Unlike the title for Twilight, which Stephenie Meyer’s publisher helped to choose, the author picked the title for the sequel. Meyer decided on "New Moon" as a reference to the darkest phase of the lunar cycle – an apt title for a book about the darkest time in lead character Bella’s life.
The title is introduced in a central moment in Chapter 3, when Edward leaves Bella in the forest after breaking up with her:
Was it always so dark here at night? Surely […] some little bit of moonlight would filter down through the clouds. […] Not tonight. Tonight the sky was utterly black. Perhaps there was no moon tonight – a lunar eclipse, a new moon. A new moon. I shivered, though it wasn’t cold. (3.217)
Throughout the story, celestial bodies like the moon and the sun pop up to help us visualize how characters feel about each other and the world around them. In particular, Bella’s descriptions of Jacob are laden with celestial similes, or comparisons (see "Symbolism, Imagery, Allegory" for more details on Jacob’s heavenliness).
In Chapter 9, the image of the moon then appears in Bella’s description of herself as she recognizes that, despite her love for Jacob, part of her will always long for Edward: "I was like a lost moon – my planet destroyed in some cataclysmic, disaster-movie scenario… that continued… to circle in a tight little orbit around the empty space left behind" (9.2).
Finally, Edward references the title when he explains to Bella how his life felt without her: "Before you Bella, my life was like a moonless night. […] And then you shot across my sky like a meteor" (23.128).
The celestial trend seems fitting for a story that deals a lot with ideas of going to heaven versus going to hell. It’s also interesting that, throughout the story, Bella feels cold, like a moon, seeking the warmth of Jacob’s sun. Of course Edward is always cold. So do two cold moons fit together? Or wouldn’t the sun and the moon be a better fit? Or are we pushing this a bit too far? You tell us.