One sure sign that summer is almost over and longer autumnal nights are taking over from more balmy late-summer evenings is the grand appearance in September’s late-evening southern sky of the Great Square of Pegasus, the winged-horse’s familiar asterism (star pattern). The Square is well up in the eastern sky as astronomically dark skies fall at mid-month and its centre culminates at around 1am BST (midnight UT) at a very favourable altitude of 60 degrees.
The four stars marking the corners of the Square all shine at second- or third-magnitude, so it’s easy to see it even from light-polluted locations. Beginners often remark that the Square appears larger than expected; it spans roughly 15 degrees along each side and is about 20 degrees across diagonally.
Only three of the stars actually belong to Pegasus. The top-left (north-eastern) star of the Square is alpha (α) Andromedae (Alpheratz), the brightest star in the constellation Andromeda. At one time it had two designations, alpha Andromedae and delta (δ) Pegasi, but when the constellation boundaries were defined by the International Astronomical Union (IAU) in 1922, Alpheratz was assigned to Andromeda and the name delta Pegasi fell into disuse (at least by professional astronomers). The three other stars of the Square are among the brightest in the constellation: magnitude +2.45 Markab (alpha [α] Pegasi) lies at the bottom right (south-western) corner), magnitude +2.4 Scheat (beta [β] Pegasi) sits at top right (north-west), and magnitude +2.8 Algenib (gamma [γ] Pegasi) marks the bottom left (south-eastern) corner.
Lying within the Square, about 2.5 degrees north-west of Algenib, is NGC 7814 (Caldwell 43), a very nice edge-on galaxy shining at magnitude +10.5. It is sometimes called the ‘Little Sombrero’, as it is reminiscent of the magnificent Sombrero Galaxy (Messier 104) in Virgo, but NGC 7814 is much smaller, spanning only 6.3 × 2.6 arcminutes. NGC 7814 is an easy capture for a 150mm (six-inch) telescope.
NGC 7741 is another galaxy located within the confines of the Square, found 6 degrees south-west of Alpheratz. It’s an attractive imaging target, but is a real challenge for a 250mm (10-inch) telescope, glowing at magnitude +11.4, a magnitude or so fainter than NGC 7814, across its 4 x 2.8 arcminute form.
One final target of note within the Square is the galaxy grouping of NGC 7769, 7770 and 7769. The former and latter are nice eleventh-magnitude spirals that can be observed through apertures in the 250-300mm (10- to 12-inch) range, but the real appeal is the group’s attractiveness in deep images.
A good measure of your sky quality is to count the number of stars that you can see within the Great Square of Pegasus – not including the corner stars of the Square itself. You can work out your limiting magnitude according to the number of stars seen using the following table.
| Visible naked-eye stars within square | Limiting magnitude | Sky conditions |
| 1 | 4.5 | Poor |
| 4 | 5 | Average |
| 7 | 5.5 | Good |
| 13 | 6 | Very good |
| 35 | 6.5 | Excellent |
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