In a recent stargazing trip with a wonderful set of fellow stargazers and Milky Way chasers, I understood the importance of Bortle’s number and how it was to be utilized. A remote location away from city lights may have good skies, but there are other factors that need to be considered too. In an attempt to understand Bortle and his invention, I wrote this piece.
John E. Bortle is an amateur astronomer who is the genius behind designing Bortle’s dark sky scale in the year 2001. The scale is a representation of the darkness of the night sky and the visibility of celestial objects from a particular location, based on a set of criteria such as sky brightness, light pollution, and the number of visible stars. It ranges from Class 1 (highest visibility) to Class 9 (lowest visibility).
John Bortle developed the Bortle Dark-Sky Scale in response to the growing problem of light pollution, which was making it increasingly difficult to observe celestial objects from many locations around the world. He identified nine different levels of darkness based on his own observations and experiences. He then assigned specific criteria to each level, such as the magnitude limiting to the naked eye, the visibility of the Milky Way, and the degree of light pollution. Bortle’s aim was to create a scale that was simple and easy to understand but also accurate and reliable.
The Bortle Dark-Sky Scale is primarily based on visual observations of the night sky rather than on precise measurements. However, there are some quantitative criteria associated with each level of the scale.
- Class 1: The sky is so dark that the Milky Way casts a faint shadow on the ground. The naked eye limiting magnitude is at least 7.6. There is virtually no light pollution.
- Class 2: The sky is very dark, with the Milky Way visible in all its splendor. The naked eye limiting magnitude is around 7.1 to 7.5. There is minimal light pollution, mostly from sources below the horizon.
- Class 3: The sky is dark, but the Milky Way is not as prominent as in Class 2. The naked eye limiting magnitude is around 6.6 to 7.0. There is moderate light pollution from nearby towns and cities.
- Class 4: The sky is moderately dark, with the Milky Way only faintly visible. The naked eye limiting magnitude is around 6.1 to 6.5. There is significant light pollution from nearby urban areas.
- Class 5: The sky is moderately bright, with only a few of the brightest stars visible. The naked eye limiting magnitude is around 5.6 to 6.0. There is severe light pollution from nearby cities and suburbs.
- Class 6: The sky is bright, with only the brightest stars and planets visible. The naked eye limiting magnitude is around 5.1 to 5.5. There is very severe light pollution from cities and suburbs.
- Class 7: The sky is very bright, with few stars visible. The naked eye limiting magnitude is around 4.6 to 5.0. There is extremely severe light pollution from cities and suburbs.
- Class 8: The sky is extremely bright, with no stars visible. The naked eye limiting magnitude is around 4.1 to 4.5. There is no true dark sky.
- Class 9: The sky is completely washed out by light pollution, with no stars visible. The naked eye limiting magnitude is less than 4.0.
The criteria for each level of the scale are based on the "naked eye limiting magnitude," which is a measure of how faint an object can be seen with the unaided eye under a given set of observing conditions. It can also be affected by several other factors, including sky brightness, atmospheric conditions, and observer sensitivity.
There are other factors that play an important role in celestial visibility, and thanks to Bortle, we are now able to measure it quantitatively. It gives amateur astronomers like us a chance to soak up the beauty of the starry sky.
