The lunar limb is the edge of the visible surface (disc) of the Moon as viewed from Earth. Seen from afar, it looks like a circular arc, but with higher resolution, it can be seen that the Moon’s surface is not flat and therefore its profile is irregular. In the 17th century, the irregularity of the lunar limb was observed by Galileo Galilei using a telescope, and the discovery was mentioned in his 1610 treatise titled Sidereus Nuncius.