Bortle Scale Description

One of the requirements for obtaining International Dark Sky Community certification for Westcliffe and Silver Cliff was the implementation of a Sky Quality Monitoring program. Dark Skies began this effort in February, 2014. A total of 30 readings are taken quarterly with a Unihedron SQM Meter at 10 varied locations within the cities' boundaries on nights with no clouds and no moon. Converted to the "Bortle Scale" used by astronomers, these reading averaged together reveal that the two towns usually rate a Class (3) "rural sky". The scale runs from Class (1) "an excellent dark-sky site" to Class (9) "an inner city sky". The most recent readings taken in June resulted in an average of Class (2) "typical truly dark site". These are exceptional readings, especially considering they are taken within city limits, and show that efforts by Dark Skies and local citizens and organizations to control wasteful lighting has really paid off. Some of the characteristics of Class (2) are: the zodiacal light is distinctly yellow ish and bright enough to cast shadows at dusk and dawn; the summer Milky Way is highly structured; the Andromeda Galaxy and many Messier objects and globular clusters are naked-eye objects.

Dark Skies members have begun taking readings in more remote areas of the Wet Mountain Valley. Many of these readings are easily reaching Class (1) which has these characteristics: the zodiacal light is visible and colorful; the gegenschein is visible, the zodiacal band is visible; the Scorpius and Sagittarius regions of the Milky Way cast obvious shadows; Venus and Jupiter affect dark adaptation; the Andromeda Galaxy is a direct vision naked-eye object; some of the fainter constellations are barely recognizable due to the large number of stars that are visible.

There are still many poor quality lights in the Valley that waste light and energy by shining up into the sky, onto neighboring properties where the light is unwelcome, and shine so bright that it makes it harder to see what is needs to be illuminated, an effect similar to cars coming at you on a dark road with their bright lights on. If you still have such a light Dark Skies stands ready to provide technical assistance in shielding or finding a better light fixture for the purpose, and will also provide financial assistance to correct poor lights. Please contact us.