The Sky This Week
U.S. Naval Observatory's Weekly Blog

The Sturgeon Moon, Stars, and the Seeliger Effect

by Geoff Chester, USNO Public Affairs
09 August 2022

The "Seeliger Opposition Effect" on Saturn, imaged January 2006
Note the brightness of the rings in the image on the right, taken half a day after opposition.

The Moon begins to climb northward along the ecliptic this week, passing into the rising stars of the autumnal constellations.  Full Moon occurs on the 11th at 9:36 pm Eastern Daylight Time.  August’s Full Moon was known as the Sturgeon Moon in Native American skylore due to the large number of these ancient fish that gathered in the Great Lakes to spawn at this time of the year.  Other names are the Grain Moon, Corn Moon, and Lightning Moon.  Look for Saturn to the north of the Sturgeon Moon on the night of the 11th.  On the 14th Luna rises with bright Jupiter in the southeastern sky during the late evening.

As we mentioned last week, the bright Moon washes out the majority of the annual Perseid meteor shower, which peaks on the night of the 12th/13th.  That said, if you find yourself out away from city lights in the wee hours of the 13th take a few moments to look for some of the brighter shooting stars that are part of the shower.  Many Perseids are brighter than second magnitude, and bright “fireballs” are not uncommon sights.  The meteors are quite swift, zipping across large swaths of the sky in a few seconds.  The stream of meteoroids that produce the shower is quite broad, so the shower will remain active through the end of the month.

By now the change in the length of daylight has become quite noticeable.  Each day is about two minutes shorter than its predecessor, and the  total time that Old Sol is above the horizon is now one hour less than it was at the time of the summer solstice here in Washington.  The daily rate of change will continue to grow larger as we approach the autumnal equinox, when nearly two more hours will be swallowed up by the encroaching night.

The light of the Sturgeon Moon washes out our view of the summer Milky Way this week, but you can trace out its location in the sky by noting the placement of bright stars.  As twilight ends you should be able to locate the bright red star Antares in the southern sky along with three stars in a near vertical line just to the west.  These stars form the “heart” and “head” of Scorpius, the Scorpion, and if you have a clear view of the southern horizon, the Scorpion’s “tail” should be visible as well.  To the east of Scorpius you should be able to make out the “teapot” asterism formed by the brighter stars of Sagittarius, the Archer.  Moving up from the horizon, you will see three bright stars that for the Summer Triangle asterism.  The southernmost star, Altair, leads the constellation of Aquila, the Eagle.  The brightest star, Vega is the lead star in the diminutive constellation of Lyra, the Harp, while the northernmost, Deneb, marks the “tail” of Cygnus, the Swan.

As midnight approaches, a glittering array of stars in the shape of a “W” climbs into the northeastern sky.  This is Cassiopeia, the Queen, which is one of the central characters in an ancient story that plays out in the autumn sky.  This is actually a good time to get to know these stars since they are scattered in front of the myriad stars that make up the Milky Way.  Once the Moon moves into the early morning they will guide you to some of the splendors offered by the softly glowing star clouds of our home galaxy.

Saturn reaches opposition on the 14th, rising at sunset and setting at sunrise.  This is a particularly interesting time to look at the ringed planet through the telescope.  On the nights leading up to and just after opposition you may notice a curious lighting effect.  During this time the planet’s rings become much brighter than the planet’s disc in much the same way that the Moon’s surface becomes much brighter around the time of Full Moon.  Known as the “Seeliger Effect”, it is only visible when Saturn is close to opposition. 

Bright Jupiter rises at around 10:00 pm and joins Saturn as a telescopic attraction by midnight.  Look for the Moon close to the giant planet late in the evening of the 14th.  Ruddy Mars is still best seen in the pre-dawn sky.  This week the red planet moves into the constellation of Taurus, the Bull, closing in on the Pleiades star cluster.  Venus is now starting her slow fall toward the Sun.  You can see her low in the east as morning twilight announces the pending sunrise.
 
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