Rayleigh is a lunar crater that lies along the northeast limb. This feature is seen edge-on from Earth, making it difficult to see much detail. In addition, libration effects can completely hide this crater from view. It lies just to the north of Lyapunov, and to the northwest of the large Joliot. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On the Moon they are named after deceased scientists, polar explorers, astronauts or cosmonauts.
