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Roaming the Sky With a Square

by Geoff Chester, USNO Public Affairs | 28 November 2023

by Geoff Chester, USNO Public Affairs | 28 November 2023

Jupiter and four moons, imaged 2023 November 24, 01:12 UT
from Alexandria, Virginia with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a TeleVue 2X "Big Barlow" lens

A waning Moon brightens the late night and early morning skies this week.  Luna starts the week high above the familiar outline of Orion, the Hunter, then passes close to the bright star Pollux in Gemini on the evening of the 30th.  Last Quarter occurs on the 5th at 12:49 am Eastern Standard Time.  

This is the week when we enter the winter solstice “season”.  Beginning on December 2nd, we experience the earliest sunsets of the year here in Washington, DC.  Between the 2nd and the 12th sunset will occur at 4:46 pm EST.  That said, the latest sunrises won’t occur until early January of 2024.  In between we will have the winter solstice, which occurs on the 21st, giving us the shortest day of the year.

If we still kept time by sundials the solstice would also be the date of the latest sunrise and earliest sunset, but modern society demands more precise timekeeping that’s independent of the apparent position of the Sun.  

Earth’s orbit around the Sun is slightly elliptical, and we are approaching the time of year when we reach the closest point to Old Sol.  We are thus moving at our fastest orbital speed for the year.  However, Earth’s rotation speed is essentially constant, so we have to correct for the Sun’s apparent position in the sky to ensure that each day lasts exactly 86,400 precisely-defined seconds.  This correction factor, also known as the Equation of Time, is graphically depicted by a figure-8 diagram often placed over the Pacific Ocean on globe maps.  Known as the “analemma”, it tells you how fast or slow the Sun’s apparent (sundial) time of noon transit is compared to a “Mean Sun”.

The waning Moon rises progressively later during the week, allowing the brighter constellations of the autumn sky to shine before they are replaced by the bright stars of winter.  High in the south at 7:30 pm you should be able to see the “Great Square” asterism formed by the brightest stars of Pegasus, the Winged Horse.  You can use the square as a guidepost to other autumnal sights.  

Draw an imaginary line through the two western stars, then extend it to the south.  Here you will find Fomalhaut, the most isolated of the sky’s first-magnitude stars.  It is the brightest star in the obscure southern constellation Pisces Austrinus, the Southern Fish, but was once a part of Aquarius in the epic work of Ptolemy’s “Almagest”.

Follow a similar line through the eastern side of the square to point out the five stars that form the “W”-shaped figure of Cassiopeia.  A diagonal line from the lower right star Markab through the upper left star Alpheratz will lead you past the brighter “chain” of stars that mark the constellation of Andromeda to the star Mirfak, the brightest star in Perseus.  Take a look at Mirfak through a pair of binoculars; you will see the bright star set on the northern edge of a loose gaggle of blue-tinted stars.  This is another naked-eye star cluster, although it is often overlooked due to its more widely-known neighbors, the Pleiades and the Hyades.  It is about 560 light years away, and contains about 500 stars, most of which are very faint dwarf stars similar to the Sun.

Saturn is gradually slipping out of the evening sky.  You can still see the ringed planet in the southwestern sky as evening twilight fades, so you have a few hours to enjoy a view of him before he sets at around 11:00 pm.  Try to catch him in the telescope before 8:30 pm, though; after that the view becomes more perturbed by Earth’s atmosphere.

Jupiter shines brightly in the east and becomes visible shortly after sunset.  The giant planet is inn prime viewing position for most of the evening and early morning hours.  Old Jove is a great target for small- to medium-aperture telescopes, and a good four-inch glass will keep you entertained for hours.  The planet’s dark equatorial cloud belts mark the presence of high-speed jet streams in the jovian atmosphere, and the planet’s rapid rotation will let you see changes in their structure over the course of a few minutes.  On the evening of the 29th, watch the innermost Galilean moon Io drag its shadow across the cloud tops between 5:40 and 7:50 pm EST.

Venus continues to greet early risers with her brilliant glow in the pre-dawn sky.  If you have been watching the dazzling planet for the past few months you have probably noticed that she is moving a bit farther southward on each successive morning.  This week she drifts by the bright star Spica, one of the signature stars of the spring sky.  She will be visited by the waning crescent Moon next week.


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

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