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%
14%
71%
7%
14
flagAustria
18%
9%
63%
4%
22
flagPakistan
27%
9%
63%
0%
11
flagAustralia
22%
7%
59%
11%
27
flagCanada
12%
18%
56%
12%
16
flagthe Philippines
22%
22%
50%
5%
18
flagthe United States
26%
15%
49%
7%
95
flagGermany
20%
16%
48%
12%
85
flagIndia
29%
18%
41%
8%
217
flagthe United Kingdom
26%
25%
40%
7%
67
flagPoland
19%
25%
37%
16%
3627
flagRepublic of Ireland
18%
36%
36%
9%
11