The so-called Lunar Cannon is a large crossbow in the branches of the Mother Tree on Alais. It was built by the Stickmen so they can "blast off to Luna", although why they would want to do this is never explained. It is also never explained why they refer to the moon as "Luna", its Latin name.
April completes the Lunar cannon by providing the bowstring with Q'aman's fishing line. She then uses the Lunar Cannon to launch a grappling rope across the distance dividing the road to Tama'a from the Mother Tree, where an old bridge had collapsed.