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

Shining Light on the Harvest Moon

by Geoff Chester, USNO Public Affairs
06 September 2022

Saturn, imaged 2022 September 2, 02:08 UT at the U.S. Naval Observatory

The Moon waxes toward her full phase as she begins to climb northward from the southern reaches of the ecliptic.  Full Moon occurs on the 10th at 5:59 am Eastern Daylight Time.  Look for the Moon near Saturn on the evenings of the 7th and 8th.  On the 10th and the 11th she shares her nook of the sky with bright Jupiter.

September’s Full Moon is known to most of us as the Harvest Moon due to a phenomenon that occurs at this time of the year, at least for some of us.  Skywatchers who live in northern temperate zones experience an interesting effect of orbital geometry during the time of the Full Moon that occurs closest to the autumnal equinox.  On average Luna moves about 13 degrees per day along her apparent orbital path, but at this time of the year her orbit makes a very shallow angle with the eastern horizon.  The net effect of this is that the time between moonrise on successive nights is around 25 minutes around the dates of Full Moon.  If you lived in Scotland the difference is only 10 minutes per night.  Residents of Anchorage, Alaska will see Luna rise at the same time on the nights of the 8th, 9th, and 10th, then rise a minute earlier for three nights after Full Moon.  This phenomenon has been a boon to farmers in the Northern Hemisphere for centuries, who take advantage of the light of the Moon to have a bit of extra time to bring in their crops.  Since our sky lore is heavily derived from northern European traditions, the “Harvest Moon” is almost universally associated with September’s Full Moon.  However, our friends in the Southern Hemisphere see things a bit differently.  For them, the Moon’s orbit intersects the horizon at a steep angle, so intervals between successive moonrises are closer to an hour at this time of year.  They will see their “Harvest Moon” phenomenon in March as we observe the vernal equinox.

The bright Moon washes out most of the faint stars of the autumnal constellations, but there is one lonely sentinel that stands out during autumn evenings.  The first-magnitude star Fomalhaut shines low in the southeastern sky at around 10:00 pm local time.  As the night passes the star forms a large triangle with Saturn and the rising Jupiter.  Despite its seeming isolation in the sky, the star is one of the most-studied of the near solar neighborhood.  Located at a distance of 25 light years from us, it is now known to be a triple star system, with two widely separated dwarf companion stars.  The brightest component, Fomalhaut A, is surrounded by a ring of dusty debris that may harbor several “exoplanets”.

One other constellation that heralds early autumn is Pegasus, the Winged Horse.  This large star pattern is best identified by a large asterism called the “Great Square”, which takes up much of the eastern sky.  Formed by four second-magnitude stars, the Square reminds me of a baseball diamond, which seems appropriate for the time of year.  In ancient Greek mythology Pegasus was one of the most enduring figures, featured prominently in the tale of Andromeda and Perseus.  He leads the entire cast of characters in the myth into the sky each fall.

Saturn now appears low in the southeastern sky as evening twilight ends and steadily works his way to the meridian by 11:30 pm.  This gives ample time to view the ringed planet, which invariably elicits genuine amazement when people see him through a telescope for the first time.  The broad, flat rings appear as solid bodies, but in reality they are composed of billions of bits of frozen water ice.  Their distinctive flat shape and striated gaps are corralled by the gravity of the planet’s inner moons.  They are not permanent, though.  In a few million years the ring particles will either spiral into Saturn’s atmosphere or get kicked by gravity into interplanetary space.

Jupiter appears about half an hour after sunset, looking like a glowing coal in the eastern sky.  As the giant planet rises his color turns a creamy white, and by 11:00 pm he’s entering prime observing time.  Old Jove’s atmosphere is in constant seething turmoil, a stark contrast to the relative calm of Saturn.  Jupiter’s cloud belts are continually changing in their appearance, and the planet’s rapid rotation presents a new view every few minutes.

Morning is still the realm of ruddy Mars, who is currently nearing his most northerly part of his journey around the Sun.  The red planet spends the week cruising through the stars of Taurus, the Bull, rivalling the constellation’s brightest star, Aldebaran.
 
 
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