Flag Sundial

Flag Sundial: an altitude dial, formed by "unwrapping" the scale of a shepherd's dial into a flat plane which can be shaped like a flag or pennant and is positioned perpendicular to the sun.

Cylinder Sundial: (also known as a shepherd's dial or pillar dial): a portable, altitude dial in which hour lines for different dates are delineated around the surface of a cylinder, which is allowed to stand or hang vertically. A horizontal gnomon projects radially from the top of the cylinder, and is adjusted to the appropriate date around its periphery. Sometimes two gnomons are supplied; a long one for winter and a short one for summer. The dial is held with the gnomon facing the sun so that the shadow falls vertically. Latitude specific.

Altitude Sundial: (or elevation dial): any dial which uses the sun's altitude, rather than its azimuth, for indicating the time. Usually does not need to be aligned N-S. Examples are ring dials, flag dials and shepherds' dials. Altitude dials were often incorporated in quadrants and folding rules.

Altitude (of the sun) (a, ALT): the angular distance of the centre of the sun's disk above the observer's horizon (negative numbers indicate that the sun is below the horizon). It is measured along the principal plane to the sun's centre, and is the complement of the zenith distance. It is part of the horizontal co-ordinate system.

The layout of the hour lines for a flag sundial is identical to that of a cylinder sundial. Please go to the cylinder sundial page for a description of how to determine these lines and the length of the gnomon. Instead of wrapping the dial plate around a cylinder it is left flat.

Figure 1 illustrates a flag sundial that will indicate local apparent time. The gnomon is perpendicular to the dial plate and must be able to slide along the date scale. The entire sundial is rotated to allow it to be aligned with the sun. If the mounting pole is stationary, then the "flag' must be able to rotate around the pole for alignment.

Figure 1: Flag Sundial (SONNE/CAD)

Figure 2 illustrates the hour lines for a flag sundial (left) that  indicates locate apparent (solar) time and one (right) that indicates zonal solar time, which is corrected for longitude. As both these types of dials have symmetrical hour lines for each the two periods between the solstices, the dial plate can show the entire year on one side. The start date at the left of the dial plate is June 21 and the end date at the right is December 21. 

Figure 2: Flag Sundial Hour Lines - LAT & Zonal Solar Time (SONNE/CAD)

Figure 3 illustrates the hour lines for a flag sundial that indicates clock time and the hour lines are corrected for both longitude and the Equation of Time. The hour lines are not symmetrical in this case. For this sundial the dial plate would be inscribed on both sides and the movable gnomon would extend out on both sides of the sundial.

Figure 3: Flag Sundial Hour Lines - Clock Time (SONNE/CAD)

For an image complete with shadow click here.