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Double Moons and Double Stars

by Geoff Chester, USNO Public Affairs | 25 July 2023

by Geoff Chester, USNO Public Affairs | 25 July 2023

Venus close to inferior conjunction with the Sun, 2004 June 3
Imaged with the USNO 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
and a Canon EOS Rebel T2i DSLR

The Moon waxes in the evening sky, skirting the southern horizon as she moves through the southern summer constellations.  Full Moon occurs on August 1st at 2:32 pm Eastern Daylight Time.  August’s Full Moon is traditionally known as the Sturgeon Moon here in North America, since the continent’s largest freshwater fish are beginning their spawning season.  Other names include the Grain Moon, and Corn Moon.  

This year, though, we get a “bonus” Full Moon on the 30th.  This second one is one of two types of “Blue Moon”.  This term applies to either the second full Moon in a calendar month (as we have this year) or the third full Moon to occur in an astronomical season (i.e., between equinoxes and solstices) that has four full Moons.  This second type of Blue Moon will occur on August 24th next year.  Both of this month’s Full Moons also happen to occur close to the time of the nearest lunar perigees for the year, so don’t be surprised to see popular media spinning up the “Super Blue Moon”.

August 1st was once a special day in pre-Christian Gaelic culture, one of the four “cross-quarter” days that marked the mid-points of the astronomical seasons.  Known in Gaelic as Lughnasadh, it was widely observed in Scotland, Ireland, Wales, and the Isle of Man as the first of three harvest festivals.  When Christianity arrived in these regions the day became the feast day of Saint Peter in Chains, and has subsequently evolved into Lammas, or “Loaf Mass”.  On this day it is traditional to bring the first loaf of bread made from the first crop of the season’s grain to the local church for all to share.  Unlike the other cross-quarter days that we know as Groundhog Day, May Day, and Halloween, Lammas isn’t very widely observed here in the U.S., but it is still popular in Great Britain and marks the “traditional” start of summer holidays in many parts of Europe.

The bright Moon washes out the summer Milky Way, which to me is the showpiece of the season.  While the Galaxy’s misty star clouds fade from view, we still have a number of bright stars and deep-sky objects to look at.  By midnight the Summer Triangle asterism is beginning to cross the meridian, giving us three bright stars intrinsic to guide our exploration to other objects.  

The highest and brightest is Vega, which leads the diminutive constellation of Lyra, the Harp.  If you have a pair of binoculars, look to the northeast of Vega for a closely-spaced pair of stars.  This pleasing pair belies a bigger secret.  If you gave a good telescope of 3 or more inches in aperture, you will see that each component of the binocular pair is itself a close double star.  A recently discovered companion of one of the two closer pairs brings the total number of stars in the system to five.  They are all located some 160 light years away.

If you turn your gaze to the Summer Triangle, you should be able to spot a third-magnitude star almost at the center.  This star is called Albireo, which, loosely translated from ancient Greek and Arabic, means “The Hen’s Beak”.  It in fact represents the head of Cygnus, the Swan, but there is much more to it than a confusing name.  Any small telescope will show Albireo as a pleasing, wide double star with a wonderful color contrast.  The brighter component shows a golden yellow hue, while the secondary component is a striking blue color.  Located some 420 light years from Earth, astronomers have seen little change in the component stars’ relative positions since the first telescopic observations over 300 years ago, so the stars may just be aligned toward our line-of-sight as an “optical double”.  

Venus has now for all intents and purposes vanished from the evening sky.  Technically she is still visible, but only in very bright twilight early in the week.  She will pass between the Erath and the Sun on the 13th.

Mars is also very difficult to spot in evening twilight.  The red planet is also moving toward conjunction with the Sun, and he will linger in bright twilight until well into the fall.  His next good evening apparition will occur in the winter of 2024-2025.

Fortunately, Saturn is poised to take center stage in the evening sky.  Currently the ringed planet rises just before 10:00 pm local time, so by midnight he’s high enough to point the telescope for a pleasing view.  He will rise 4 minutes earlier each night and will reach opposition, rising at sunset and setting at sunrise, in late August.

We still have to wait until the wee hours to spot giant Jupiter, but he, too, is steadily advancing to the evening sky.  The best time to see Old Jove is still before sunrise, when you will find his bright glow leading the bright stars of the Great Winter Circle into the gathering twilight.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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