You take your clock for granted today, but it’s only been in the last couple of centuries that machines (and electronics) have been accurate enough to be used for timekeeping. Before that, people had to use other ways to tell the time of day. One of the most useful and easy to make is a sundial.

In its simplest form, a sundial consists of a style – a thin rod or sharp straight edge – that casts a long shadow onto a flat surface. As the Sun moves in the sky, the shadow moves as well in a perfectly predictable way. By putting marks on the flat surface, you can know what time it is by the position of the shadow.

For a sundial to work, it must be aligned with the axis of the Earth’s rotation. The style must be pointed towards North, and the style’s angle with horizontal must be equal to the sundial’s latitude.

NASA’s Mars Exploration rovers are equipped with miniature sundials on top of their color calibration targets. Scientists use these to fix colors in images based on the known colors in these calibration targets. The sundials are decorative, but they also help locate the Sun’s direction compared to the rovers.

This article on Universe Today talks about the sundial attached to NASA’s Mars Exploration rovers.

Would you like to make your own sundial? NASA has a cool page that shows you how to construct and use your own sundial. This page gives you a template so you can construct your own sundial (warning, it’s a PDF document, not a web page).

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.