As we've already mentioned elsewhere, the title refers to our solar system's sun and moon—and their correspondents among the book's humans are Anna Wetherell and Emery Staines. It's very important that the reader understand those characters' relationships to the heavenly luminaries, since it sheds a lot of, well, light on their relationships with the rest of the characters and to the plot.
For example, by discerning that Anna is supposed to represent the sun, you know that pretty much everything is revolves around her, long before you can even begin to understand her full role in the mess the Crown council is discussing. We know that we should keep an eye on her, even when she's not at the front and center of the plot. And the same goes for Emery; we actually know long before we "meet" him that he's one of the luminaries, and so he's probably going to be a big deal.
In short, the title does a great job of letting us know that two characters who seem secondary at first are actually the two most important characters in the book. Even before we know the "Why," the title tells us the "Who."