The month of September affords stargazers a last chance to see the Milky Way and all its attendant splendors. The rich constellations of Scorpius and Sagittarius are setting for the year, moving a little westward each night, but the lengthening nights keep these stars accessible for a little longer, at least for observers in the northern hemisphere. Southern observers still enjoy the thickest part of the Milky Way almost overhead. In the east, the relatively star-poor constellations of Pegasus, Capricornus, and Piscis Austrinus are moving into view. Observers with very dark sky get the chance this month to see the zodiacal light, a faint wedge of white sunlight reflected from fine dust in the inner solar system. Also this month, low in the southwestern sky at sunset, Jupiter slowly fades from view while Saturn still hangs on, its rings casting dramatic shadows on the disk of the planet. Mercury makes its best showing of the year in the eastern sky before dawn. And the Sun crosses the celestial equator heading south, marking a change of seasons. Here’s what to see in the night sky this month…
4 Sept. Neptune reaches opposition today. This blue-green ice giant, the most distance major planet from the Sun in our solar system, shines at magnitude 7.8 and spans an apparent diameter of 2.4″. Its tiny disk is visible in the constellation Aquarius less than 1º east-southeast of the orange 4th magnitude star Hydor (λ Aquarii). While the planet is plenty bright enough to see with a telescope, or even binoculars, resolving its disk requires some magnification, at least 75x to 100x. The planet’s disk gets larger with more magnification while the images of the stars do not. Visually, the planet has a very pale blue-green color.
6 Sept. Full Moon, 07:03 UT
11 Sept. Jupiter passes about 3º north-northeast of the bright white star Spica low in the southwestern sky after sunset. Jupiter, while still brighter than any star, is essentially well past its prime for the year and lies too low in the sky for serious telescopic observation.
12 Sept. Mercury reaches greatest western elongation 18º from the Sun. This is the best time to see Mercury this year. It appears in the morning sky and shines at magnitude +0.0 at its brightest before it moves back towards the horizon and dims as September progresses.
13 Sept. Last Quarter Moon, 06:25 UT
14 Sept. The planet Saturn reaches eastern quadrature today as it lies exactly 90º east of the Sun. While the planet appears to slowly shrink in size this month, it’s still a breathtaking sight in a telescope. The rings are tilted by some 26º, the most in 15 years, and the long shadows cast by the Sun give the planet a stunning ‘3D’ appearance in a telescope. While Saturn lies in the constellation Ophiuchus and remains relatively low for northern observers this year, it still looks remarkably beautiful on a night of steady air. Southern-hemisphere observers get the best views this year and for the next several apparitions of Saturn.
15 Sept. From now until the end of the month, northern-hemisphere observers who have very dark sky can see the zodiacal light in the eastern sky about 90-120 minutes before sunrise in the northern hemisphere. This whitish glowing wedge of light appears to thrust upward from the horizon. The zodiacal light, sometimes called the “False Dawn”, is simply sunlight reflected off tiny dust particles in the inner solar system.
18 Sept. Wake early and look eastward before sunrise to see a vertical line of celestial objects that include, from high to low, Venus, Regulus, a very slender waning crescent Moon, Mars, and Mercury. A pair of binoculars will help you get the best view of these objects in the brightening morning sky. The angle of this vertical celestial stack relative to the horizon favors northern-hemisphere observers. At magnitude -3.9, Venus outshines all. Venus is moving away from Earth this month as it swings around the far side of its orbit.
20 Sept. New Moon, 05:30 UT
22 Sept. The September Equinox arrives at 20:02 UT as the Sun crosses the celestial equator moving south. This marks the beginning of autumn in the northern hemisphere and the beginning of spring in the southern hemisphere.
28 Sept. First Quarter Moon, 02:54 UT
(All maps produced with SkyX Serious Astronomer by Software Bisque).