This is a quote from Paul Valéry's L'idee fixe, 34. Valéry was a French poet, essayist and philosopher (1871-1945).
The title of Book Two, The Earth's Sharp Edge, is taken from a line at the close of Chapter 20 when Port and Kit are in a bus heading for El Ga'a. Port is now showing signs of being seriously ill and is not responding to her entreaties:
She looked out at the windswept emptiness. The new moon had slipped behind the earth's sharp edge.