Man has always struggled to keep accurate track of the time. In ancient Egypt and Mesopotamia, the first time measurement systems were sundials, which were completely dependent upon the Sun. Today we no longer define time by the Sun, and have atomic clocks which are able to accurately measure time to the billionth of a second.
The 24 Hour Day
A day is a natural unit of measurement with which to measure time. Our sleep cycles follow the rotation of the earth and its impact upon sunlight: during daylight hours we are awake, in the dark of night we sleep. The day is the most fundamental element of our timekeeping system, which we use to define weeks (7 days) months (28, 30 or 31 days) and the year (365 days).
The practice of following the days has always been with the man, but it was not until about 3500 BCE in ancient Egypt that a system for breaking up the day into smaller units first developed. To keep better track of time, the Egyptians separated day and night each into 12 units, resulting in an ultimate 24 hour day.
The hours of the day were not even from one day to the next under this system, however. The length of daylight changes every day, growing longer with each passing day in the spring and summer then receding after the summer solstice. The Egyptians, however, were content with their system and their uneven hours.
It was in Mesopotamia about 3000 BCE that a standardized hour first developed. The ancient Sumerians, whose base number for their counting system was 12, followed a 24 hour day like the Egyptians. (Roughly 12 day hours and 12 night hours). However, they standardized the hours so that they would no longer precisely correspond to the Sun, but would remain consistent throughout the year.
It is from these origins that we receive our own 24 hour day, which we now use throughout the entire world.
Counting the Minutes and Seconds
As timekeeping developed, more and more fundamental units were needed to measure time. Eventually, two smaller units get developed: what are now called the minute and the second. We define a minute as 1 60th of an hour, and a second as 1 60th of a minute. Why we use the number 60 is not entirely sure, although it most likely has to do with the Babylonians use of 60 as the base number of their counting system.
Devices to Keep Track of Time
It is possible to keep track of the time without the help of any devices. All one need do is look up at the position of the Sun, and it can give a fair estimate of the time of day. One can also examine the stars at night to provide an estimate of the time when you can no longer see the Sun.
Sundial
To keep more accurate track of the time, however, men have sought to build devices for the purpose. The first device used to keep track of the time was the sundial. First used in Mesopotamia ca. 3500 BCE, it was comprised of a rod or plate fixed to a disc. The Sun would cast a shadow from the rod onto the disc, telling the Sun’s position and thus the time of the day.
Water Clock
Another early developed timekeeping device was the water clock. Its exact origins are unknown, but water clocks have been found dating as far back as 1500 BCE in ancient Egypt. The device works by allowing water to pass through a small aperture of the clock at a specific rate. By watching this flow of water, you will be able to tell what time it is, even at night as it was not by the Sun. Water does not always flow at a consistent rate, however, so this device was not entirely accurate.
Hourglass
The origins of the hourglass are not precisely sure. They might have been used as early as the 11th century CE, although we do not have assured evidence of their existence until the 14th century. Hourglasses are composed of a single piece of glass separated into two bowls. There is a narrow opening allowing sand to fall from one bowl to the other. The amount of time taken for all of the sand to fall from the top to the bottom would be about an hour or whatever amount of time the hourglass maker wanted to keep track.
Mechanical Clock
It is also in the 14th century that we see the first mechanical clocks. Although very crude in design, these devices were the first able to measure a consistent unit of time accurately. The original mechanical watches ran on a system of falling weights. Mechanical clocks are still in use today.
In the 16th century, a new development arose. It was the mechanical clock, which ran on a coil mainspring. It allowed individuals to carry the time with them for the first time in the history of the world. The watch is a vital element in the lives of people today, and mechanical clocks are still in use.
Pendulum Clock
The pendulum clock was invented in the middle of the 17th century by a Dutch physicist named Christiaan Huygens (1629-95). The passing of time was by the natural swing of a pendulum, which would give an even more accurate showing of the time than the mechanical clock.
Quartz Clock
The development of the clock has been a constant struggle to create the most accurate clock possible. In 1927, a man by the name of W.A. Marrison invented the quartz clock.
When exposed to an electronic field, quartz vibrates at a constant ultrasonic frequency that would provide the frictionless beat needed to operate the clock. It was the most accurate clock yet invented and could keep time to the thousandth of a second.
Atomic Clock
The quartz clock was beating for accuracy in 1948 with the atomic clock. It keeps track of time by monitoring the vibrations of a caesium atom and can keep track of time to the billionth of a second.
Defining the Second
It would seem that determining the second would be an easy task. It is 1 60th of a minute, and therefore 1 86,400th of a day. In fact, until the 20th century, this was the official definition of the minute.
It had not always been the definition. After all, for the first few millennia of timekeeping, no exceedingly perfect device existed, and there was no need for an exact description of the second. The original hours did not even have strict definitions.
Unfortunately, the Earth is not an exceedingly accurate timekeeper either. The rotation of the Earth is continually slowing. While only fractions of fractions of a second, it is slowing down and providing longer and longer days.
Trying to follow the length of the day by the position of the Sun is also tricky, as there are always variations between one day and the next.
In 1967 it was decided that a new exact definition which would never change needed for the second. The 13th General Conference of Weights and Measures determined that the new definition of the second would be 9,192,631,770 vibrations of the caesium atom (the same measurement used to assess time with the atomic clock). It is now the official definition of the second, and our minutes, hours and days are also upon this definition.
Leap Seconds
Because the rotation of the Earth no longer defines the second, it is required to occasionally make adjustments to the clock to keep us in line with astronomical time. Like leap years, we also have leap seconds, where clocks are put back by a single second. These leap seconds generally go unnoticed being so minor, although they have been happening since 1972.