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Cosmic Challenge: The Great Square

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Cosmic Challenge: The Great Square


October 2023

Phil Harrington

This month's suggested aperture range:

Naked Eye









Stars in the Great Square

23h 38m





Pegasus is one of the best-known autumn constellations. Depicting the winged horse that Perseus used to rescue Princess Andromeda from the clutches of Cetus the Sea Monster, Pegasus flies high in our southern sky during October and November evenings.


Pegasus is usually drawn upside down in our sky, with four stars Alpheratz (Alpha [α] Andromedae), Scheat (Beta [β] Pegasi), Markab (Alpha [α] Pegasi), and Algenib (Gamma [γ] Pegasi) -- marking the corners of a great square that frames the horse's torso. (Although always shown as marking the northeast corner of the Great Square of Pegasus, Alpheratz is technically assigned to the neighboring constellation Andromeda. In fact, Alpheratz is Alpha (α) Andromedae.)


Above: Autumn star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington




Above: The Great Square rises over trees from the author's Long Island backyard.

Above: Chart for this month's Cosmic Challenge.



A line of three stars Homam (Zeta [ζ] Pegasi) Baham (Theta [θ] Pegasi), and Enif (Epsilon [ε] Pegasi) -- hooking to the southwest of Markab is usually shown as the horse's neck and head, with Enif marking its nose.


Unfortunately, this means that poor Pegasus is flying upside down from a Northern Hemisphere perspective. To make matters worse, we only see the front half of the horse; its hindquarters are nowhere to be found (there is a political joke in here somewhere, but I'll leave that to you).


Rather than strain to see a horse flying across the sky, planetarium lecturers often tell their audiences that the Great Square marks a baseball diamond, a perfect allusion for this months World Series. In our cosmic baseball diamond, Scheat and Alpheratz, both 2nd magnitude, mark home plate and first base, respectively. The remaining two stars, Algenib and Markab, both 3rd-magnitude stars, are second and third base, respectively.





Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window




Let's fill in the rest of the team players out in the field. Pitching today's game is number 71; 5.3-magnitude 71 Pegasi, that is. It looks like he's about to be joined on the mound by 4.4-magnitude catcher Upsilon (υ) Pegasi and team manager Tau (υ) Pegasi, at magnitude 4.6. It would also appear that the home plate ump, 4.8-magnitude 56 Pegasi, is heading out to see what's going on. With a time-out called, the batter, 3.5-magnitude Mu (μ) Pegasi has headed back to the on-deck circle to talk to Lambda (λ) Pegasi, at magnitude 4.0, who is next at bat.


Meanwhile, at first base, we have Psi (ψ) Pegasi, a 4.7-magnitude sun that appears to be playing in as if looking for the batter to bunt. Second baseman Chi (χ) Pegasi, at magnitude 4.8, is playing in the hole between first and second. Shortstop Phi (φ) Pegasi, at magnitude 5.1, and third baseman 70 Pegasi (magnitude 4.6) round out the infield, while HD 216489 is the 5.6-magnitude third base coach.


Depending on the darkness and transparency of your observing site, you may be able to see the full complement of players on our cosmic ball field, or possibly just the four bases. From dark, rural sites, however, the infield looks to be overrun by fans racing in from the stands, as if this must have been the final game of the World Series and the game has just ended.

Stars within the Great Square of Pegasus

Naked-eye Limiting Magnitude

Number of stars

Chart panel




















How many stars can you count inside the Great Square? The table above lists the number of stars visible in half-step increments beginning at magnitude 5.0, while the corresponding panels in the chart above plot the locations of those stars.


Wait for the Great Square to reach culmination, its greatest altitude above the horizon, before attempting a star count.  That occurs at about 10 PM local standard time (11 PM local daylight saving time) on October 1 and 8 PM local standard time (9 PM local daylight saving time) on November 1.


Post your result in this columns discussion forum. Or contact me directly through my web site.

Until next month, remember that half of the fun is the thrill of the chase. Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2023 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


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