Of the five brightest planets, three are pretty much out of the loop for viewing. Mercury and Mars are too close to the sun to be seen. Venus is also too close to the sun to be seen, although during the last few days of the month it will begin pull just far enough away from the bright twilight glow to be glimpsed about a half hour after sunset near the western horizon. That leaves only brilliant Jupiter in the southwest sky and Saturn in the south at dusk. But by month’s end, both of these big worlds are out of sight by midnight.
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.
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Planet Viewing Guide
After solar conjunction on September 4, Mercury will re-appear low in the western sky after dusk for the rest of September and October. The lengthy evening apparition will be a poor one for Northern Hemisphere observers, but very good one when viewed from southerly latitudes. Visually, the planet will continually drop in brightness. In a telescope, its disk will grow slightly in apparent size and wane slightly in illuminated phase. On the evenings around September 13, Mercury and Venus will appear very close together, with very bright Venus sitting about half a finger’s width (20 arc-minutes, or two-thirds of the full moon’s diameter) above dimmer Mercury. On September 29, the very young crescent moon will form a picturesque triangle above Mercury and Venus.
Mercury — is too difficult to glimpse for observers at mid-northern latitudes this month. It stands at superior conjunction on September 3rd. Moving into the evening sky, it will set only 45 minutes after the sun at month’s end, as seen from latitude 40° north. Mercury will be better placed at more southerly latitudes, especially for observers south of the equator.
Venus — passed through superior conjunction with the sun on August 14th. It is therefore setting too soon after the sun to sight until late September when it creeps back into the evening sky. On September 30th, Venus sets about 30 minutes after the sun for viewers at latitude 40° north. But the planet is only about 5° high at sunset, so spotting it will be quite difficult even if your western horizon is unobstructed. Only in the Southern Hemisphere will this brilliant planet be high enough in bright twilight to detect easily.
Earth – On September 23rd, at 3:50 p.m. EDT, the sun crosses the celestial equator, heading south. This equinox marks the beginning of autumn in the Northern Hemisphere and spring in the Southern.
Mars — is in conjunction with the sun today and too close to the sun to be seen all of this month. On August 26th, Mars arrived at the aphelion of its orbit: its farthest point from the sun for this Martian year. Our own planet is most distant from the sun in July, but our orbit is so nearly circular that aphelion and perihelion have little effect on temperature and climate. Mars, on the other hand, follows a more elliptical orbit. The red planet is now receiving only 69 percent as much sunlight as it will early next August at perihelion. This situation creates a seasonal asymmetry on Mars. The Martian southern hemisphere always suffers greater extremes between summer and winter than the northern hemisphere. In early October it will be wintertime in the south latitudes of Mars, a cold winter even for this frigid world.
The earlier sunsets of September will permit Jupiter to remain a good evening target in the southwestern sky all month. The very bright planet will be moving eastward through the stars of southern Ophiuchus. As Earth moves farther from it, the planet will decrease slightly in brightness (from magnitude -2.21 to -2.0) and apparent disk size (from 39 to 36 arc-seconds). On the evening of September 5, the first quarter moon will be positioned less than four finger widths to the upper right (or 4 degrees to the celestial northwest) of Jupiter. From time to time, the little round shadows cast by Jupiter’s four Galilean moons are visible as they cross the planet’s disk. On Wednesday, September 4 from 9:21 to 11:33 p.m. EDT, observers in the Americas can watch Io’s shadow transit Jupiter, accompanied by the Great Red Spot. On Wednesday, September 18 from 8:41 to 11:10 p.m. EDT, observers in the Americas can watch Europa’s shadow transit the northern hemisphere of Jupiter. And on Friday, September 20 from 7:40 to 9:52 p.m. EDT, they can watch Io’s shadow transit Jupiter.
Jupiter — the first planet that most people will see as twilight fades. But look early; it’s getting lower all the time, and by September’s end it sets as early as 10 p.m. daylight-saving time. Jupiter begins September 7° from sparkly, red Antares across the border in Scorpius. By month’s end it will have edged 9½° away from Antares. On September 5th, take note of Jupiter, glaring at magnitude -2.2 about 4° to the left of the first quarter moon in southern Ophiuchus.
Saturn — shines with a steady yellow light due south in the constellation Sagittarius. Don’t linger too long in getting a telescope set up to view it. Saturn is ideally placed after dusk but soon starts to sink toward the southwest. Saturn’s rings are tilted wide open, inclined 25° to our line of sight. This is almost exactly how they have appeared for the last five years, but take a good look for detail within the rings while you still can. Next year they will start to close up, on their way to becoming edge-on in 2025. We won’t see them this open again until around 2029. Saturn’s largest moon Titan can be spotted with small telescopes four ring lengths west of the planet around September 13th and 29th, and a similar distance to its east around the 5th and 21st. On the evening of September 8th, you’ll see Saturn shining about 5½° to the right of the waxing gibbous moon.
Saturn will be visible during all of September as a medium-bright, yellowish object in the lower part of the southern and southwestern evening sky. It is positioned just to the left (celestial east) of the Milky Way, above the stars that form Sagittarius’ teapot-shaped asterism. On Wednesday, September 18, Saturn will complete a westward retrograde loop with respect to the distant stars and resume regular eastward motion. The ringed planet will decrease in brightness from visual magnitude 0.33 to 0.47 due to Earth’s steadily increasing distance from Saturn. The planet’s rings, which subtend an angular size of about 40 arc-seconds, continue to be well open because the planet’s northern pole is tilted roughly sunward. On the evening of September 8, Saturn will be positioned a palm’s width to the right (or 6 degrees to the celestial west) of the bright, waxing gibbous moon. Hours earlier, observers in eastern Africa, Madagascar, southern Indonesia, western and northern Australia, western Micronesia, and western Melanesia can see the moon occult Saturn.
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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. Follow us on Twitter @Spacedotcom and on Facebook.