The Brightest Planets in September’s Night Sky: How to See them (and When)Space.com
By Joe Rao, Space.com Skywatching Columnist | September 1, 2017 06:20pm ET
As we make the transition this month from summer to fall, the planet that was prominent all summer long falls into the sunset fires and becomes increasingly more difficult to observe. It is accompanied by the bluish first-magnitude star, Spica, serving as sort of a "lieutenant" to the king of the gods. Star and planet have engaged in a sort of a celestial pas de deux since the start of the year - a triple conjunction - with their third and final meeting on Sept. 5 before they finally part company for a dozen years.
Meanwhile, the "lord of the rings," Saturn glows in the south-southwest sky as darkness falls.
In the morning sky, Venus reigns supreme, accompanied by two other planets: Mars which finally emerges back into view after the spending the summer hidden in the glare of the sun and Mercury, the innermost planet to the sun, which manages to move just far enough out of the sun's glow to put on its best morning apparition of 2017. All three planets gather not far from the bright star Regulus in the Leo, the Lion and on the morning of the 18th, a crescent moon joins this star and trio of planets in an eye-catching array for those who are up before the sun.
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 has its best morning apparition of the year for mid-northern latitudes, though one almost as good will happen in late December. The planet is too dim to be seen the first several days of September, but it rapidly brightens and can be spotted with binoculars by Sept. 5.
Look about 45 minutes hour before sunup, 10 degrees north of due east along the horizon. On Sept. 5, Mercury is just 2.5 degrees to the upper right of Regulus (magnitude +1.4) and slightly brighter. Mercury brightens rapidly from day to day as it moves away from the sun and shows us more of its lighted side.
If observed daily in mid-twilight, both Mercury and Regulus get progressively higher at first. The pair is closest on the morning of Sept. 10, with Mercury 0.6 degrees to the lower right of Regulus. Regulus continues to gain altitude, but Mercury quickly levels off that week. An even more dramatic conjunction with much dimmer Mars occurs on Sept. 16 (see below under Mars).
Greatest elongation with Mercury 18 degrees west of the sun, is attained on Sept. 12. Mercury is then almost directly above the sun, making this a very favorable apparition from mid-northern latitudes. At magnitude -0.2 and four times as bright as Regulus, the planet rises around the time twilight begins.
For the rest of the month, Mercury brightens another magnitude as it heads back into the twilight glow. By Sept. 26 it has brightened to -1.3 (nearly as bright as Sirius, the brightest star), but this speedy planet is only 9° from the sun and rises in mid-twilight and may then be too low in the bright morning twilight to find.
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 degrees 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 the constellations Cancer and Leo during September and has a few notable conjunctions with stars. On the morning of September 1st the Beehive Cluster, M44, is centered about 1.5 degrees north (left) of Venus. The next morning, the dazzling planet passes about 0.3 degrees from faint, 4th-magnitude Delta (δ) Cancri, known also as Asellus Australis.
On the morning of Sept. 20, Venus blazes at magnitude -3.9 just 0.5 degrees north (lower left) of 1st-magnitude Regulus. Their proximity for several mornings intensifies the contrasting yellow-white of the planet and bluish white of the star. Venus ends September rising only 2 hours before the sun.
Mars – after having been hidden in the solar glare since spring, returns in September from its summer travels behind the sun. The Red Planet can be glimpsed this month low in the eastern sky before sunrise.
Because Mars was at conjunction with the sun on July 26 and will arrive at aphelion on Oct. 7, it is now unusually faint, magnitude +1.8. During the next 10 months, Mars will brighten 70-fold, to -2.9 at the perihelic opposition due in late July 2018. Use Mercury to locate Mars on the morning of the 16th; binoculars will be a big help. The planets will be separated by less than 0.3 degrees. Mars will sit just below Mercury, appearing only 1/11th as bright.
On the morning of Sept. 18, check out this "celestial quintet:" The waning crescent moon, just a 4-percent illuminated sliver, will lie more-or-less midway between Venus and Regulus above it and Mars and Mercury below it. Mars and Venus begin a slow dance in the dawn on Sept. 28. Brighter Venus falls back toward the sun’s glare while the much fainter planet creeps up and out of it; they will pass on Oct. 5.
Jupiter – is low in the west-southwest at dusk as September begins; only about 10 degrees high 40 minutes after sunset (for observers at mid-northern latitudes). Binoculars help show the sparkle of Spica 3 degrees to the lower left of the planet; they will engage in their third and final conjunction in 2017 on Sept. 5. Jupiter keeps appearing lower each night; by late September this planet is hard to find in the bright sky before it sets 45 minutes after the sun. A very narrow waxing crescent moon, only 3-percent illuminated, lies 7 degrees to Jupiter's right on Sept. 21.
Saturn – shines in the south-southwest as twilight fades. It is a steady yellow light about 12 degrees to the upper left of Antares, which is fainter and twinkles.
In a telescope Saturn's rings are still tilted wide open – so much so that they hide the planet’s south pole. A fat waxing crescent moon sits less than 3 degrees above Saturn on Sept. 26.
https://www.space.com/33619-visible-planets-guide.html
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