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- The Bortle Scale is a nine-level numeric scale that rates the night sky’s brightness, or how well celestial objects can be seen, on a scale of 1 to 9.
- The lower the number, the darker the sky and the better the conditions are for stargazing. A “1” on the Bortle Scale is an excellent, dark sky; a “9” is an urban sky so bright that the Milky Way is barely visible.