Symbolism, Imagery, Allegory
Not since Margaret Wise Brown said goodnight to the moon has the lunar satellite caught the symbol bug so hard. And let's face it: Rourke ain't subtle about it. Not only is the moon in the title (you don't need to be Sherlock to see that one), it's also, uh, literally everywhere else in the book. Makes sense, given that the moon is literally everywhere in real life, too. No matter where you are at night, clouds permitting, you can see the moon. In Shooting the Moon, it's a heavy-handed but successful way to connect Jamie to her brother who's thousands of miles away.
When she's developing his photos, she tells us:
I discovered that in some of them wispy clouds were sliding by a full moon's eyes, and in others crescent moons stood suspended in the night sky like slivers of light, Venus twinkling beneath them. There were quarter moons and waning gibbous moons, every sort of moon there was, sometimes with stars peeking out from the corner of the frames, sometimes framed by circles of light. (16.6)
Jamie might not get the big deal about the white ball in the sky at first, but soon she comes around. Notice how she thinks of the moon as if it's a person—it has eyes that look right at her. (Fancy literary detour: this is called personification.) This is her way of learning to appreciate how detailed and nuanced things can be up there in the sky.
As Jamie starts examining TJ's photos more closely, she gains a deeper appreciation for them and the moon. She starts to look at the details and grooves of the moon, whereas before, she didn't really get TJ's fascination with it. As the war becomes more real to Jamie, she starts to notice the little things in life more, to pay attention in more nuanced ways, which we see as her appreciation for the moon develops.
Jamie also realizes that the moon is always changing. It goes through different stages all month long, constantly waxing or waning. But the moon is much more than a sign of constancy and change to TJ. Check out what he says about it when Jamie asks why it's so important:
"It's got shadows in it," TJ explained. "From the craters. I think the shadows are interesting. And I like the idea that now there are human footprints on the moon's surface. There's something pretty cool about that." (5.9)
There is something cool about that, especially since the moon landing has basically just happened. For TJ, the moon represents light and dark and their dynamic relationship, as well as the role humans play in nature. Considering the fact that his life is on the line for pretty much the whole book—he stands to have a person kill him before he'd naturally die—this is one powerful symbol for his experience as a soldier.