Venus will be the first to go, dropping into the sunset fires early this month and transitioning to the morning sky by month's end. Jupiter will be next, and while it will not disappear completely, it will spend much of October gradually getting lower in the west-southwest sky, while setting progressively earlier. Saturn is next in line, but for much of this month still remains well positioned for viewing, though by late evening as it nears the horizon, atmospheric turbulence will likely cause its image to become rather shaky. Mars is still a fiery, eye-catching object in the south-southeast at dusk, but continues to fade and grow smaller as it continues to recede from Earth. By Halloween it will shine with only one-eighth of the radiance it displayed in July when it was unusually close to the Earth. Finally, there is Mercury which is very low in the southwest sky all month, but very difficult to perceive until the latter days of the month when it will be in the general vicinity of Jupiter. [Night Sky, October 2018: What You Can See This Month (Maps)]
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 directing you as to where to look to see them.
Mercury — is above the southwestern horizon after sunset during all of October but never high enough to be easily visible. The best time to look is the last week of the month. Use binoculars to scan for it, 45 minutes after sunset. On October 30th, Jupiter (magnitude -1.7) will be hovering about 3 degrees to the upper right of Mercury (magnitude -0.2). Can you glimpse either with the naked eye? Both planets may seem fainter than they are because they are very close to the horizon.
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 -4.8, but it appears much dimmer because it’s low in bright evening twilight. This evening it sets about an hour after the sun, but it stands only 9 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 even on the opening nights of the month. By October 9th, 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 October 26th, then shoots up gloriously into the dawn sky during early November.
Mars — is still in the south-southeast when twilight ends, and it stays right there at nightfall all month. It is traveling eastward with respect to the stars, or perhaps we should say the stars are sliding westward behind it. It spends all of October moving across the stars of Capricornus, the Sea Goat. As Mars’ distance from us increases from 56 to 73 million miles during October, the planet fades from magnitude -1.3 to -0.6. The telescopic view of Mars, so fine during midsummer, has become relatively mediocre again. In July the red planet was only 3.2 light minutes away from Earth and more than 24 arc seconds across. By the end of October it is more than twice as far and less than half the apparent size. Mars is that very bright, fiery colored object well to the right of the waxing gibbous moon on October 18th.
Jupiter — Shining at magnitude -1.8, Jupiter is still easily seen as October begins. Look for it low in the west-southwest in early twilight. During the final days of the month it will be in the vicinity of Mercury and by October 31st the king of the planets is so low that you’ll probably need binoculars to help you find it. Three days past new phase and just 10-percent illuminated, a slender waxing crescent moon passes a little over 3 degrees north of Jupiter on October 11th.
Saturn — is in the south-southwest after dusk and this is probably your last chance to get a good telescopic view of Saturn’s disk, rings, and moons. The planet will remain visible in early evening almost to the end of the year, but it is getting near the horizon where atmospheric seeing wreaks its worst effects – assuming you have an unobstructed view to the southwest at all. The crescent moon has noticeably widened since it visited Jupiter on the 11th, when it passes only 1½ degrees to the right of Saturn on October 14th.
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, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's Lower Hudson Valley.