Evaluating the summer night sky brightness at a research field site on Lake Stechlin in northeastern Germany

TL;DR

This study measures the natural and artificial brightness of the night sky at Lake Stechlin, Germany, demonstrating minimal light pollution and establishing it as an ideal site for ecological light pollution research.

Contribution

It provides detailed luminance measurements at a rural site, showing very low artificial skyglow, and combines photometric and imaging methods to assess light pollution sources.

Findings

Sky brightness on clear nights is close to natural levels.

Cloud cover significantly darkens the sky, indicating minimal artificial influence.

Distant towns are visible but contribute little to skyglow.

Abstract

We report on luminance measurements of the summer night sky at a field site on a freshwater lake in northeastern Germany (Lake Stechlin) to evaluate the amount of artificial skyglow from nearby and distant towns in the context of a planned study on light pollution. The site is located about 70 km north of Berlin in a rural area possibly belonging to one of the darkest regions in Germany. Continuous monitoring of the zenith sky luminance between June and September 2015 was conducted utilizing a Sky Quality Meter. With this device, typical values for clear nights in the range of 21.5-21.7 mag$_{SQM}/$arcsec$^2$ were measured, which is on the order of the natural sky brightness during starry nights. On overcast nights, values down to 22.84 mag$_{SQM}/$arcsec$^2$ were obtained, which is about one third as bright as on clear nights. The luminance measured on clear nights as well as the darkening with the presence of clouds indicate that there is very little influence of artificial skyglow on the zenith sky brightness at this location. Furthermore, fish-eye lens sky imaging luminance photometry was performed with a digital single-lens reflex camera on a clear night in the absence of moonlight. The photographs unravel several distant towns as possible sources of light pollution on the horizon. However, the low level of artificial skyglow makes the field site at Lake Stechlin an excellent location to study the effects of skyglow on a lake ecosystem in a controlled fashion.