The sundial is the oldest known method for keeping time. The Egyptians were the first group of people that took timekeeping seriously as a culture, at least according to reasonable proof. Many historians believe that the Sumerians kept time thousands of years before the Egyptians, but their proof for this theory is merely speculative.
Around 3500 B.C., the Egyptians built obelisks, which were tall four-sided tapered monuments. They placed them in strategic locations so they would cast shadows from the sun. Their moving shadows formed a kind of sundial which indicated noon, which allowed citizens to divide their day into two parts.
These obelisks also indicated the year’s longest and shortest days by the length of the shadows they cast. Later on, the early Egyptians added markers around the base of the monument to indicate further time subdivisions.
Then, around 1500 B.C., the Egyptians took the sundial to the next level by creating a more accurate shadow clock. The sundial was divided into ten partitions, which also included two twilight hours.
This sundial was only capable of keeping accurate time (by today’s standards) for a half a day. Therefore, around noon the device needed to be turned 180 degrees in order to measure the afternoon hours.
A sundial works by tracking the movement of the sun around the earth’s celestial pole by casting a shadow or a point of light on a surface that is marked with hour and minute lines. The shadow-casting device, which is called the gnomon or the style, must point towards the north celestial pole, which is very close to Polaris, also known as the North Star. The gnomon serves as an axis around which the sun seems to rotate.
The thinner the shadow line is, the greater the accuracy of the sundial. In other words, the larger the sundial is, the greater its accuracy will be. This is due to the fact that the hour line is capable of being divided into smaller portions of time.
However, if the sundial gets too large, a point of diminishing returns will be reached. This happens because, due to the diffraction of light waves and the width of the sun’s face, the shadow will spread out and become fuzzy. This diffraction makes the sundial quite difficult to read.
Throughout the years, sundials evolved from flat horizontal or vertical plates to more complex forms. These changes were driven by the search for greater accuracy.
One new version was the hemispherical dial, which was a bowl-shaped depression cut into a block of stone. It contained a central vertical gnomon and was scribed with different sets of hour lines for each season.
This sundial evolved into the hemicycle, which most scholars believe to have been invented around 300 B.C. The hemicycle removed the unnecessary half of the hemisphere, which gave it the appearance of a half-bowl cut into the edge of a squared block.
Throughout time, the sundial has evolved and changed to more accurately tell the time. Today, however, with modern timekeeping technology, the need for a sundial has completely diminished. However, they still make an attractive addition to any backyard, mostly for decorative purposes.
Amy Martin is a freelance author who regularly writes about sundials