Due to librations in latitude(about 5° due to the tilt of the moon's orbit relative to that of the earth) and longitude(somewhat less, and due to the ellipsoidal nature of the lunar orbit), the portion of the moon's surface that always is visible from the earth is less than a full hemisphere. If the lunar station were on the edge of this area, the astronauts usually would have a good line of sight to the earth, never be out of view of the earth, and would occasionally be on the limb, which of course moves on the lunar surface with the librations.