(Looking for last month’s ‘Night Sky’? Find it at this link…)
The thick band of the Milky Way in Scorpius and Sagittarius has moved westward, but the lengthening nights keep it accessible to stargazers until the end of September before it sets for the year. In the east, the relatively star-poor constellations of Pegasus, Capricornus, and Piscis Austrinus are moving into view along with hundreds of galaxies accessible with a small telescope. The latter constellation’s lone bright star, Fomalhaut, shines alone in the southern sky for much of the night along with Saturn to the west, which is still in excellent position for observing. Also in September, Venus blazes in the eastern early-morning sky, Jupiter reaches prime observing season, and the zodiacal light emerges in the morning for observers in dark sky. Here’s what to see in the night sky this month…
Comet C/2023 P1 Nishimura
In August 2023, the Japanese amateur astronomer and comet hunter Hideo Nishimura captured an image of a new comet now called Comet C/2023 (P1) Nishimura. The comet was a relatively bright 11th magnitude at the time of discovery in Gemini. It was a happy bit of luck for Nishimura since most comets are now captured by automated telescopes at professional observatories. The comet is brightening quickly and may reach naked-eye visibility by the middle of September. It will lie in the morning twilight for northern hemisphere observers, so a pair of binoculars or a wide-field telescope will help you spot the comet more easily. The image below shows the path of the comet through the month as seen from the northern hemisphere. It appears in the east-northeastern morning sky at morning twilight.
Southern-hemisphere observers get their best chance to see the comet at month’s end. The image below shows where to find it in the bright evening twilight in the western sky (the cyan circle in the image shows a 2º field of view).
Comet Nishimura is on a hyperbolic orbit on its way the inner solar system and – if it survives its encounter with the Sun – will head out to interstellar space. It won’t be back this way again. Read more background on the Comet Nishimura at this link.
Sky Events in September 2023
4 September 2023. The waning gibbous Moon and Jupiter rise together in the east in the late evening about 7º apart from each other. Jupiter, now shining at magnitude -2.6, begins retrograde motion today. It’s now well-placed for observation from now through the end of the year in the constellation Aries about 15º north of the ecliptic, its most northerly position in eight years and ideal for observing with binoculars or a telescope.
5 Sept. The Pleiades star cluster leads the Moon by about 5º in the early-morning sky. All this month, make an effort to rise early before twilight and gaze at the bright stars of Taurus, Orion, Gemini, and Canis Major along with the bright planets Venus and Jupiter. It’s an impressive display.
6 Sept. Last Quarter Moon, 22:21 UT
6-20 Sept. The arc of the Milky Way splits the sky in half in the mid-evening hours making for a great photo opportunity or for old-fashioned visual observing in dark sky as the Moon gets out of the way for the next couple of weeks. The plane of the galaxy appears to thrust at a right angle from the southwestern horizon in Sagittarius, passes overhead through the constellations Scutum and Cygnus, then arcs again down to the northeastern horizon into the rising constellation Auriga.
11 Sept. Look eastward before dawn to see the waning crescent Moon and the Beehive cluster (Messier 44) about 3.5º apart. A lovely sight, for sure, but Venus steals the show here as it shines at magnitude -4.7 to the south. The planet is quickly moving away from Earth now and over the course of September its apparent diameter shrinks from 49” to 32”. However, its elongation from the Sun increases from 28º to 44º during the month.
12 Sept. Over the next couple of weeks, northern-hemisphere observers who have very dark sky can see the zodiacal light in the east 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.
15 Sept. New Moon, 01:40 UT
19 Sept. Neptune reaches opposition in the constellation Pisces near the 5.5 magnitude star 20 Piscium. The planet itself, which lies today at a distance of 4.3 billion kilometers, shines at magnitude 7.8 and spans a diameter of just 2.4”. You can spot the planet in binoculars, but you need a telescope at about 150x or more to reveal its pale blue-green disk.
19 Sept. Venus reaches its greatest illuminated extent today and shines as the “Morning Star” at magnitude -4.8, as bright as it ever gets, and bright enough to cast a shadow in dark locations.
20 Sept. Look to the southwest to see the crescent Moon just a degree or two (depending on your location) from brilliant Antares in Scorpius.
22 Sept. First Quarter Moon, 19:32 UT
22 Sept. Northern-hemisphere observers enjoy the best morning apparition of Mercury in 2023. Look for the speedy little planet in the east at dawn in the constellation Leo. Today the planet marks its greatest western elongation about 18° from the Sun and well above the eastern horizon. The planet shines at a bright magnitude -0.3 today and brightens to magnitude -1.0 in the next few days.
23 Sept. The September equinox arrives at 6:50 Universal Time as the Sun crosses the celestial equator moving southward. This marks the first day of autumn in the northern hemisphere and the first day of spring in the southern hemisphere.
29 Sept. Full Moon, 09:58 UT