Description
Pillar Dial

The pillar dial was one of the earliest portable sundials to be made. It is also called the cylinder dial, and the shepherd’s dial, because it was regularly used by shepherds right up until the beginning of the 20th Century. The earliest pillar dial known is a Roman dial from the first century AD.

Pillar dials normally consist of a cylinder marked with date and hour lines. On top of this cylinder is a cap with a horizontal pointer (the gnomon) which can be turned around on the top of the cylinder.

Pillar dials are altitude dials. That means that they tell the time from the height (altitude) of the sun in the sky. Different date lines are needed for different dates in the year, because the sun is not at the same height in the sky at the same time every day in the year. In the winter it is lower in the sky and in the summer it is higher. You can imagine that the pattern of lines on the cylinder is like a graph of the time plotted against the date and the gnomon must be set over the correct date line to tell the time correctly.

This positioning of the gnomon is the first thing that must be done to use the pillar dial. The cap is turned round until the gnomon is over the correct vertical date line. Then the dial is suspended from a ring in the top of the cap. It is turned towards the sun so that the shadow of the gnomon falls straight down the date line. The tip of the shadow shows the time of day.