Astronomers dream of nights where thousands of stars dazzle in pitch-black skies with the band of the Milky Way flowing like a great river from one side to another. Alas for most of us this is often just a dream. We live in towns and cities polluted by artificial lighting from streets, building and factories that significantly diminish our view of the night sky.

However, it's not all that bleak as things do seem to be improving. Recently more efficient street lights have been installed in many areas, significantly reducing the amount of light beamed up to the heavens rather than down on the ground. Of course there are always out of town regions, especially forests and mountainous areas, away from the bulk of civilisation where the beauty of the night sky still rules. But have you ever wondered, how dark are your night skies? One way to check is by looking towards the constellation of Pegasus.

Pegasus is the 7th largest constellation in the sky covering an impressive 1,121 square degrees. For Northern Hemisphere observers, it's easily recognisable during the autumn months. The prominent asterism known as "Great Square of Pegasus" dominates the constellation. It's made up of reasonably bright stars between 2nd and 3rd magnitude:

Scheat (β Peg) - mag. 2.44
Markab (α Peg) - mag. 2.49
Algenib (γ Peg) - mag. 2.83
Alpheratz (α And) - mag. 2.07

Strangely not all stars are located in Pegasus. The northeastern star and brightest of the four, Alpheratz, lies in neighbouring Andromeda. For a long time, Alpheratz was considered both a part of Pegasus and Andromeda. Eminent 16th century celestial cartographer Johann Bayer catalogued it as α Andromedae and δ Pegasi in his classic star atlas, Uranometria. Finally, when the official constellation boundaries were drawn up by the International Astronomical Union (IAU) in 1922, Alpheratz was assigned to Andromeda.

Inside the Square

The square of Pegasus is large and the four corner stars easily identifiable. Despite this, there are few easily visible stars inside the square, but many at the fringe of naked eye visibility. Therefore counting the total number of stars inside, is an excellent guide to how dark your skies are.

Seeing conditions

Limiting MagnitudeNumber of naked eye stars visible in the squareSeeing
6.535Exceptional
6.2521Excellent
613Superb
5.759Very good
5.57Good
5.255Above Average
54Average
4.753Below Average
4.51Poor
≤ 4.000Very Poor

Great Square of Pegasus (credit:- freestarcharts)

Great Square of Pegasus - pdf format (credit:- freestarcharts)

Stars inside the Square of Pegasus

IndexHenry Draper Catalogue (HD)Hipparcos Catalogue (HIP)BayerFlamsteedRA (J2000)DEC (J2000)Visual Mag. Brighter Than
1220657115623Upsilon Pegasi6823h 25m 23s23d 24m 15s4.42mag 4.5mag 4.75mag 5.00mag 5.25mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
2220061115250Tau Pegasi6223h 20m 38s23d 44m 25s4.58---mag 4.75mag 5.00mag 5.25mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
3224427118131Psi Pegasi8423h 57m 46s25d 08m 29s4.63---mag 4.75mag 5.00mag 5.25mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
4218356114155---5623h 07m 07s25d 28m 06s4.76------mag 5.00mag 5.25mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
5223768117718Phi Pegasi8123h 52m 29s19d 07m 13s5.06---------mag 5.25mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
6221615116264---7123h 33m 28s22d 29m 56s5.33------------mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
7222133116611---7523h 37m 57s18d 24m 02s5.49------------mag 5.50mag 5.75mag 6.00mag 6.25mag 6.50
8448729---8700h 09m 02s18d 12m 43s5.57---------------mag 5.75mag 6.00mag 6.25mag 6.50
9218792114449------23h 10m 43s17d 35m 40s5.68---------------mag 5.75mag 6.00mag 6.25mag 6.50
10224930171---8500h 02m 10s27d 04m 56s5.80------------------mag 6.00mag 6.25mag 6.50
11223461117500---7923h 49m 39s28d 50m 33s5.95------------------mag 6.00mag 6.25mag 6.50
12218396114189------23h 07m 29s21d 08m 03s5.97------------------mag 6.00mag 6.25mag 6.50
13220933115806---6923h 27m 40s25d 10m 02s5.99------------------mag 6.00mag 6.25mag 6.50
14166544------00h 06m 37s29d 01m 17s6.07---------------------mag 6.25mag 6.50
15221662116307------23h 33m 55s20d 50m 27s6.10---------------------mag 6.25mag 6.50
16223755117710------23h 52m 23s21d 40m 16s6.13---------------------mag 6.25mag 6.50
17218235114081------23h 06m 18s18d 31m 04s6.16---------------------mag 6.25mag 6.50
18224303118048------23h 56m 42s22d 38m 53s6.18---------------------mag 6.25mag 6.50
19218935114526---6023h 11m 49s26d 50m 50s6.19---------------------mag 6.25mag 6.50
20417716------00h 08m 52s25d 27m 47s6.24---------------------mag 6.25mag 6.50
21225276399------00h 04m 56s26d 38m 56s6.25---------------------mag 6.25mag 6.50
22222098116592---7423h 37m 40s16d 49m 32s6.26------------------------mag 6.50
23220318115407---6523h 22m 41s20d 49m 43s6.28------------------------mag 6.50
24222683116972------23h 42m 44s16d 20m 09s6.30------------------------mag 6.50
25219310114742------23h 14m 36s24d 06m 10s6.36------------------------mag 6.50
26220288115389------23h 22m 29s25d 55m 07s6.38------------------------mag 6.50
27221394116119------23h 31m 43s28d 24m 13s6.41------------------------mag 6.50
28221493116187------23h 32m 29s23d 50m 37s6.41------------------------mag 6.50
2922475834------00h 00m 24s26d 55m 05s6.43------------------------mag 6.50
30218101113994------23h 05m 06s16d 33m 46s6.43------------------------mag 6.50
31218261114096------23h 06m 32s19d 54m 39s6.44------------------------mag 6.50
32221905116465------23h 35m 56s24d 33m 40s6.44------------------------mag 6.50
33221113115915------23h 29m 06s23d 02m 53s6.45------------------------mag 6.50
34225292410------00h 05m 01s27d 40m 29s6.47------------------------mag 6.50
35219196114686------23h 13m 59s19d 38m 02s6.47------------------------mag 6.50

Sky Highlights - September 2017

Opposition
Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
Evening
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
Midnight
South:- Neptune
East:- Uranus (mag. +5.7)
Morning
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
Evening
West:- Jupiter
Northwest:- Saturn
East:- Neptune
Midnight
West:- Saturn
North:- Neptune
Northeast:- Uranus
Morning
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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