Other Cities Spectral Analysis of Sky-glow The New Sky-Glow Spectra The Sky-Glow Story
Given our large and modern urban populations, outdoor lighting is necessary. However, excessive illumination than simple lighting is frivolous, wasteful and sometimes dangerous. There is also exploitation involved by lighting contractors & installers, and cities cannot ignore the significant consumption costs by having polluting, dusk-to-dawn lighting.
Despite the many advances in the field of excellent, non-polluting lighting products and practices, the brightening of the night sky continues to increase over many cities. And now the color of the night-sky is shifting for the second time in 25 years. The kaleidoscopic spectrum at the top was easily obtained from an overcast sky over Montreal, Quebec.
The January 1996 image was taken on a misty night while the 2017 image was shot on a much drier evening. Can you spot any changes over the 21 years? The big changes are at street level.
We know what sky-glow looks like over our gleaming cities, but what does the spectrum of sky-glow look like?
Most amateur astronomers are well-versed on the basic causes of urban sky-glow - at least this is what is believed. Frequently, limited or very dated information has been available. It's likely that countless of amateurs may very well have an incomplete knowledge of light-pollution compelling the need for an analysis such as this. That analysis is finally here - this is Your Light-Pollution.
When anyone asserts that a certain type of contamination is caused, that claim is often backed up by a chemical analysis which may also expose the cause. For any discussion of a "pollution of light", a spectral analysis is obligated. As in any field of spectroscopy, a spectral analysis of sky-glow will focus on a fundamental question: What is it made from? Certainly, an analysis of sky-glow may have been done for professional observatories typically isolated from huge amounts of urban sky-glow. Having little knowledge of urban spectra, city stargazers are forced to cope blindly.
The difference between my clear night sky-glow spectrum and my overcast spectrum over my back yard in Ville-Saint-Laurent, Quebec, Canada:
Shot in 2008, The spectra above were obtained less than 24 hours apart; top part on a clear night, bottom on the following overcast evening.
Note the missing blue band compared to the logo pic at top. You can also calculate that the bottom overcast exposure was 10x shorter than the top exposure. Clear-night spectra are difficult to obtain, but not impossible.
More Up-Light City Spectra.