August 21, 2021

AstroCal – September 2021


     Let’s begin this month’s AstroCal with the evening planets.  Venus will be the star of the show near the western horizon shortly after sunset.  Shining at magnitude -4.3 (remember,  the lower the number, the brighter the object) will make it the brightest object in the fall sky after the Sun and the Moon.  During the first half of the month, a much dimmer Mercury (mag -0.71) can be seen hovering just above the horizon and to the lower right of Venus.  On Sept 8, Mercury will be just 5 degrees below and slightly left (or south) of the young Crescent MoonJupiter and Saturn appear in the SE at sunset and offer prime viewing all night long.  Both are in retrograde motion, meaning they appear to be moving backwards, or westerly, against the background field of stars.  This is an illusion caused by the Earth catching up to and passing these two Gas Giants as they orbit far beyond the orbit of Mars.  Jupiter starts the month at mag -2.6 while Saturn is dimmer at +0.3 and both will slowly diminish in brightness as the fall continues.  By contrast, the dimmest stars one can see with the unaided eye have a magnitude of +6.5.

     Neptune reaches opposition, or the point where it is on the opposite side of the Sun as seen from the Earth, on Sept 13.  Neptune will be traveling the night sky in the constellation of Aquarius, the Water Bearer and will pass just 1.6 arc-minutes S of the 6.3 mag star, Phi Aquarius.  An arc-minute is one sixtieth of a degree or the same angle one would see between the top and bottom of a 9 inch diameter soccer ball viewed from a distance of 8.25 football fields away.  If observed on this date with a six inch telescope at 150-power, Neptune would appear as a bluish disk next to the smaller-appearing orange-colored star.  Uranus, at mag 5.7, will be visible in the constellation of Aries, the Ram from late evening until morning.  This month’s viewing challenge will be to find the variable star Mira in Cetus, the Whale.  Mira reached its peak brightness in August and should still be visible with binoculars 20 degrees SSW of Uranus.  Mars is in conjunction and too close to the Sun to be seen this month.  The Red Planet will next appear in the morning sky in November.

     The big Solar event for the month takes place on September 22 at 3:21 p.m. when the vertical ray of the sun crosses the Equator.  As viewed from locations on the Equator, the Sun would appear straight overhead at noon and marks the beginning of fall in the northern hemisphere and spring in the southern hemisphere.  This event, known as the Autumnal Equinox, means we have reached the halfway point between the longest hours of daylight in June to the shortest period of daylight in December in the northern hemisphere.  The fall and spring (or Vernal) equinoxes are the only times of the year when there are 12 hours of both daylight and night across the globe.  Because the Earth is tilted 23.4 degree from the Earth – Sun plane, locations near the Equator always have 12 hours of daylight and darkness.  The Earth’s poles vary from 0 hours to 24 hours of daylight between June and December at the North Pole and vice versa for the South Pole.  This tilt is also responsible for the changes we see in the northern United States marked by the winter, spring, summer, and fall seasons we take for granted in the Great Lakes region.

     The New Moon (or Dark of the Moon if you prefer) will take place on Monday, September 6 with the First Quarter appearing on Sept 13, the Full (or Harvest) Moon taking place on Sept 20 (7:55 p.m. EDT), and the Last Quarter Sept 28.  On Friday, Sept 17, the Gibbous phase of the waxing Moon will be just below Jupiter (this is the shape of the Moon when it is between the Quarter and Full stage).  Also sometimes called the Barley Moon or the Full Corn Moon, the Harvest Moon is so named because it is found nearest the autumnal equinox during the time of the traditional fall harvest season. 

     Compiled by Ken Raisanen of WOAS-FM – information provided by Abrams Planetarium Sky Calendar, Michigan State University.  More information and subscription information can be found on the their website at or on Twitter at  Yearly subscriptions cost $12 and can be started anytime.


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