A Fire in the Heavens — Mary Robinette Kowal

On a tidally locked world, Katin is aboard a ship fleeing from religious persecution in her homeland. Her people worship the seven-sisters, a group of dieties that resides in the night sky and are visible as a tight cluster of stars. This exploration ship has sailed farther West than anyone has sailed before. Some think that they will sail off the edge of the world, but she is confident that the ancient stories of a land far to the West are true. She dismisses the tales that accompany the stories of light in the night sky called “the moon” as exaggerated superstition.  

Until they sail over the over the curve of the world and sees the moon for the first time.