Never heard about it nor can I find references about it with google
Hmm, it reminds me of the ecliptic plane: though it' a flat line, it appears to us as a gigantic curve in the sky.
This has to do with our location on Earth (a sphere); when we're not exactly in the same line, because we are on a sphere, that line appears curved. The straight line between the Moon and the Sun would also eppear curved