# How does a Sundial work?

## What is a Sundial?

Long before the age of clocks and the time of pocket watches, mankind had to come up with a simpler way to tell the time of day – a sundial is a device that does just that. The concept is simple: a flat surface adorned with a style is placed in plain view of the sun as it moves across the sky, and as the day goes by and the sun’s angle changes relative to the sundial, the shadow cast by the style changes appropriately.

But wait, let’s take a step back. Style? What is that? Well, in this sense, it isn’t an aspect of writing, fashion or artistic sense – it’s an old word, a stepping stone to the word stylus, and refers simply to a rod. On a sundial, this rod is also sometimes substituted for a triangular blade called a gnomon (this means “one that knows” or “examines” in Greek, and is pronounced as “no-mon”).

To properly work, a its gnomon must be aligned parallel (or along) the axis (or centerpoint of rotation, like the middle of a spinning top) of the Earth as it rotates – otherwise, the gnomon would have to be adjusted based on location due to the Earth’s changing position throughout the year, to make up for a lack of accuracy. As we’ve progressed mathematically, more sophisticated sundials have been built, including ones that incorporate hour lines that make up for the time differences, as well as ones with cylindrical, spherical or conical surfaces.

Yet, before these things were figured out, sundials provided a fairly accurate time of day, being off by only about 15 minutes throughout certain periods of the day, depending on the season. They are also heavily dependent on location – dial makers have to take into account the longitude and latitude of their location, as well as the direction of true North (for dial makers in the Northern Hemisphere) or true South (for dial makers in the Southern Hemisphere). In the Northern Hemisphere, gnomons have to point north – the opposite is true in the Southern Hemisphere. Hour indicators on the face have to be set according to the dial’s location, to avoid miscalculations.