A light-coloured, linear feature extending radially from a crater. A number of lunar craters are surrounded by extensive and conspicuous ray systems that show up particularly at full Moon, when they can be seen with the unaided eye. Ray systems are associated with the youngest craters, such as Tycho and Copernicus. They may be due to rock surfaces exposed in relatively recent impacts; such surfaces would be more reflective than old surfaces that have been subjected to radiation for millions of years. Alternatively, rays may consist of glassy, reflective ejecta. Rays are also found around craters on several other bodies in the solar system.