Here is the full list of all 22 NightSky achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Completed Skymning Alternative
Completed Harara Mountains Alternative
Completed The Beach
Completed The Beach Alternative
Completed Harara Mountains
Completed Skymning
Completed Ranna Caverns
Completed The Void
Completed Aurora's North
Completed Slightly Nonsense
Completed Old Ruins Alternative
Completed Perpetuum Factory Alternative
Completed Ranna Caverns Alternative
Completed The Void Alternative
Completed Murky Depths Alternative
Completed Giant Leaf Alternative
Completed Aurora's North Alternative
Completed Slightly Nonsense Alternative
Completed Old Ruins
Completed Perpetuum Factory
Completed Murky Depths
Completed Giant Leaf