The oldest type of clock is a sundial clock, also called a sun clock. The shadow of the sun points to a number on a circular disk making it possible to measure time. Originally people determined the time by looking at the sun as it crossed the sky. When the sun was directly overhead in the sky, it was the middle of the day, or noon. When the sun was close to the horizon, it was either early morning (sunrise) or early evening (sunset).

Then they observed that a cast shadow was longer in the morning and it grew considerably shorter till it no longer existed when the sun was right above during mid day. As night came, the shadow lengthened on the alternative side of the object. The first simple sundial was known to be discovered by an individual who placed a gnomon or vertical stick in the ground and marked the shadow’s length to show the time.

Obelisks built by the Egyptians and Babylonians were monuments that were tall and four-sided, designed to cast shadows from the sun. These shadows created a type of sundial, which indicated noon and thus separated the day into two halves. Later, additional marks were added to further divide the daylight hours.

The gnomon or style of a true sundial is oriented toward Polaris and is the axis around which the sun appears to rotate. Larger sundials provided smaller and smaller divisions of time up to a point of diminishing returns. The gnomon of one astronomical observatory constructed in India in the 1700s contained a 30-metre high gnomon.

A stick shaped like a ‘T’ or the “time stick" was used by early Egyptians. It was made by using a singular vertical stick with a crossbar attached to a base where 5 hours were marked off. The stick was put in such a position that it was directed to the east at morning and west towards afternoon. The hours were denoted when the shadow moved across the base.

The sundial described by Berossus, who was an author and a Babylonian priest is a cube-shaped block. At the centre a small bead is placed and the shadow cast by the bead measures the hours. The disadvantage of this type of sundial is that since the day’s length changes with the season, the length of the hours also varies.

The advances in mathematics, including the concept of trigonometry invented by the Greeks, allowed mathematical calculations rather than geometry to perfect the construction of sundials. Perga’s Apollonius developed the hemicyclium by making use of a conic-shaped surface and inscribing it with hour lines. Ptolemy used the analemma, which projects shadows geometrically at different angles onto flat surfaces.

The concept of “equal hours" is credited to Ab û al-Hasan in about 1200 AD who wrote a treatise regarding hour lines on cylindrical, onical, and other surfaces.

Even up until the late 1800s, when mechanical clocks became not only more accurate, but less expensive, sundials were commonly used to check the accuracy of and to adjust mechanical clocks.