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
8%
16%
66%
8%
12
flagPakistan
27%
9%
63%
0%
11
flagAustria
16%
11%
61%
5%
18
flagCanada
12%
18%
56%
12%
16
flagAustralia
26%
8%
52%
13%
23
flagGermany
14%
20%
47%
15%
63
flagthe Philippines
30%
15%
46%
7%
13
flagthe United Kingdom
25%
22%
44%
7%
54
flagIndia
30%
19%
41%
8%
186
flagthe United States
30%
17%
41%
8%
79
flagPoland
19%
26%
37%
16%
3410
flagRepublic of Ireland
20%
40%
30%
10%
10