When an astronomical object is tidally locked, it means that one side of this object always faces the body it orbits around. For example: Luna is tidally locked to Earth; the fictional planets Aurelia and Aranel are tidally locked to the red dwarf stars they orbit around; the dwarf planet Pluto and its moon Charon are tidally locked to each other. Contrary to the popular belief, Mercury is NOT tidally locked to the sun!