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Under the Harvest Moon

by Geoff Chester, USNO Public Affairs | 26 September 2023

by Geoff Chester, USNO Public Affairs | 26 September 2023

Saturn, imaged 2023 September 21, 01:14 UT at the U.S. Naval Observatory, Washington, DC
with the historic 30.5-cm (12-inch) f/15 Clark/Seagmüller refractor
and a ZWO ASI183MC CMOS color imager

The Moon starts the week in the company of Saturn, then moves eastward along the ecliptic through the dim autumnal constellations.  Full Moon occurs on the 29th at 5:58 am Eastern Daylight Time.  Look for Luna just above the bright glow of Jupiter late on the evening of October 1st.  On the following night you will find her just to the south of the Pleiades star cluster.

September’s full Moon is almost universally known to Northern Hemisphere dwellers as the “Harvest Moon”.  This is not just a traditional name; it describes an astronomical phenomenon that only occurs in the autumn.  

As the Moon moves around the sky, it advances about 13 degrees along its orbital path from night to night.  In the autumn, if you watch Luna’s progress from new to full phase, you will see that it passes its lowest point above the southern horizon at the time of its first quarter phase, then begins to climb northward as it waxes to full. Its orbital path intersects the eastern horizon at a shallow angle, which means that at the time of full Moon it rises at almost the same time from night to night.  Before the invention of artificial lighting, the light of the bright rising Moon added a bit of extra light after twilight, giving farmers some extra time to bring in their crops.

Here in Washington, the Moon rises about 30 minutes later on successive nights around the time of the Harvest Moon.  The effect is accentuated as you move to more northern latitudes.  In Great Britain the difference between successive moonrises is just 10 minutes, while in Stockholm the difference is 2 minutes.  Above the Arctic Circle the Moon actually rises earlier on successive nights!

The bright light of the Harvest Moon washes out the fainter stars that make up the bulk of the autumnal constellations.  Capricornus, Aquarius, and Pisces between them don’t have any stars that are brighter than third magnitude.  The lone exception is the star Fomalhaut, which lies about 20 degrees southeast of Saturn.  This lonely beacon is the brightest star in the constellation of Pisces Austrinus, the Southern Fish.  Located about 25 light years from Earth, Fomalhaut was one of the first stars found to have a surrounding proto-planetary disc.  In 2008 this disc was imaged by the Hubble Space Telescope, and a tantalizing knot in the disc may be a planet in the throes of formation.

The early evening finds the stars of the Summer Triangle passing through the zenith.  By midnight Vega, Deneb, and Altair are heeling over the western horizon, giving way to the small, W-shaped constellation of Cassiopeia, climbing high into the northeastern sky.  

South of the zenith, a large square asterism is crossing the meridian.  This almost perfect square is part of the sprawling constellation of Pegasus.  If you look between the square and Cassiopeia, you should be able to spot two “chains” of stars that diverge from the northeast corner of Pegasus.  These stars form the constellation of Andromeda.  Looking between Cassiopeia and the horizon you will see a wishbone-shaped group of stars that represent the hero Perseus.  If you can find all of these constellations, you will have found one of the greatest stories from Greek mythology.  Each of these constellations are associated with the cautionary tale of Andromeda, who nearly paid the ultimate price for the vanity of her mother, Cassiopeia.

Saturn greets evening skywatchers as the brightest object in the southeastern sky during the early evening hours.  The ringed planet is located among the stars of the large but obscure constellation of Aquarius, the Water Bearer.  The Moon lies just to the southwest of the ringed planet on the evening of the 26th.

Jupiter continues to rise earlier each night, and this week you can see him crest the eastern horizon at around 8:30 local time.  Old Jove is very bright, and once he has cleared the tree line you should have no trouble finding him in the late evening.  He is perhaps the most rewarding planet for small telescope owners thanks to his four bright moons that change their positions from night to night.  The planet’s disc is large enough to show its dark equatorial cloud belts in a three-inch telescope, and occasionally one might sight the famous Great Red Spot.

Venus rises at around 3:30 am and dominates the eastern sky during the hours before dawn.  If you are up and about at 5:00 am, look for the bright star Regulus below the dazzling planet.  She will pass the star next week.


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

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