Skylights: September 2022

September brings some notable seasonal changes in our sky. The last 7:00pm sunset occurs on September 12. Sunsets will remain earlier than 7:00pm through March 22, 2023. Equinox occurs at 9:04pm on the 22nd.

The Full Harvest Moon rises 6 minutes after sunset on the 9th. 

September isn’t the best month for viewing Mercury. Before it passes inferior conjunction on the 23rd, it lies low in the west-southwest after sunset. During the last few days of September, it will become visible in the morning sky, overtaking Venus in altitude above the eastern horizon on the 27th. By month’s end, it rises just an hour before the Sun. 

Venus is now just about as far from Earth as it will get, as we are looking past the Sun to see it on the far side of its orbit. Its elongation angle decreases from 13° in early September to just 6° at the end of the month, though its steep angle with respect to the horizon should make it observable throughout most of the month. On the 5th, Venus shines just 0.6° northeast of Regulus, in Leo. By the 10th, it is rising less than an hour before the Sun, and a challenging 29-day waning crescent Moon is 2.8° northeast of Venus on the 25th. 

If you haven’t watched Mars in a while, September marks a great time to get reacquainted with the Red Planet. It begins the month blazing at magnitude -0.1 only 5° from Aldebaran. Note the similarity in color between Mars and Aldebaran while they are in close proximity. Outshining all the stars visible in our sky except Sirius, it continues moving eastward through Taurus, and crosses into the Winter Hexagon on the 11th. Turning a telescope on Mars reveals a gibbous disk approaching 12 arcseconds by month’s end, large enough to reveal details in medium and large instruments. 

Jupiter rises due east in early evening and is the most prominently bright star-like object in the sky until dawn. Having spent the past several weeks in Cetus the whale, it moves westward back into Pisces on the 1st. Through a telescope, Jupiter shows a very large 49 arcsecond globe and many of its cloud features, as well as the shadows of its moons, will appear in small to medium-sized telescopes.

Saturn continues to move westward in Capricornus. Have you noticed how far it has moved with respect to Deneb Algedi and Nashira? It no longer makes a neat isosceles triangle with them, and will continue moving west for several more weeks. Through a telescope, it is also becoming three-dimensional again, with a bit of shadow appearing on the ring behind and to the east side of Saturn. Several of Saturn’s moons can be seen with a medium-sized telescope. Besides Titan, there are, in order of distance from the planet, Tethys, Dione, and Rhea. It is notable that all four of Saturn’s brightest moons appear roughly in line extending to the west of the planet on the 3rd.

Uranus, in Aries, rises just a few minutes before the Pleiades. To find it, locate 4th magnitude Botein (Delta Arietis) about 9° southwest of the Pleiades, then look 3° to the south-southwest. Uranus is the fourth “star” at the end of a shallow zigzag line with 6th magnitude stars 53 and 54 Arietis in between. Uranus is slightly brighter than these stars, and its small positional change with respect to them should make it stand out. If you need additional help finding Uranus, the  waning gibbous Moon lies 2.7° to its east on the 14th. Later in the month, when bright moonlight is out of the sky, you may try to find Uranus without optical aid. Find a dark sky and wait until about 3:00am, when Uranus is high in the south.

September is the best time to observe Neptune, as our most distant planet reaches opposition on the 16th. At 28.9 AU from Earth, Neptune shines at magnitude 7.7, putting it within view of binoculars. Neptune is 5.3° south of Gamma Piscium, and about ¼ of the way between Jupiter and Saturn.

If you haven’t observed Pluto yet this year, September is your last good opportunity to do so. Wait until the second half of the month when the bright Moon is out of view, and Pluto will be due south just after evening twilight. Use a 12-inch telescope to spot it visually, or take an image using a telephoto lens or small telescope to  reveal the magnitude 14.4 dwarf planet about 3.2° west-southwest of globular cluster M75 in western Sagittarius.

After having been out of view for the summer, Ceres comes back into view in the predawn sky. The magnitude 8 dwarf planet can be found with a small telescope near the sickle asterism of Leo.

Asteroid 2 Pallas, at magnitude 9, is visible in Orion, just 2° north of Saiph (kappa Orionis) on the 1st, and moves southeastward through Monoceros and into Canis Major as the month progresses. It appears to move directly towards SIrius, and in early October it will pass just 1/3° to the northeast of our night sky’s brightest star.

Asteroid 3 Juno spends most of September in Aquarius, just north of the ecliptic between Saturn and Jupiter, and reaches opposition on the 7th. The magnitude 7.7 asteroid lies just over 1.3 AU away 

Looking to the stars beyond, we note some seasonal changes in September. The Summer Triangle is poised high overhead at the beginning of darkness, and will be part of our evening sky for the next few months, but in the southwest, Scorpius is leaving the sky earlier and earlier. By the end of the month, Antares has set by 9:00pm. 

The best time to observe the clusters and nebulae of Sagittarius is now behind us, but rising high in the northeast, the richness of the Milky Way through Cepheus and Cassiopeia is waiting to be explored.

Our star that signals the beginning of spring, Arcturus, now serves as a golden beacon of autumn as it appears lower in the west with each departing evening of late summer.

To the east and northeast, Pegasus and Andromeda have come fully into view, and with them, views of our Milky Way’s closest neighboring spiral galaxy, the Andromeda Galaxy, M31.

To the northwest, the Big Dipper begins to assume its proper “upright” orientation, as any imagined stardust confections or cosmic concoction within its bowl would be neatly contained without spilling out. During September, the pointer stars of Big Dipper also align with the cardinal points of the clock, making it one of the four times in the year when the Big Dipper itself can be used as a simple clock. At about midnight local time, the two pointer stars, Merak and Dubhe, which point to Polaris any time of the night and year, are aligned due north, directly below Polaris. If you can imagine them as an hour hand on a 24-hour clock, they are due east of Polaris at 6:00am, and if you could see them at noon, they would be directly overhead. At 6:00pm, they would be due west.  

And while the Big Dipper’s pointer stars are telling you that it's midnight, follow the pointers back up to Polaris, and continue across the zenith and into the southern sky. You will find that the pointer stars line up with the western edge of the Great Square of Pegasus, and all the way south, to Fomalhaut, in the Southern Fish, Pisces Austrinus. 

As the midnight sky rolls on into the AM hours, Auriga and Taurus begin to bring up the stars of the Winter Hexagon. Orion is fully visible at about 2:00am, and Sirius, at the southernmost point of the hexagon, is visible a little over an hour later.

If you’re out early in the AM, before dawn, you will even see the stars of spring making their comeback already, with Cancer, Hydra, and Regulus, in Leo, making their appearance low in the east before the onset of twilight. And if you’re still observing at this hour, turn to the northwest to see Cygnus assuming its Northern Cross orientation, and look again to the northeast to note the position of the Big Dipper’s clock hand as the time approaches 6:00am.