018. Mars has two conjunctions with the nearly full Moon in October. On October 2, the waning Moon will be positioned just 2.25 degrees to the celestial south of the planet. On October 29, the waxing Moon will be positioned 5.5 degrees to the southeast.
Read the September/October edition of SkyNews magazine for more on the opposition.
During October, Jupiter will continue to be well-placed for early evening observing, although it will be setting in the west at 10:30 p.m. at month-end. The earlier sunsets of autumn will keep Jupiter in sight, even as it slides farther into the west every night. In early October, Jupiter will already be shining in the lower part of the southern sky after dusk, with dimmer Saturn appearing soon afterward about a palm’s width to Jupiter’s left. Jupiter will be moving eastward through the stars of northeastern Sagittarius, its faster orbital speed reducing its angular separation from slower-moving Saturn until they meet on December 21. During October, Jupiter will decrease slightly in brightness from magnitude -2.35 to -2.17. Its apparent disk diameter will shrink from 40.4 to 36.9 arc-seconds. The Great Red Spot will be visible every second or third night. On October 17, observers in the Eastern Time Zone can see the diffuse shadow of Callisto; the crisp, round shadow of closer-in Io; and the great Red Spot all completing a group transit event between 5:25 p.m. and 7:25 p.m. EDT. On October 22, the waxing, half-illuminated Moon will form a neat triangle below 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 October, although they will remain rather low in the sky for mid-Northern observers. The ringed planet will be moving eastward through the stars of northeastern Sagittarius, allowing faster Jupiter to creep closer until they meet up on December 21. The rings, and many of Saturn’s moons, are easily visible in backyard telescopes. During October, Saturn will diminish slightly in apparent size, and it will diminish from magnitude 0.47 to 0.58. On October 22, the waxing, half-illuminated Moon will form a neat triangle below Jupiter and Saturn — a grouping that will make a beautiful wide field image.
During October, blue-green Uranus (magnitude 5.7) will be visible all night long while it travels slowly westward in southwestern Aries, about 11 degrees south of Aries’ brightest star Hamal, or 5 degrees north of the stars that form the top of Cetus’ head. Uranus will reach opposition on October 31, when it will be closest to Earth for this year at 1.75 billion miles, 2.81 billion kilometres, or 156 light-minutes away. It will appear slightly larger in telescopes for a week or two.
Neptune (magnitude 7.8) will be visible all night during October while it moves retrograde westward through the stars of eastern Aquarius. Neptune’s apparent disk size be 2.35 arc-seconds. Throughout the month, the planet will be travelling toward that constellation’s naked-eye star Phi (φ) Aquarii, which is located about 1.25 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.