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

Shine on, Harvest Moon!

by Geoff Chester, USNO Public Affairs
14 September 2021

Saturn & Jupiter, imaged at the U.S. Naval Observatory, 2021 September 8
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
and a ZWO ASI183MC CMOS imager

The Moon skims the southern horizon this week, waxing through her gibbous phases before reaching her full phase on the 20th at 7:55 pm Eastern Daylight Time.  Luna will be a few degrees south of Saturn on the evening of the 16th.  On the following night she lies just southwest of bright Jupiter.
 
As this Full Moon is the one that occurs closest to the autumnal equinox it is almost universally known as the Harvest Moon.  I use the word “almost” because the phenomenon is only visible to residents of the Northern Hemisphere in September.  Our friends “down under” see the same phenomenon, only for them it occurs six months later near the March equinox.  It is a consequence of the Moon’s orbital geometry and one’s geographic location.  The Harvest Moon is so named because before artificial lighting began to dominate the night farmers could use the light of the rising Full Moon to give them some extra time to bring in their fall harvests.  At this time of year the plane of the Moon’s orbit intersects the horizon at a shallow angle for Northern Hemisphere observers, and the time between successive moonrises is about half of its more usual times during the rest of the year.  Here in Washington the Moon rises only 29 minutes later on the nights before, during, and after Full Moon.  The effect becomes more pronounced as you move to higher latitudes.  Our friends at the Royal Observatory in Edinburgh, Scotland see successive moonrises that occur only 9 minutes later on the nights around Full Moon.  Travel to Tromsø, Norway and you will find that Luna rises 20 minutes earlier on the nights around Full Moon!  Similar circumstances occur around the time of October’s Full Moon, and in folklore this was seen as a boon to hunters, who could use the extra light from the Moon to pursue game across the stubble of the harvested fields.

The brightening Moon washes out the ghostly glow of the summertime Milky Way, but you can still see the path that the Galaxy traces across the sky.  The season’s brighter stars delineate the plane of the Milky Way, forming the signature constellations of summer from the south to the north.  In the early evening look for the red star Antares in the southwest.  This is the brightest star in the constellation of Scorpius, the Scorpion, and you can trace out the other stars in Scorpius on either side of Antares.  Just east of Scorpius is the “Teapot” asterism formed by the brightest stars in Sagittarius, the Archer.  Halfway to the zenith in the south lies Altair, southernmost of the three stars that delineate asterism called the Summer Triangle, which lies directly overhead at 9:30 pm.  The faintest of the three stars is Deneb, the brightest star in the constellation of Cygnus, the Swan, which is also known as the Northern Cross.  Deneb is one of the most luminous stars in the sky, beaming across some 2500 light-years of space to grace our nights.  Continuing northward, look for a group of five stars that trace out the letter “W”.  This asterism forms part of the constellation of Cassiopeia, the legendary queen of Ethiopia memorialized in Greek mythology along with Perseus, whose stars are now rising in the northeast.

Venus continues her eastward trek along the ecliptic, steadily moving toward the stars of Scorpius.  You can spot Venus in the southwest shortly after sunset and as twilight deepens her presence becomes more evident.  She will continue to be a fixture in the early evening sky for the rest of the year.

The Moon visits Saturn and Jupiter this week, and the two giant planets figure prominently during the nighttime hours.  Saturn shines with a subdued yellow tint, and crosses the meridian at around 10:00 pm.  This is the best time to have a look at the ringed planet since it is the time when he’s highest above the horizon.  
 
Jupiter’s follows Saturn across the sky, and you should have no trouble spotting him in the southeast during evening twilight.  The giant planet’s glow rivals that of Venus, making him an easy target for telescopic observation.  You will be rewarded for tracking him down as almost any telescope will show his four bright moons first documented by Galileo in 1610.  Larger telescopes will reveal more detail in the planet’s turbulent atmosphere where winds blow with incredible ferocity.  Watch Old Jove for an hour and you will see his rapid rotation and the constantly changing configuration of the four Galilean moons.
 
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