Quote:
Originally Posted by Merlin66
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Hi Ken
Your source, cleardarksky reports St Leonards (based on 2016 data -see top of page on your link) as what appears as Bortle 4 based on the blue/violet? colour code. I just had a look on
LightPollutionMap https://www.lightpollutionmap.info/#...FFFFFFFFFFFFFF ,a source that I usually use for such info and found sort of similar results (see excerpt image below) for St Leonards. It also shows based on 2015 data this time, St Leonards as a Bortle Class 4 area, but for now take these results with a grain of salt we will correct them somewhat...
First an aside.....
I like the Light Pollution Map site in preference as it reports the SQM reading, Brightness and Bortle Number of the
exact location IF YOU SELECT the World Atlas 2015 Dataset at the top right hand corner. If you use the dropbox and select any of the VIIRS datasets you will get, to my mind, the less useful/less intuitive Sky radiance reported and a colour code. Of course my preference to use the 2015 World Atlas data means that it is 6 years out of date, but there is a work around to that which I use. I take the
VIIRS 2020 dataset sky radiance data & the
VIIRS 2015 dataset sky radiance data to help correct the
World Atlas 2015 Brightness data to give me effectively
World Atlas 2020 Brightness data. I will report the result of this in the following.
Anyway.......
When I started doing that for St Leonards I found something interesting....
Between 2015 and 2020 the sky radiance had gone from 0.84 bananas* to 2.17 bananas* (2.58 times original)....
No wonder you feel that the sky is not as dark as reported, it's become significantly brighter!
How much brighter is it NOW is the question? Well aside form measuring it, which is the subject of another one of my discussions, the best we can do is use the available data, that means 2020 data.
OK from LightPollutionMap.....
World Atlas 2015 Brightness data
Brightness=0.286 mcd/m^2 , SQM=21.44 mag./arc sec^2, Bortle Class=4
VIIRS 2015 dataset sky radiance data
Radiance=0.84 x10^-9 W/cm^2*sr, Teal / Darker Green colour code
VIIRS 2020 dataset sky radiance data
Radiance=2.17 x10^-9 W/cm^2*sr, Light Green colour code
Ok so ... if you ask me it's therefore reasonable to take (extrapolate) the
World Atlas 2020 Brightness data to be 2.17/0.84 = 2.58 times the
World Atlas 2015 Brightness data so that The
World Atlas 2020 Brightness is 0.286 x 2.58 = 0.738 mcd/m^2.
Now using this Sky Brightness to SQM reading converter (
http://www.unihedron.com/projects/darksky/magconv.php)on the Unihedron SQM Site that means our brightness of 0.738 mcd/m^2 equates to a reduced SQM reading of 20.41.
Now an SQM reading of 20.41 mag./arc sec^2 equates to a Bortle 5** ish sky (just on the border between 4 and 5, whereas previously in 2015 it was firmly a Bortle 4)
OK That's too much writing, colours, &c. for me, for now
Best
JA
* - Sorry I couldn't resist the bananas, but nonetheless the SkyRadiance units are W/cm^2 * sr
** Bortle /Approx SQM Equivalence Ranges
https://en.wikipedia.org/wiki/Bortle_scale