03 July 2017

A New Tool for Describing Sky Darkness

Sky darkness is the subject of a lot of research worldwide. Not only are astronomers interested in measuring the darkness (or brightness) of the night time sky, but governments and institutions are too, especially as it effects humans, animals and plants. Surprising results are being published constantly, including an important one for astronomers last year.

A peer reviewed article* (Atlas) was published in 2016 that included a world map of artificial light. The researchers measured sky brightness for most of the world, although they skipped the very high latitudes in both hemispheres. Measurements were made from satellite- and earth-based equipment. The light values reported in the study were reported in micro- and milli-candelas per square meter. You can read about the study at http://cires.colorado.edu/Artificial-light ; the paper is published at http://advances.sciencemag.org/content/2/6/e1600377 

Candelas are a measure of luminance, not the usual unit of measure used by astronomers. We are more familiar with magnitudes per square arc minute (MPSAS) and naked-eye limiting magnitude (NELM). Converting values among these units helps us understand the relationship between the familiar units (MPSAS or NELM) and the Atlas unit (mcd/m2). Further, the color codes used in the Atlas provide a quick, consistent way of assessing sky darkness.

Introducing Zone Calculator

SQM Reader Pro 3 includes Atlas zone color along with SQM reading values in MPSAS and NELM (and Deep-Sky Planner 7 will too, soon). While developing the feature, a testing tool was developed which is now released to the public. The software is called Zone Calculator and it is available for free at http://knightware.biz/sqm/zonecalc.htm

Hopefully this little tool will help astronomers describe the darkness of their sky in a simple, more uniform way.