Throughout history, human beings have expressed the desire to measure the passage of time. In this tireless quest, which led them to conceive modern clocks and watches, the sundial was the very first step, relying on the path of our brilliant star across the sky to determine the time of day. Here is its history.
Can one even imagine what the invention of the sundial represented? Using a rod and a graduated surface, the Ancients had found the most accurate method of their time for knowing the hour of the day; and they had conceived the idea by observing the path of the solar body across our sky. Admittedly, this system could only function during daylight, which limited its use. But the sundial was supplemented by only two mechanisms: the water clock and the hourglass, which could not calculate hours but could “merely” measure durations.
The Egyptians undoubtedly already possessed knowledge of the sundial, but no description has reached us. Other writings mention the use of this instrument: the Bible refers to the sundial owned by Ahaz, king of Judah, in Jerusalem (8th century BC), Pliny the Elder mentions the one invented by the Greek Anaximander (6th century BC), and Herodotus attests to the use of the gnomon among the Babylonians (around the 5th century BC)—a use that undoubtedly dates back to a much older invention, perhaps transmitted to the Egyptians in very ancient times (2nd millennium BC). Using a gnomon to establish the height of the sun, the Greek Eratosthenes employed it to demonstrate the roundness of the Earth.
But among primitive sundials, it is the one created by Berossus, in Greece, that we know best. In the 3rd century BC, the Chaldean had invented a vertical dial composed of a hollow hemisphere, turned toward the earth, and equipped, at the very center of its cavity, with a button whose shadow, under the sun, marks the hours on the walls of the half-sphere. This type of dial bears a name: the scaphe (“boat”).
In Europe, churches and monasteries appropriated the measurement of time. The canonical dial, which appeared in the 7th century, was used to indicate to members of the religious community the beginning of liturgical acts; the problem of nocturnal hours would be circumvented with the aid of fire clocks. As for the (questionable) precision of these horizontal sundials, installed perpendicular to the walls of churches, abbeys, and monasteries, it mattered little. Not because approximation would be acceptable, but because the Church made it a point to impose this calculation of hours as being perfect.
The sundial, in fact, lacks precision. The fault lies with an hour that is not constant throughout the year. For the Ancients, the interval separating sunrise from sunset is always composed of twelve hours, regardless of the time of year—thus the dials of gnomons are graduated twelve times, at equal distances. Yet, depending on the distance between the star and the Earth, the duration of days increases and decreases. As a result, ancient dials indicate “unequal hours,” or “temporary” hours, varying according to location and season.
It would take the invention of the inclined style to move to diagrams displaying equal hours. The inclination of the traditional rod stemmed from an awareness dating to the 15th century: the fact that the Earth revolves around the Sun. In a sense, the effectiveness of the inclined style, also called “polar,” helped demonstrate the veracity of the theses supported by scholars such as Copernicus, Kepler, and Galileo: it turned out that by tilting the style to make it parallel to the planet’s axis, the measurement of hours became accurate…
The appearance of clocks, toward the end of the 14th century, did not immediately bury the sundial. On the contrary, the two instruments became complementary: the sundial, with its now inclined style, was capable of measuring the hour with a certain accuracy; and the clock, for its part, could preserve that hour. This duo would have fine hours ahead of it, before the increasing precision of mechanisms relegated the sundial to museum shelves.
The operation of the sundial is based on that of another instrument: the gnomon, an astronomical tool whose shadow’s length and direction serve to point to the hours traced on a flat, horizontal surface. The gnomon is the entire device comprising the rod planted or fixed, always vertically, bearing the name “style,” and the support on which it is placed—in a base or directly in the ground.
In itself, the gnomon is intended to establish the height of the sun in the sky, always by means of its projected shadow. Originally a simple stick planted vertically in the ground, known to the Greeks and Chaldeans, it is the origin of the science of sundials, called gnomonics. In short, the gnomon is the ancestor of the “modern” sundial reinvented by the Babylonians and Greeks.
The principle of the gnomon applies very simply to the sundial: the shadow of the style is projected onto divisions created artificially on the flat surface, indicating the passage of hours. The shadow follows the course of the sun in the sky and progressively passes over each division, in the manner of the hand on the dial of a watch.
The typical sundial is divided into twelve hours, which run from sunrise to sunset. But even with the later inclination of the style, the dial retains its two main drawbacks: it indicates only local time, which changes according to longitude… and it remains subject to climatic uncertainties!
The First Time-Measuring Instrument
How a Sundial Works: The Gnomon
How a Sundial Works: The Division of Hours