by Geoff Chester, USNO Public Affairs | 17 October 2023 The Moon, imaged 2022 April 11, 02:35 UT, from Alexandria, Virginia with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor, 1.6X Antares 2-inch Barlow lens, and a ZWO ASI183 CMOS imager The Moon waxes through her crescent phases as she skims along the southern horizon. First Quarter occurs on the 21st at 11:29 pm Eastern Daylight Time. Early in the week Luna passes through the setting summer constellations, then slowly turns northward as she traverses the sparse autumnal star-fields. Look for Luna near Saturn as the week comes to a close. The evening of the 21st is International Observe the Moon night, an evening devoted to the study of our only natural satellite. If you have a telescope, you are encouraged to set it up and invite friends and neighbors over for a close-up look at Luna’s battered surface. Most of us barely notice the comings and goings of the Moon as she makes her monthly journey around the Earth. Few of us have seen her as a world with an astonishing variety of surface details and alien landscapes. Her airless surface is a testament to the incredible violence that occurred during the early stages of the solar system’s creation. The large, relatively smooth circular basins that were dubbed “seas” by early telescopic viewers are the remnant signatures of impacts by large proto-planetary bodies, while the thousands of smaller craters indicate the vast number of smaller bodies that eventually accreted into the planets and moons that we see today. Most of Luna’s exposed surface is far older than the oldest rocks on the surface of the Earth, which tells us that our planet is a dynamic, evolving world that bears little resemblance to its initial form. Don’t have a telescope? Here in the Washington area you can enjoy viewing lunar vistas with the National Capital Astronomers at the Rock Creek Nature Center in Washington or with the Northern Virginia Astronomy Club at the National Air and Space Museum’s Udvar-Hazy Center in Chantilly, Virginia. The evening skies of mid-October are dominated by two prominent geometrical asterisms. In the early evening you can still find the Summer Triangle passing through the zenith. Delineated by the bright stars Vega, Deneb, and Altair, this area of the sky encompasses a number of smaller constellations as well as one of the denser regions of the Milky Way. It is a fine area to inspect with binoculars or small telescopes, revealing a number of star clusters and glowing gaseous nebulae. By 11:00 pm the “Great Square” of Pegasus stands high in the south. You will immediately notice that there are fewer bright stars in this part of the sky since we are looking away from the plane of the Milky Way. There are few nearby star clusters here; the “deep sky” is instead dominated by the faint wispy glimmers of distant external galaxies. The brightest of these is located about 15 degrees northeast of the Square’s brightest star, Alpheratz. Under dark skies you can see this object as a small fuzzy patch that looks like a detached piece of the Milky Way. Binoculars will show an oval patch of diffuse light, and telescopes will show a glowing ellipse with a star-like center. This is the Andromeda Galaxy, the closest large galaxy to us. “Close”, however, is a relative term. The faint light we see from the Andromeda Galaxy has taken some 2.5 million years to traverse the gulf of space that separates us. Saturn is well-placed in the evening sky, appearing in the southeastern sky as evening twilight deepens. The ringed planet reaches his highest point in the south at around 9:30 pm local time, so you have the entire evening to inspect him through the telescope. Saturn’s ring system is one of the most enigmatic sights that you can see in the heavens. Their origin is still a subject of scientific debate, but their fate is more certain. In a few million years they will dissipate as their constituent particles decay into the planet’s atmosphere. Jupiter is now easily seen as a bright object in the eastern sky by 8:00 pm. Even though Old Jove is currently 600 million kilometers (372 million miles) from Earth, he outshines all of the planets except for much closer Venus. Jupiter’s brightness is due to his vast size and bright atmosphere, which reflects over 50 percent of the incident sunlight that strikes him. Venus reaches her greatest elongation west of the Sun on the 23rd. At this time she lies 46 degrees from the Sun and appears at her highest above the eastern horizon in the pre-dawn sky. If you’re up before the Sun you should have no trouble spotting her as she glides below the rising stars of Leo, the Lion.