Figure 45: Shepherd's dial, set for a latitude of 52ºN
Figure 46: Shepherd's dial made from an empty soup can
Figure 47: Hour lines on a shepherd's dial
Altitude dials are dials that use the altitude of the Sun to tell time. The best known altitude sundial is the shepherd's dial.
The shepherd's dial is one of the simplest and most widely used portable dials. It is also called the pillar's dial, the traveler's dial, and the cylinder. Although the shepherd's dial is not very accurate, it is easy to make and inexpensive. It indicates the time of day from the Sun's altitude, which depends not only on the time of day, but also on the latitude and time of year. It is designed for a particular latitude and is adjustable for the date.
Figure 45: Shepherd's dial, set for a latitude of 52ºN
Figure 46: Shepherd's dial made from an empty soup can
Figure 47: Hour lines on a shepherd's dial
The shepherd's dial consists of a cylinder capped by a movable top to which a gnomon is attached. The cylinder is usually hollow to contain the gnomon when it is not in use. The hour lines are either drawn on a paper which is glued to the cylinder or inscribed directly on the surface of the cylinder. The hour lines appear as curves on the rounded face of the cylinder and they lie close together in winter, and also near noontime, so that the instrument is least accurate at those dates and times.
The months of the year are traced around the base of the cylinder.
The length of the shadow of an object depends on the altitude of the Sun in the sky. (See Figure 48.) Since the altitude of the Sun is dependent on the time of the year, the gnomon has to be swung to the correct position of the year. Letters around the base of the cylinder indicate the months. (See Figures 46 and 47.)
In use, the dial is hung by a string fixed to the tip of the cap, with the gnomon extended toward the Sun. The shadow of the gnomon then falls straight down and ends somewhere between the hour lines.
In the morning, when the tip of the shadow just touches the curved hour line labeled 11a.m. - 1p.m., it is 11 o'clock. An hour later the shadow will be longer and will just reach the line labeled noon at which point the Sun will begin to go down. At 1 o'clock the shadow will again reach the 11am-1pm line.