Peering over the limb

For observers, the Moon is like a circular billboard in the sky. We never see its entire back side from Earth, but with the subtle effects of the Moon’s orbit known as libration, we can peek a few degrees around its edge, allowing us to view nearly 60% of the Moon’s surface. Within that just-around-the-edge zone, during favourable librations we can observe all of , even and , both of which extend beyond 90°E.