2020 events

Sun 14:30-15:00
Sunset Room

Measuring sky brightness in the presence of the Moon

James Irish

Citizen astronomer, ASV

Astronomers, unless they are lunar aficionados, don't like moonlight or clouds. But, just as the Sun illuminates the daytime, excites solar observers and enables sun photometry, the Moon can also be used as the luminance source for measuring sky brightness. SQM measurements under moonlight reveal how much trees or other obstructions affect data in the absence of the Moon. Example plots will be presented. Data from multiple sites has been used to devise a simple model of the relationship between the zenith distance of the Moon and the SQM measurement. Departures from that model indicate changes in extinction coefficient for scattered light or obstructions such as trees. I'll describe more sophisticated models from the literature on the brightness over a narrow field of view as a function of the arc distance to the Moon and the phase of the Moon; these enable visual and imaging astronomers to plan observations close to the Moon's trajectory. Finally, I'll note that the standard sun photometer (CIMEL318) is now available in a version which can be used with the Moon as the luminance source, enabling measurement of the aerosol optical depth at multiple wavelengths for about two weeks each lunar cycle.