What does the Bortle scale refer to?

spectrum of the stars
types of nebulae
brightness of the night sky
size of the planets
The Bortle scale is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. It was created by John E. Bortle in 2001.
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Answers by country

spectrum of the stars
types of nebulae
brightness of the night sky
size of the planets
answers
flagFrance
7%
15%
69%
7%
13
flagAustria
15%
10%
65%
5%
20
flagPakistan
27%
9%
63%
0%
11
flagAustralia
23%
7%
57%
11%
26
flagCanada
12%
18%
56%
12%
16
flagthe United States
26%
16%
47%
7%
89
flagGermany
19%
18%
46%
14%
77
flagthe Philippines
30%
15%
46%
7%
13
flagIndia
29%
18%
42%
8%
203
flagthe United Kingdom
27%
22%
41%
8%
62
flagPoland
19%
25%
37%
16%
3560
flagRepublic of Ireland
20%
40%
30%
10%
10