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
13%
13%
66%
6%
15
flagAustria
12%
15%
63%
6%
33
flagAustralia
19%
9%
58%
12%
31
flagthe United States
23%
14%
54%
7%
112
flagPakistan
33%
13%
53%
0%
15
flagthe Philippines
25%
20%
50%
5%
20
flagCanada
11%
27%
50%
11%
18
flagGermany
17%
16%
47%
16%
136
flagRepublic of Ireland
11%
27%
44%
16%
18
flagIndia
29%
18%
42%
8%
246
flagthe United Kingdom
26%
23%
40%
9%
76
flagPoland
19%
25%
39%
16%
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