August 24, 2023

AstroCal – September 2023


     Start your observations for September in the eastern sky in the hours before sunrise.  Venus and Jupiter will be easy to find shining at -4.8 and -2.5, respectively.  They will be the brightest objects in the sky not counting the Full Moon and the Sun.  Venus will be due east in the lower part of Gemini – the Twins while Jupiter will reside to the right of the Pleiades star cluster located in the shoulder of Taurus – the Bull.  During the course of the month, the crescent of Venus as seen from Earth will increase from a slim 13 percent to 36 percent by the end of September which will increase its brightness even more.  

     Mercury reaches inferior conjunction (where it will be directly between Earth and the Sun) on September 6.  When it emerges about a week later, it will be near the horizon just below and to the left of Venus.  Your observing challenge for the month is to find the earliest day you can spot the Winged Messenger when it makes its appearance in the morning sky.  Mercury will be easier to spot as it reaches its greatest elongation from the Sun (about 18 degrees on September 22).  It will appear 51 percent illuminated with its greatest brightness reaching -0.3.

     Saturn will be the easiest evening planet to find in the SE sky two hours after sunset.  Located between Aquarius, the Water Bearer and Capricornus, the Sea Goat, Saturn will continue in retrograde motion until November 2023.  During retrograde motion, the planet will appear to be moving east to west among the background stars.  When Saturn reached opposition in late August, it was 73 light-minutes from us (or 8.8 astronomical units) with a magnitude of 0.4.  In September, the Ringed Planet will appear as a bright oval-shaped disk when viewed with binoculars but one will need a small backyard telescope to see the rings which will be tilted at 10 degrees relative to the Earth.  One can see why first attempts to view the planet with early telescopes (which would have been fuzzier images compared to modern telescopes) led some to say the planet looked to have ‘ears’.  Mars will be difficult to see low in the western sky as it nears conjunction with the Sun.  It will be lost in the Sun’s glare in November and re-emerge in the morning sky early in 2024.

     The August Blue Moon (the second Full Moon in a calendar month) took place on the 30th.  This means the Lunar phases in the early part of September will be a waning cycle ending with an old Crescent Moon that will pass Venus and Mercury in the morning sky Sept 11-13.

The New Moon takes place on Sept 14, so you can look for the Young Crescent Moon low in the western sky an hour after sunset a couple of days later.  The First Quarter (9-22), and Full Moon (9-29) will follow.  The center of the Milky Way galaxy can be located on 9-22 as the First Quarter Moon crosses the Tea Pot Asterism in Sagittarius, the Archer.  There is too much distance and interstellar material between us and our galactic center to actually see it, but if you want to travel there in a flight of imagination, that is the direction you will need to go. 

    Sept 23 will also mark the Autumnal Equinox when the Sun’s vertical ray crosses the Equator at 2:50 a.m. EDT.  The Equinox marks the official start of Fall in the Northern Hemisphere.  All locations on the planet will experience 12 hours of daylight and 12 hours of darkness at this time.  This places us three months past the longest daylight period and three months closer to the shortest daylight (and thus, the longest night) of the year.   

      This month’s historical item honors lawyer and amateur astronomer Johann Bayer (1572-1625) who lived in Augsburg, Germany.  In September 1603, he first published Uranometria, a book featuring 51 copper-plate engravings covering both the northern and southern celestial hemispheres.  These star maps were drawn using a trapezoidal projection where the longitude and latitude are projected as straight lines, with lines of constant declination remaining parallel, but lines of constant right ascension converging at the poles.  On the back of each map was a corresponding catalog of stars.  This work became the standard reference for astronomers throughout the 17th and 18th century. (Historical information provided by Jason Ybarra for the American Astronomical Society Historical Astronomy Division).  

     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 their website at or on Twitter at  Yearly subscriptions cost $12 and can be started anytime.


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