With the singular exception of Mars, October will provide a very good view of the four other naked-eye planets.
Right after sunset we have Venus, slowly ramping up in brightness as well as slowly lifting a little higher into the southwest sky compared the recent months; by month’s end it remains above the horizon for more than two hours after sundown. Saturn and Jupiter are conveniently placed in the southern sky as darkness falls and are available for telescopic scrutiny during the first half of the night. And finally, for early risers, Mercury puts in its best morning sky appearance of 2021 during the second half of the month.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm’s length measures roughly 10 degrees. Here, we present a schedule below which provides some of the best planet viewing times as well as directing you as to where to look to see them.
Mercury — passes inferior conjunction on Oct. 9, then rapidly enters the morning sky. This little hummingbird of a planet levitates quickly up into its best dawn apparition of the year for mid-northern latitudes. By Oct. 17, having brightened to 1st-magnitude, it will rise just south of east more than an hour before the sun. It reaches greatest western elongation, at magnitude -0.6 on Oct. 25. Although only 18 degrees from the sun, for several days surrounding this date the planet will rise before the beginning of morning twilight, making this the year’s most favorable morning apparition for observers in mid-northern latitudes. Brightening to -0.8, Mercury will pass 4 degrees to the left of sparkly Spica on November 2nd.
Venus — resplendent at magnitude -4.6, arrives at its greatest eastern elongation, or greatest angular distance east of the sun (47 degrees) on Oct. 29. But the planet is so far south on the celestial sphere that it remains fairly low, just 12 degrees above the southwest horizon 45 minutes after sunset. Soon after sunset on the evening of Oct. 9, look low toward the southwest during evening twilight to catch a view of the 3½-day-old waxing crescent moon hovering just a couple of degrees above and slightly to the left of Venus, the brightest planet. Using binoculars and wide-field telescopes will also show the 2nd-magnitude star Delta (δ) Scorpii, also called Dschubba, a mere 0.75 degrees above Venus. On the evening of Oct. 16, take a look for 1st-magnitude Antares shining south (lower left) of Venus by just 1.5 degrees, about a finger’s width at arm’s length. Antares twinkles red, though only 1/130 as bright. Telescopically, Venus’s gibbous phase wanes to half-lit by October’s end, while the planet’s apparent diameter swells in size by 25%. Venus glides through Libra, Scorpius, and Ophiuchus during October, entering Sagittarius on Nov. 2. Ever since late last spring this flaming planet has remained at about the same low altitude in the dusk (for viewers at mid-northern latitudes). But during October Venus climbs just a bit higher, and now stays above the horizon until twilight is well and truly over. The interval between sunset and Venus-set grows significantly during October — from about 1¾ to 2½ hours. It will be highest and best in late November and early December.
Mars — is in solar conjunction on Oct. 8, behind the sun’s glare and invisible all of this month.
Jupiter — resides on the eastern side of dim Capricornus, well to the left of Saturn, and is the brightest light in the sky once Venus sets. Jupiter reaches its highest point in the south soon after dark. Its disk shrinks by about 9% during October but still looms large enough to show many telescopic details if the air is steady.
Saturn — is the bright yellow “star” in the south during early evening. It lies on the western side of the dim, boat-shaped pattern of Capricornus, which is composed of 3rd and 4th-magnitude stars that this planet — along with brilliant Jupiter to the east — quite overshadows. If you check around Saturn with binoculars during the course of October, you’ll find that it remains almost motionless with respect to the background stars. This is because Saturn reaches a stationary point on Oct. 10. It ceases its westward or “retrograde” motion (the direction all outer planets appear to move for a few months around opposition) and resumes its eastward travel against the stars. On Oct. 14, the waxing gibbous moon forms a stretched-out isosceles triangle with Saturn and Jupiter. The two gas giants of the solar system are separated by 15 degrees; Saturn (magnitude +0.5) shines about 9 degrees to the upper right of the moon, while the much-brighter Jupiter (magnitude -2.6) burns some 9 degrees to the moon’s upper left. Viewed at high power, Saturn is an excellent test of both atmospheric steadiness and telescope quality. The beautiful ring system is tilted about 17.5 degrees to our line of sight.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and other publications in New York’s lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.