Interesting - I guess it must be linked with the gender of the sun and moon - like in Germanic languages (except English, of course, where the genders are usually switched - French or Roman influence, perhaps? Er, I guess one of our linguists can answer that) ... actually, I've always wondered why northern Europeans assigned the genders of the sun and moon the opposite of those in the south, and whether this tells us anything about gender roles in these cultures, but I guess that's completely off topic, so I'll be quiet now.
OK, more on topic - why did Tolkien assign the gender of the sun and moon the way he did? Forgive me if that's a dumb question...