An instrument for telling time by the sun. A sundial consists of a gnomon, which may be a rod or triangular plate, and a graduated plate on which the gnomon casts a shadow. The place on the dial plate where the shadow falls indicates the apparent solar time. Three types of sundials are distinguished according to the position of the dial plate: equatorial, horizontal, and vertical. In all three types, the rod or the edge of the gnomon is made parallel to the earth’s axis and intersects the dial plate at the dial’s “center, ” which is the point from which the hour lines radiate. The line corresponding to noon is located in the plane of the meridian passing through this center.
In equatorial sundials, the plane of the dial plate is parallel to the plane of the celestial equator. The dial plate is calibrated with equally spaced graduations on the basis of 360° being equal to 24 hours.
In horizontal sundials, the dial plate is horizontal and is calibrated in accordance with the equation
tan = (tan )(sin φ)
where is the angle formed at the center of the dial by the given hour line and the noon line, is the hour angle of the sun (the apparent solar time), and φ is the geographic latitude of the place where the dial is located. The hour lines for6 A.M. and 6 P.M. are always perpendicular to the noon line.
Vertical sundials are usually found on the walls of various structures. Consequently, the plane of the dial plate may have any azimuth. In such sundials, the hour lines are symmetric with respect to the noon line only when the dial plate is perpendicular to the meridian. In this case, the equation for the calibration of the dial has the form
tan = (tan )(cos φ)
Various types of portable sundials are also known.
The position of the shadow on the dial plate indicates the apparent solar time. To obtain the mean solar time, the equation of time must be added to the apparent solar time. Standard, or zone, time can be obtained by taking into account an additional correction that depends on the time zone and the geographic longitude of the place where the sundial is located. The accuracy of sundial readings is generally not better than several minutes.