Time is, well, timeless. It has been regulating mankind’s existence since the advent of civilization. But measuring it down to the last second is a relatively recent accomplishment.

Using a system that the Sumerians had developed circa 2000 BC, the Babylonians established the foundations of modern timekeeping. They divided the daylight hours into 12 parts and the nights into 12 more, defining the 24 segments that we now refer to as hours.

Babylonian astronomers used a sexigesimal (base 60) system to split each of these 24 divisions into 60 units (minutes), which were subdivided in turn into 60 more parts (seconds). It is not known why 60 was chosen, although it may be because 60 is the smallest number divisible by the first six counting numbers.

Although the Babylonians could divide time into minutes and seconds using calculations, they lacked the mechanical means to accurately measure it in such precise quantities. It wasn’t until the 1300s that the early mechanical clocks had minute hands, and the technology for marking seconds did not appear until the last half of the 16th century.

### Jost B¸rgi and Tycho Brahe

The earliest known example of a timepiece with a hand for marking seconds is an unsigned spring-driven clock dated between 1560 and 1570. In 1579, Swiss clockmaker and mathematician, Jost B¸rgi, made a clock for William of Hesse that marked seconds. Two years later, in 1581, Danish nobleman and astronomer Tycho Brahe redesigned his clocks, which already displayed minutes, to track seconds as well, although their accuracy was questionable. (Brahe noticed an average four second discrepancy between his four clocks.)

### Pendulum Clock Design

It wasn’t until pendulum clocks were invented that measuring seconds became possible. In 1644, French philosopher and mathematician, Marin Mersenne, calculated that a 39.1 inch long pendulum would take two seconds to complete an arc: one second for the forward swing, and another for the return. In 1670, clockmaker William Clement put Mersenne’s theory into effect when he modified the pendulum clock designed by Dutch scientist Christiaan Huygens in 1656. This new timekeeper, the original grandfather clock, displayed seconds in a small subdial using a special anchor escapement mechanism with the seconds pendulum. This new innovation was more accurate than the previous measurement mechanism, now allowing seconds to be reliably measured for the first time.

### Hats Off to Ancient Civilizations

Thanks to the ancient civilizations that divided time into smaller parcels and inventors like B¸rgi and Clement, who made it possible to mark time down to the last second, modern society experiences 24-hour days, 60-minute hours, and 60-second minutes. Changes in the science of timekeeping, however, have actually redefined these units.

The Babylonians and their successors calculated seconds by dividing astronomical occurrences into smaller units. At one time, the International System of Units (SI) defined a second as a fraction of the mean solar day. When this measurement proved faulty (because of irregularities in the rotation of the Earth), it was changed to relate to the tropical year instead. Finally, in 1967, the second was measured in terms of the energy transitions of the cesium atom. This change introduced the era of Coordinated Universal Time (UTC) and atomic timekeeping.

This change has resulted in an interesting anomaly. To properly sync atomic time with astronomical time, leap seconds are added to UTC. Therefore, not all minutes contain exactly 60 seconds: approximately eight per decade actually contain 61.

Eric FleschJanuary 3, 2016 / 8:29 pmIt’s likely that the second corresponded to a heartbeat and was thus in practice one of the oldest measures of time. Their equivalent time-length is probably by design, not coincidence.