Long Baseline Sky Darkness Measurements: A Statistical Approach

Long Baseline Sky Darkness Measurements: A Statistical Approach

Kenneth J. Grant, Andrew D. Cool and David Bennet

River Murray International Dark Sky Reserve (RMIDSR)

There is growing interest in the preservation of dark skies. This stems not only on from concern about the effect of light pollution on optical astronomy, but also on human and wildlife health and well-being. Astrotourism is a growth market, with over 250 designated International Dark Sky Places (IDSPs) currently established across more than 22 countries on six continents. Accreditation requires measurement of the sky darkness on an annual basis to ensure that it maintains its darkness to a suitable level. However, the methodology behind this is only loosely defined, allowing for a wide interpretation of the readings.

We present on a long baseline measurement of the sky darkness at the River Murray International Dark Sky Reserve (RMIDSR), the first such Reserve to be accredited in Australia. Readings were taken every minute and filtered for environmental factors such as the presence of the Moon or clouds. Values in excess of 21.9 mag/square arc second were recorded, with 22.0 usually taken to be the value for a ‘perfectly’ dark sky. Rather than using a simplistic averaging of values, which is meaningless for logarithmic values such as these, we developed a statistical method to estimate sky darkness to a pre-defined percentile. We believe that this has the potential to be the basis for dark sky estimations internationally.