The brightest planets in September's night sky: How to see them (and when)

Evening skywatchers can view two planets this month, though one of them will be a bit challenging to spot. That will be the planet Mars which is battling the glow of evening twilight and low altitude in order to be seen in the west-southwest sky shortly after sunset. Saturn, in contrast, will be much easier and will be visible in a dark sky low in the east-southeast just a couple of hours after sunset. It reaches opposition on Sept. 21, meaning it will be in the sky all night and will appear highest in the south soon after midnight.

The morning sky boasts the two brightest planets. Jupiter rises soon after midnight and is well up in the south-southeast sky at the break of dawn. Meanwhile, dazzling Venus continues to put on a show in the east-northeast 2 to 3 hours before sunrise. Be sure to set your alarm clock for around 5:30 a.m. on Sept. 19.

If you have a clear and unobstructed view toward the east, you'll see a very striking array of Venus, a slender crescent moon and the bright blue star Regulus all crowded into a relatively small space on the sky. While the view will be striking enough to the unaided eye, binoculars will only enhance the view. The only planet out of the viewing loop this month is Mercury as it will be too close to the sun's glare to be visible.

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.

The sun

The sun crosses the celestial equator heading south on Sept. 22 at 2:19 p.m. EDT. This equinox ushers in autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

Mercury

Mercury is heading rapidly into the sun's glare, reaches superior conjunction on Sept. 13, and transitions from the morning into the evening sky. It is hopelessly low in the solar glare for anyone at mid-northern latitudes during all of September.

Venus

Venus rises in the east-northeast less than 3 hours before the sun — and simultaneously with Sirius — during the first few days of September for observers around 40° north latitude. It shines with its usual powerful white radiance, though in telescopes it is now a rather small and featureless gibbous disk.

Venus speeds across much of Cancer and Leo during September and during these first couple mornings of September have some notable conjunctions with stars. On the morning of Sept. 1, the Beehive Cluster, M44, is centered about 1.5 degrees north (upper left) of Venus. The next morning, the dazzling planet passes only 0.2 degrees from the faint, 4th-magnitude star Delta (δ) Cancri, known also as Asellus Australis.

Early on the morning of Sept. 19, 90 minutes before sunrise, a strikingly tight conjunction between the moon, a planet and a bright star will be visible very low in the eastern sky. Two days before the new phase, a narrow waning crescent moon will sit less than one degree to the upper left of Venus. At the same time, this dazzling planet will blaze at magnitude -3.9 just 0.5 degrees north (upper left) of 1st-magnitude Regulus. Their proximity to each other will intensify the contrasting silver-white of the planet and bluish hue of the star. Venus ends September rising only 2 hours before the sun.

Mars

Mars is now in its final full month of visibility. It's now setting before the end of evening twilight, and is becoming increasingly difficult to see as it drops lower in the west-southwest sky and closer to sunset.

On Sept. 12 it will be passing about 2 degrees to the upper right of the bluish 1st-magnitude star Spica in the constellation Virgo. The star appears twice as bright as the yellow-orange planet. Start searching low to the horizon about 45 minutes after sunset. Binoculars will prove most beneficial in locating them.

On Sept. 24, about a half hour after sunset, using binoculars, scan very low near the west-southwest horizon for a view of a 3-day old waxing crescent moon. If you can find it against the bright backdrop of the evening twilight sky, see if you can also catch a glimpse of Mars about 7 degrees to the moon's upper right. This will probably be your last opportunity to see this planet in the evening sky before it completely disappears into the sunset glow.

Jupiter

Jupiter, the solar system's largest planet, rises after midnight and shines brightly at magnitude -2.1, with its four moons visible in the smallest telescope and will provide a dramatic end to many late-night observing sessions in the September chill.

On the morning of Sept. 16, a few hours before sunrise, check out a lovely waning crescent moon in the eastern sky, and shining about 4.5 to its lower right will be Jupiter, while shining to the left of the moon are the two stars marking the heads of Gemini, the Twins, Pollux and Castor.

Saturn

Saturn is now very near the borderline separating the zodiacal constellations of Pisces and Aquarius, rises right around sunset. It reaches opposition on Sept. 21, meaning we see Saturn at its closest and biggest in this month as the sunlit face of its rings are closing from 2.5 degrees to 1.5 degrees from edge-on. In a telescope the rings should appear as a bright line of radiance on either side of the golden globe. So thin are the rings in September that the brightest Saturn can get overall is magnitude +0.6.

But on nights of good seeing, even small telescopes should reveal glimpses of the narrow ring system. On September 8th, look low toward the east beginning around 8:30 p.m. for a view of the moon, one day past full phase. Saturn will be the bright yellowish-white star situated about 5.5 degrees to the moon's right.

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 and other publications.

Editor's Note: If you get a great photo of any of the planets and would like to share it with Space.com's readers, send your photo(s), comments, and your name and location to spacephotos@space.com.