Monthly summary of stellar events - September Skies -Rick Wagner
1 – 15 Sep – look for the zodiacal light in the eastern sky during morning twilight. If you’re under very dark transparent skies at midnight you can try to see the even fainter gegenschein – the backscatter of sunlight from dust particles in the plane of the solar system. It will appear as an extremely faint glow about 10º across centred near Neptune about 12º south of the circlet of Pisces. If you don’t see either of them you’ll have another good opportunity in early October.
06 Sep – look for an extremely old (only 14 hours before new) extremely thin crescent Moon just slightly above the eastern horizon shortly before sunrise. This will be an extremely challenging, near world-record observation.
07 Sep – New Moon, 20:51EDT
08 Sep – the crescent Moon is 6 degrees to the upper left of Mercury (mag 0) very low above the western horizon shortly after sunset; much brighter Venus (mag -4.1) is 16º left of the Moon. See if you can see Spica (mag 1) 4º to the lower right of the Moon.
09 Sep – the crescent Moon has waxed to a slightly chubbier crescent and is now 4º to the upper right of Venus, forming a nearly equilateral triangle with Venus and Spica.
10 Sep – minor planet (2) Pallas is at opposition southwest of the circlet if Pisces.
13 Sep – First Quarter Moon
14 Sep – Neptune reaches opposition. If you have a telescope of 20cm or (preferably) larger aperture this is the season to try to spot Neptune’s largest moon Triton. It is about mag 13.5 and varies from 10-17 arcsec from the mag 8 planet. It is most distant from Neptune and easiest to see every ~3 days. If you watch it from night to night you can watch its retrograde movement – the only major moon in the solar system that orbits backwards. You can get a chart for your chosen observing time at https://skyandtelescope.org/wp-content/plugins/observing-tools/neptune_moons/neptune.html.
14 Sep – Mercury is at greatest elongation east, visible in the evening sky. But not a favourable apparition as the ecliptic lies close to the horizon at sunset and so Mercury will remain low and set soon after sunset.
20 Sep – Full Moon, 19:54EDT
22 Sep – autumnal equinox – astronomical autumn begins in the northern hemisphere; 15:21EDT