Sundials not only told time for many centuries, they also told a story. Individuals for centuries calculated time based on the simple progression of the sun in the sky combined with a basic knowledge of astronomy and math. As sundials evolved from sticks to more elaborate mechanisms, they also acquired great beauty of design. Markings found on the flat surface of a sundial represent each hour of daylight. As the sun moves across the sky from east to west, the gnomon or vertical rod at the center of the dial casts a shadow across this flat surface. The position of the shadow on the hour markings is an indicator of the current time of day.
Sundials not only tell time, they are educational and give us a sense of history. They are an artistic addition to our outdoor spaces which bring us closer to nature and history, providing a rare moment of reflection.
There are two main groups of sundials:
1. Altitude dials determine the time from the sun’s altitude (the sun’s height above the horizon). These dials either must be correctly oriented to the compass directions or they must be aligned to the sun.
2. Azimuth dials determine the time from the hour angle of the sun (the sun’s angle on its daily arc). These dials must be correctly oriented and therefore a magnetic compass is often incorporated in the instrument.
The working of a sundial is not so simple due to the tilted axis of the earth. If this is not considered, the sundial will have a different time each week. By aligning the gnomon with the earth’s axis, the difference can be compensated. Different sundials have different principles for calculating time. In a horizontal sundial, the gnomon or needle is tilted in line with the earth’s axis while the base is kept horizontally. In an equatorial sundial, the base plate where the shadow falls is kept at an angle that is parallel to the equator while the gnomon is perpendicular to the plate. The most common type is the vertical sundial where the base plate is vertical and the gnomon is aligned to the earth’s axis.
A sundial’s position in latitude in one hemisphere should be the opposite in the other hemisphere. For instance, a direct south vertical sundial in the northern hemisphere becomes a direct north vertical sundial in the south hemisphere. Positioning the sundial accurately requires finding true north or south and positioning them parallel to the axis in terms of longitude. The rotation of the sun is also calculated when building and placing a sundial.