by Geoff Chester, USNO Public Affairs | 19 September 2023 Treasures in Cassiopeia: star cluster Messier 52 and NGC 7635, the "Bubble Nebula", imaged 2023 September 17 at Blackwater Falls State Park, Davis, West Virginia with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a ZWO ASI183MC CMOS color imager The Moon skirts the southern horizon this week, waxing through her crescent and gibbous phases as she sidles through the southern summer constellations. First Quarter occurs on the 22nd at 3:32 pm Eastern Daylight Time. Look for the Moon a few degrees west of the red-hued star Antares on the evening of the 20th. On the 22nd you will find her perched atop the “spout” of the Teapot asterism formed by the brightest star in Sagittarius. The autumnal equinox occurs on the 23rd at 2:50 am EDT. This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 180 degrees and enters the southern hemisphere of the sky. At this moment the Sun will be directly overhead as seen from the equator south of the tip of the Indian peninsula. The term “equinox” translates from Latin to “equal night”, implying that the day is evenly split to 12 hours of daylight and night. However, a quick glance at an almanac will show that this isn’t exactly the case. The date when we have exactly 12 hours between sunrise and sunset is the 26th. This is due to the Sun having a tangible disc, and that we measure sunrise as the first appearance of its upper limb and sunset by the disappearance of the same limb. Refraction in the Earth’s atmosphere also makes the limb appear a tad higher than it actually is, so the combined effects delay the actual time of “equal night”. Enjoy these few minutes of extra daylight while you can. We won’t see days longer than the nights until a few days before the vernal equinox next year. The night sky of early autumn is one of the best times to learn to identify bright stars and their associated constellations. The evenings turn crisper and lack the haze that often scatters moonlight that brightens the space between the stars. From suburban skies, the fainter stars associated with the bright stars of the Summer Triangle become easier to see, and you can begin to see the figures associated with the constellations’ names. Looking at the Triangle’s brightest star, Vega, you may notice a small parallelogram of third-magnitude stars nearby. These stars represent Lyra, the Lyre of Orpheus in Greek Mythology. Crossing through the center of the Triangle is a chain of stars trending southwest from Deneb, the faintest star of the Triangle. Clearer skies should show a cross-shaped stick figure that delineates the figure of Cygnus, the Swan, one of the many guises assumed by Zeus in his romantic dalliances on Mount Olympus. Looking northeast later in the evening, you will see a small, W-shaped group of stars that heralds the arrival of the autumnal constellations. This is Cassiopeia, the mythical Queen of Ethiopia, who plays a central role in a story that involves many other nearby constellations. Her vanity, which caused her to declare herself more beautiful than the daughters of Neptune, led to the near-demise of her daughter Andromeda. The latter was dramatically rescued by the hero Perseus, who rode to her rescue on the winged horse Pegasus and killed the sea monster Cetus in the process. All of these figures will figure prominently as autumn plays out in the Northern Hemisphere. Saturn should be an easy object to find as evening twilight fades. The ringed planet’s pale yellow glow stands out as the brightest object in the southwestern sky during the early evening hours and crosses the meridian at around 11:30 pm local time. The planet’s rings are now tipped about 12 degrees to our line of sight; they will continue to gradually narrow until their next edge-on presentation in March, 2025. Jupiter now rises at around 9:00 pm and climbs steadily in the eastern sky as the night progresses. You should have no trouble spotting his bright glimmer against the dim stars of Aries and the “circlet” asterism in Pisces. Look for the Pleiades star cluster some 15 degrees to the left of Old Jove relative to the horizon. Venus continues to cast her bright glow over the eastern horizon as morning twilight gathers. At 6:00 am local time she will be 25 degrees high as she moves through the stars of Leo, the Lion. Look for the Lion’s brightest star, Regulus, about 15 degrees below and to the left of Venus. If you continue the line from Venus through Regulus, look about five degrees above the horizon for the elusive planet Mercury. Binoculars will be a big help in locating the fleet planet, but once you’ve found him you should be able to see him with the naked eye as he climbs in the brightening sky. He should be visible for most of the week as he reaches greatest elongation from the Sun on the 22nd.