How dark are your night skies?

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 Magnitude	Number of naked eye stars visible in the square	Seeing
6.5	35	Exceptional
6.25	21	Excellent
6	13	Superb
5.75	9	Very good
5.5	7	Good
5.25	5	Above Average
5	4	Average
4.75	3	Below Average
4.5	1	Poor
≤ 4.00	0	Very Poor

Great Square of Pegasus (credit:- freestarcharts)

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

Stars inside the Square of Pegasus

Index	Henry Draper Catalogue (HD)	Hipparcos Catalogue (HIP)	Bayer	Flamsteed	RA (J2000)	DEC (J2000)	Visual Mag.	Brighter Than
1	220657	115623	Upsilon Pegasi	68	23h 25m 23s	23d 24m 15s	4.42	mag 4.5	mag 4.75	mag 5.00	mag 5.25	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
2	220061	115250	Tau Pegasi	62	23h 20m 38s	23d 44m 25s	4.58	---	mag 4.75	mag 5.00	mag 5.25	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
3	224427	118131	Psi Pegasi	84	23h 57m 46s	25d 08m 29s	4.63	---	mag 4.75	mag 5.00	mag 5.25	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
4	218356	114155	---	56	23h 07m 07s	25d 28m 06s	4.76	---	---	mag 5.00	mag 5.25	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
5	223768	117718	Phi Pegasi	81	23h 52m 29s	19d 07m 13s	5.06	---	---	---	mag 5.25	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
6	221615	116264	---	71	23h 33m 28s	22d 29m 56s	5.33	---	---	---	---	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
7	222133	116611	---	75	23h 37m 57s	18d 24m 02s	5.49	---	---	---	---	mag 5.50	mag 5.75	mag 6.00	mag 6.25	mag 6.50
8	448	729	---	87	00h 09m 02s	18d 12m 43s	5.57	---	---	---	---	---	mag 5.75	mag 6.00	mag 6.25	mag 6.50
9	218792	114449	---	---	23h 10m 43s	17d 35m 40s	5.68	---	---	---	---	---	mag 5.75	mag 6.00	mag 6.25	mag 6.50
10	224930	171	---	85	00h 02m 10s	27d 04m 56s	5.80	---	---	---	---	---	---	mag 6.00	mag 6.25	mag 6.50
11	223461	117500	---	79	23h 49m 39s	28d 50m 33s	5.95	---	---	---	---	---	---	mag 6.00	mag 6.25	mag 6.50
12	218396	114189	---	---	23h 07m 29s	21d 08m 03s	5.97	---	---	---	---	---	---	mag 6.00	mag 6.25	mag 6.50
13	220933	115806	---	69	23h 27m 40s	25d 10m 02s	5.99	---	---	---	---	---	---	mag 6.00	mag 6.25	mag 6.50
14	166	544	---	---	00h 06m 37s	29d 01m 17s	6.07	---	---	---	---	---	---	---	mag 6.25	mag 6.50
15	221662	116307	---	---	23h 33m 55s	20d 50m 27s	6.10	---	---	---	---	---	---	---	mag 6.25	mag 6.50
16	223755	117710	---	---	23h 52m 23s	21d 40m 16s	6.13	---	---	---	---	---	---	---	mag 6.25	mag 6.50
17	218235	114081	---	---	23h 06m 18s	18d 31m 04s	6.16	---	---	---	---	---	---	---	mag 6.25	mag 6.50
18	224303	118048	---	---	23h 56m 42s	22d 38m 53s	6.18	---	---	---	---	---	---	---	mag 6.25	mag 6.50
19	218935	114526	---	60	23h 11m 49s	26d 50m 50s	6.19	---	---	---	---	---	---	---	mag 6.25	mag 6.50
20	417	716	---	---	00h 08m 52s	25d 27m 47s	6.24	---	---	---	---	---	---	---	mag 6.25	mag 6.50
21	225276	399	---	---	00h 04m 56s	26d 38m 56s	6.25	---	---	---	---	---	---	---	mag 6.25	mag 6.50
22	222098	116592	---	74	23h 37m 40s	16d 49m 32s	6.26	---	---	---	---	---	---	---	---	mag 6.50
23	220318	115407	---	65	23h 22m 41s	20d 49m 43s	6.28	---	---	---	---	---	---	---	---	mag 6.50
24	222683	116972	---	---	23h 42m 44s	16d 20m 09s	6.30	---	---	---	---	---	---	---	---	mag 6.50
25	219310	114742	---	---	23h 14m 36s	24d 06m 10s	6.36	---	---	---	---	---	---	---	---	mag 6.50
26	220288	115389	---	---	23h 22m 29s	25d 55m 07s	6.38	---	---	---	---	---	---	---	---	mag 6.50
27	221394	116119	---	---	23h 31m 43s	28d 24m 13s	6.41	---	---	---	---	---	---	---	---	mag 6.50
28	221493	116187	---	---	23h 32m 29s	23d 50m 37s	6.41	---	---	---	---	---	---	---	---	mag 6.50
29	224758	34	---	---	00h 00m 24s	26d 55m 05s	6.43	---	---	---	---	---	---	---	---	mag 6.50
30	218101	113994	---	---	23h 05m 06s	16d 33m 46s	6.43	---	---	---	---	---	---	---	---	mag 6.50
31	218261	114096	---	---	23h 06m 32s	19d 54m 39s	6.44	---	---	---	---	---	---	---	---	mag 6.50
32	221905	116465	---	---	23h 35m 56s	24d 33m 40s	6.44	---	---	---	---	---	---	---	---	mag 6.50
33	221113	115915	---	---	23h 29m 06s	23d 02m 53s	6.45	---	---	---	---	---	---	---	---	mag 6.50
34	225292	410	---	---	00h 05m 01s	27d 40m 29s	6.47	---	---	---	---	---	---	---	---	mag 6.50
35	219196	114686	---	---	23h 13m 59s	19d 38m 02s	6.47	---	---	---	---	---	---	---	---	mag 6.50

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