`cloudy_sunset.png` - "Birds retreating to the forest at dusk on a cloudy evening" 
by Arachnidly, CC BY-SA 4.0, from Wikimedia Commons
`night_sky.jpg` - by Mauro H Mathias, CC BY 2.0, from Flickr (https://commons.wikimedia.org/wiki/File:-i---i-_(24729862071).jpg)
`milky_way.jpg` - "Milky Way over Bontecou Lake" by Juliancolton, CC BY-SA 4.0 (https://commons.wikimedia.org/wiki/File:Bontecou_Lake_Milky_Way_panorama.jpg)