On sundials, hour lines are the lines that give the hour by means of the shadow cast by the gnomon. The lines may show unequal hours (when the duration of the hours varies according to the season), in which case they are referred to as ancient, Jewish, temporal, planetary, canonical, or natural; or they may show equal hours (when the day is divided into 24 hours). Equal hours are classified into equinoctial, counted from midnight to midnight, since, at the equinoxes, the duration of diurnal and nocturnal hours is identical; astronomical, counted from noon to noon; Babylonian, counted from dawn to dawn; Italian, counted from sunset to sunset.