by Geoff Chester, USNO Public Affairs | 07 December 2023 The Crescent Moon and Venus, 2023 November 9 Imaged from Alexandria, Virginia with a Canon EOS Rebel SL2 DSLR The Moon wanes in the morning sky this week, passing through the rising spring constellations as she dives southward along the ecliptic. New Moon occurs on the 12th at 6:32 pm Eastern Standard Time. Luna may be found just northwest of the bright star Spica before dawn on the 8th. On the morning of the 9th she passes just to the south of the bright glow of Venus. This is the week when we here in temperate northern latitudes experience the year’s earliest sunsets. Here in Washington Old Sol slips below the horizon at 4:46 pm EST until sunset on the 12th, which will be one minute later. Why does the time of earliest sunset occur for over a week? Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface, i.e. an ocean horizon. If the Earth had no atmosphere we could predict the rise and set times to a precision of a few seconds. However, the atmosphere refracts the light of celestial bodies near the horizon, and this refractive factor can vary with local atmospheric conditions and latitude, so we round the predicted times of rise and set to the nearest minute. The year’s last monthly observing campaign for the Globe at Night citizen science program takes place this week. The target constellation for December is Perseus, which passes overhead at around 11:00 pm local time. To me, Perseus resembles a wish bone with one tine broken off. The longer tine leads you to the Pleiades star cluster, while the shorter one ends in the eclipsing variable star Algol. Its brightest star, Mirfak, is located where the tines join. The constellation has a good roster of stars of different magnitudes, so it is useful for both urban and rural skywatchers. You can record your observations on the Globe at Night web app. One star, though, can throw you a curve ball: Algol, also known as the Demon Star, changes its brightness, fading by 1.5 magnitudes every 2.87 days. Its next two evening minima will be on the 14th at 9:27 pm EST and the 17th at 6:17 pm. As Perseus glides overhead, the stars of the Great Winter Circle are taking command of the eastern sky. Nine of the 25 brightest stars in the sky occupy this region, with two of them ensconced in the prominent figure of Orion, the Hunter. Red-tinted Betelgeuse, which marks one of Orion’s shoulders, is complemented by ice-blue Rigel, which denotes one of his knees. Between them, his three blue-hued “Belt” stars form one of the most distinctive asterisms in the sky. Most of Orion’s stars are very far away, ranging from some 2000 light years for Alnilam, the center star in the belt, to 250 light years for Bellatrix, the Hunter’s other shoulder. If the accepted distance to Alnilam is indeed the case, it is one of the most massive and luminous stars known. To appear as bright as it does in our sky, it must shine with the equivalent light of over 800,000 Suns! If we could somehow get Alnilam and Sirius, the brightest star in the sky, to exchange places, the results would be dramatic. At a distance of 2000 light years, Sirius would need a telescope to be seen at all. Alnilam, at a distance of just 8.5 light years, would shine with the apparent brightness of a full Moon. Somewhat closer to home, the pale yellow glow of Saturn is best glimpsed shortly after sunset. The ringed planet is gradually pressing westward with each passing night and now sets at around 10:30 pm. Catch him early if you want to get a good glimpse of his rings through the telescope. Jupiter is now well placed for viewing as evening twilight ends. The giant planet crosses the meridian shortly after 9:00 pm, so you will have ample time to watch him through the telescope. Old Jove offers much for small telescope owners as his large apparent disc is easily resolved in just about any instrument. On the evening of the 6th you can watch the innermost of his large moons, Io, cross onto the planet’s disc at 6:47 pm EST. At 7:36 pm, Io’s shadow enters the disc, and over the next two hours you can watch the shadow’s black dot cross the planet’s face. Venus marches southeastward along the ecliptic this week. If you’re up before dawn you can watch her motion away from the bright star Spica. Be sure to catch the close approach of the waning crescent Moon before dawn on the 9th.