Most people in brightly lit countries probably take for granted that clouds are bright at night, but this is completely unnatural. You can see it better in this pair of photos we published in a later paper (titled "Red is the new black"): https://doi.org/10.1111/j.1365-2966.2012.21559.x

This probably matters a great deal for nocturnal animals, because it's a reversal of an environmental condition that existed during the hundreds of millions of years that life evolved.

@skyglowberlin I teach large university science courses. Hundreds of fairly privileged students in the room. It has been my habit to ask them to raise their hands if they have ever seen the Milky Way.

The results have always been disappointing. Over the past 20 years it has gotten worse and worse. Recently many students don't even know what I mean by the question, and I have to explain what a dark sky looks like.

"Raise your hand if you have ever been awed by a clear dark sky full of countless, countless stars." Always less than 5%.

@skyglowberlin When I teach about retinal physiology, it breaks my heart. Often there isn't a single student who has ever experienced vision after true dark adaptation.

It turns out that—it takes time, but it's real—you can see by starlight. And the faintest stars you see? That's a single rod cell detecting A SINGLE PHOTON. Your retina is that sensitive.

Before 1879 this was an absolutely universal human experience. Now it's exotic, unimaginable.

@jameshowell @siracusa @skyglowberlin What I miss most about my hometown in rural eastern Washington State is the night sky.

It wasn't akin to being in Antarctica or anything, but I DID grow up being able to stare up to see the “band” of the Milky Way. One of my first major interests as a kid was astronomy. Even had my own telescope.