The items you need to power up Yuna's ultimate weapon - Nirvana - are called the Moon Crest and Moon Sigil. Along the same lines, Yuna's name can mean "moon" in Okinawan. If this isn't a big honking clue that we should associate Yuna with the moon, I don't know what is.

As mentioned in the name section, Yuna's name was meant to be a contrast with Tidus, which means "sun." This buys into the concept of yin and yang: the sun is the bright, strong masculine, and the moon is the quieter, soft feminine. Furthermore, only one side of the moon is ever illuminated by the sun. Even though we don't see it, we know that the other side is there. This is like Yuna's tendency to smile even when she feels sad - everyone knows that how she feels, but she tries not to show it. The sun, on the other hand, gives off light on all sides - Tidus doesn't try to hide how he feels.

The sun and moon are not often seen in the sky at the same time. Or, at least, if the moon is in the sky at the same time as the sun, it is still quite a distance away from the sun. They coexist, but are rarely together and are separated by a vast expanse of empty space. This is like Tidus and Yuna - a pair of lovers with only a fleeting amount of time spent together, and the rest Yuna spends chasing after a dream. (Of course, Tidus comes back at the end of FFX-2, but let's just ignore that for now.)

And like the moon revolves around the earth, Yuna's life revolves around Spira.

The moon's gravitational pull is the cause for the fluctuation of the earth's ocean tides. Spira is like a giant ocean - most everything in the land centers around water, and the people live on islands. Spira is a land of water, if that makes sense. Then, Yuna, as the moon, has undeniably influenced and completely altered the ebb and flow of Spira's history through the events of her pilgrimage.