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How useful is the Bortle scale ?

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FWIW, my sky in Tacande de Arriba, La Palma was supposedly Bortle 4 in 2015 with a sky brightness of 21.58 mag./arc sec^2

In Little Shelford, 10km due south of Cambridge UK, it was Bortle 5 and 20.43. So poorer, but not a lot poorer.

The MW is much brighter in La Palma for at least two reasons. One is that the galactic centre rises much higher from 28N than 52N. However, the Cygnus MW is much brighter too, at the same altitude above the horizon from each site.  The NE stellar limiting magnitude is often 6.5 to my Palmeran eyes but rarely fainter than 5 in Shelford.

The main reason, IMO, is that there are no direct lights shining on my place in Tacande so my eyes have a chance to get properly dark adapted. Lights are completely inescapable here in Shelford.

As for transparency, the sky can be clear at each and murky at each. The only time I have ever seen a true blue Moon was from La Palma during a calima. (For the benefit of those not up to speed with the nomenclature, that is when Saharan dust blows over and turns the daylight sky orange-brown.) Naked eye limiting magnitude was about 1.5, so Sirius and Betelgeuse were easy but Orion's belt very difficult. The Moon was a true steely blue colour.

On a truly dark night I can safely walk around the site in Tacande purely by starlight. Jupiter, Mars and/or Saturn at a decent altitude makes it easy. to see where I am going. I can't do that by reflected light pollution when a calima is in progress.

Edited by Xilman
Add final para
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