Here is the full list of all 30 Six Sides of the World achievements.
Complete one level in Selene
Complete all the puzzles on a level with more than one
Unlock Selene
Complete Tau Ceti
Unlock Rao
Be destroyed
Complete all levels in Rao
Unlock Altair
Complete all levels in Altair
Unlock Rho Persei
Complete all levels in Rho Persei
Unlock Sirius
Complete all levels in Sirius
Unlock the Unimpossible
Finish the game
Complete all the puzzles in the game
Complete, at least, one puzzle in each level
Complete a puzzle on the alpha, beta, gamma... levels of the game
Complete all puzzles on the alpha, beta, gamma... levels of the game
Complete all levels in Selene
Complete one level in Cygnus X-1
Complete all levels in Cygnus X-1
Watch the credits
Unlock Cygnus X-1
Complete The Dark Side without moving the lights
Find all the combinations of The Safe
Exit through the green wormhole in PAC
Exit through the yellow wormhole in Impossible Path
Complete Quadrangular