A sundial is a device used for telling the time when the sun is shining. The shadow of an upright rod falls onto a flat surface that is marked with the hours, and points to the correct hour.
sundial in British English
(ˈsʌnˌdaɪəl)
noun
a device indicating the time during the hours of sunlight by means of a stationary arm (the gnomon) that casts a shadow onto a plate or surface marked in hours
sundial in American English
(ˈsʌnˌdaɪəl)
noun
an instrument that indicates time by the position of a gnomon's shadow cast by the sun on the face of a dial marked in hours