October's sky offers a progression of planetary sights from dusk to dawn: elusive Mercury in the early evening, Saturn through most of the night, Mars after midnight, and Jupiter before sunrise — with a rare lunar occultation of Jupiter providing the month's standout event.
After becoming a fixture in the evening sky for much of the spring and summer, Venus relinquishes its title as the evening star in October. Mercury, the other "inferior" planet—one whose orbit is closer to the sun than Earth's — is also in the evening sky. However, it will be very difficult to see because it remains low in the west-southwest soon after sunset during the first 20 days of October.
Saturn is favorably placed for observation for practically the entire night when it reaches opposition to the sun on Oct. 4. It rises in the east as the sun sets, reaches its highest point in the southern sky around midnight, and sets in the west around sunrise.
A few hours after midnight, Mars becomes visible low in the east-northeast. Look for it on the morning of Oct. 11, when it passes through the popular Beehive star cluster in Cancer the Crab — a very pretty sight in binoculars.
Finally, Jupiter appears as a brilliant light in the predawn eastern sky — and on the morning of Tuesday, Oct. 6, it will take part in a rare and spectacular celestial event. Set your alarm early to see the brilliant planet and its retinue of satellites disappear behind a waning crescent moon that is 20% illuminated, about four days before new moon. Astronomers refer to this phenomenon as an "occultation," a word derived from the Latin meaning "to conceal." This head-turning sight will unfold in complete darkness across the eastern and much of the central United States, as well as adjacent southern Canada.
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
Mercury at zero magnitude reaches greatest elongation, 25 degrees east of the sun on Oct. 12. This same evening it also sits about 3 degrees to the upper right of a two-day-old crescent moon. Unfortunately, this is a very poor apparition for mid-northern latitudes. With binoculars, try scanning near the west-southwest horizon about a half hour after sunset. Observers in the southern states might be able to glimpse Mercury with the unaided eye.
Venus
Venus enters its beautiful crescent phase for telescope users every time it nears the end of one of its months-long evening apparitions. But this season it's making its lowest, poorest possible exit from the evening sky (as seen from mid-northern latitudes). As October starts, Venus is technically near its maximum possible brightness at magnitude -4.7, but it appears much dimmer because it's very low in bright evening twilight. On Oct. 1, it sets only about 35 minutes after the sun and stands only 5 degrees above the southwest horizon at sunset for viewers around 40 degrees north latitude. And things just get worse after that. If you live in a northern land, you may need a haze-free sky or binoculars to detect Venus. Just a couple of days later, this dazzling world is barely above the horizon in very bright twilight 20 minutes after sunset. Then Venus is lost from view for several weeks as it shuttles toward inferior conjunction (between Earth and the sun) on Oct. 24, then shoots up gloriously into the dawn sky during early November.
Mars
Mars is in the morning sky. On Oct. 5, any time after 2:30 a.m. local daylight time, look low toward the east-northeast for a waning crescent moon and sitting only about a degree to its upper right is a bright orange-yellow "star," which will be Mars. And about 2½ degrees below the moon is the Beehive star cluster in Cancer. Mars will be inside the Beehive star cluster on Oct. 11; time to break out the binoculars and view this very pretty sight. During this month, the red planet moves 17 degrees eastward. On the 1st, it is 5.8 degrees west of the Beehive. Crossing into Leo on Oct. 31, Mars will be 6 degrees west of Jupiter and 12 degrees west of Regulus, the brightest star of Leo the Lion. Then, at magnitude +0.9, it will be 134 million miles or 12 light-minutes from Earth. The hour before the break of dawn, when Mars is high in the southeast, is still the best time for telescopic observation. But the planet's apparent diameter is still very small, measuring only 6 arc seconds, so don't get your hopes up.
Jupiter
Jupiter, which crossed over from Cancer into Leo late last month, comes up above the east-northeast horizon shortly after 2:30 a.m. local daylight time. The best time to look at Jupiter in the telescope, however, will be as dawn begins to break, when the brilliant world stands about halfway up in the eastern sky. On the morning of Oct. 6, the crescent moon will cross in front of Jupiter, resulting in a spectacular occultation for parts of eastern and central North America. Space.com will have a detailed article concerning this major event, so stay tuned!
Saturn
Saturn arrives in opposition to the sun on Oct. 4, so through much of October it will already have risen in the east as evening twilight fades. Saturn is well up in the southeast for most of the evening. It lies to the lower left of the Great Square of Pegasus and is an entrancing sight in telescopes, with its majestic rings tilted about 7 degrees from edgewise. On Oct. 23, Saturn will be the bright "star" shining with a steady yellow-white glow well down to the lower left of the waxing gibbous moon.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.
