by Geoff Chester, USNO Public Affairs | 17 September 2024 The Moon on International Observe the Moon Night, 2024 September 14 Imaged with an 80mm (3-inch) f/6 Antares Sentinel refractor and a ZWO ASI183MC CMOS imager from Chantilly, Virginia The Moon wanes as she climbs northward through the rising autumnal constellations this week. She starts the week off with a partial eclipse on the evening of the 17th and ends the week in the company of bright Jupiter and the stars of Taurus. Last Quarter occurs on the 24th at 2:50 pm Eastern Daylight Time. The lunar eclipse on the 17th will be a small partial one as the Moon’s northern limb grazes the umbral shadow of the Earth. Between 10:00 pm and 11:17 pm EDT you will see a small circular notch drift across the top of Luna’s bright disc. Mid eclipse occurs at 10:44 pm, when about 9 percent of the disc will be obscured. The autumnal equinox occurs on the 22nd at 8:44 am EDT. This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of 180 degrees and crosses the celestial equator, moving into the southern hemisphere of the sky. The astronomical season of autumn will begin at this time, ending with the winter solstice on December 21st. It is also the time of year when we see the most rapid decline in the length of daylight, which is nearly three minutes per day. Although the term “equinox” means “equal night”, the combination of the Sun’s substantial disc, atmospheric refraction, and the way sunrise and sunset are defined, the actual date when we have exactly 12 hours of daylight won’t occur until the 25th. It’s a good thing for stargazers that we are having earlier sunsets, since this is also the time of year when the Moon seems to linger in the evening sky. As we mentioned last week, the times between successive moonrises around the time of the Harvest Moon are less than half an hour each night. This gradually increases during the week, but moonrise nonetheless occurs before midnight until next week. As Luna gradually shifts to later rise times during the week, the bright stars of the summer triangle still dominate the evening sky. By 9:00 pm the Summer Triangle crosses the zenith and offers skywatchers a number of celestial treats. Small telescope owners should have no trouble finding Albireo, which lies in the middle of the Triangle. It is one of the prettiest double stars in the sky, and the generous separation between its blue and gold-tinted components should be easy to resolve in a terrestrial spotting scope. Its striking color contrast is best seen in smaller aperture instruments; my favorite view is through my 3-inch refractor. The dual nature of Albireo may just be a lucky coincidence. Recent measurements of the distances to each component indicate that the brighter star is some 365 light years away while the blue companion is about 395 light years distant. This implies that the two stars are an “optical double” in which the stars are not physically related, but just happen to lie along our line-of-sight from Earth. To see a true double star, move to the brightest star in the Summer Triangle, Vega. Using a pair of binoculars, look just to the northeast of Vega for a close pair of stars. Now point a 4-inch or larger aperture telescope in their direction and you will see that each of the wide pair’s components is itself a close double star. In this case all of the stars are gravitationally bound, with the whole system lying about 160 light years away. Splitting the “Double-double” as the star is popularly known is often used as a test of telescope optics and the stability of our atmosphere. The bright glimmer of Venus is now beginning to appear in bright twilight shortly after sunset for viewers with low western horizons. You will find her about five degrees above the western horizon about half an hour after sunset. Use binoculars to find her bright glimmer in the bright twilight, then look for her with the unaided eye. She will continue to climb a bit higher on successive evenings, and as October begins she should be a fixture in the twilight sky. Saturn pops into view in the southeast as evening twilight fades to darkness. The planet sports a pale yellow glow and is the brightest object in this part of the sky. She climbs higher as the night progresses, and by 10:00 pm she is a good target for telescopic perusal. Bright Jupiter may be seen rising in the northeast by midnight, and each successive night he climbs a bit higher at the witching hour. Look for Jupiter rising with the waning gibbous Moon late at night on the 23rd. The giant planet will be visible all night long by early December and will keep us company throughout the winter. Ruddy Mars rises at around 12:30 am and is still best seen before morning twilight. This week the red planet begins to traverse the stars of Gemini. He gets a visit from the last quarter Moon during the early morning hours of the 25th.