He is best known for creating the Bortle scale to quantify the darkness of the night sky.
From 1977 until 1994 he authored the monthly '"Comet Digest" in Sky and Telescope magazine.
From 1970 until 2000 he edited the monthly AAVSO circular for the American Association of Variable Star Observers.
[3] The scale ranges from 1 (extremely dark rural area or national park, usually at high elevation, low humidity, and low wind) to 9 (urban inner city).
Today that scale is widely used throughout the world by the amateur astronomy community for ascertaining and relating their sky conditions to colleagues.