Lunar clair-obscur effects are tricks of the light that produce recognisable shapes on the Moon's surface. They occur when jumbled areas of light and shadow produce something that looks familiar. Most occur near the terminator and fade into and out of view in a few hours.
This isn't true for all clair-obscur effects,waning gibbous Moon under direct lighting away from the terminator for many days. It occurs when the steep sides of a mountain to the west of crater Lamèch become fully illuminated to form a bright letter ‘L’.
