A sundial is a timekeeping instrument that utilizes the apparent motion of the sun across the sky to indicate the time. Its operation is based on the principle of gnomonics, the study of the measurement of time and angles using the sun's position. The device consists of a pointer, or style, which casts a shadow onto a marked plate. The accuracy of a sundial depends critically on its geographical orientation and its alignment with the local meridian, as the length and angle of the shadow change predictably throughout the day based on the time of day and the season. Because local timekeeping is dependent on solar geometry, sundials are susceptible to adjustments related to time zones and Daylight Saving Time, requiring supplemental timekeeping methods for modern precision.