Bortle Dark-Sky Scale
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The Bortle Dark-Sky Scale is a light pollution scale created for the February 2001 edition Sky & Telescope magazine by John Bortle. It is a description of how dark the sky is in relation the observation of astronomical objects and the interference of caused by light pollution.
John Bortle created the scale to help astronomical observers judge the true darkness of the site. He based the scale on 50 years of observations.
- Class 1: Excellent dark-sky site
- Class 2: Typical truly dark site
- Class 4: Rural / suburban transition
- Class 5: Suburban sky
- Class 6: Bright suburban sky
- Class 7: Suburban / urban transition
- Class 8: City sky
- Class 9: Inner-city sky
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