Reply To: Message from NBC news – are our skies becoming more ‘starry?’

[Howard Lawrence]

I suggest that this version of VIIRS mapping is correctly correlated and validated. https://pmisson.users.earthengine.app/view/trends
It is slightly more difficult to use than the one published by https://www.lightpollutionmap.info/ but is better once the transparency of the layers are adjusted. A useful exercise is to click randomly over the London area and note the great variability in year by year trends depending upon location.
It is very difficult to draw overall conclusions about trends. The consensus within CfDS is that levels of sky brightness are either unchanged or continue to rise in suburban/urban areas, while some rural locations, in reserves for example, are getting darker, which is good news.
There are fundamental flaws with the satellite method of assessment and continuous ground based monitoring using qualified photometry is probably the most believable way of assessing trends. Are any London based observatories monitoring zenith SQM levels on a continuous basis (with offset to miss the Milky Way)? Of the two rural observatories I am familiar with, one is showing no change at zenith but great increase at lower elevations over a long period, while the other is recently starting to record an increase in sky darkness at zenith (good news but more analysis needed). My own SQM observations from a suburban area are just spot checks and so invalid for true trend analysis.