November 30, 2000

What's the Time?

The Units of Timekeeping  

Plants and animals know how to distinguish night from day, midnight from noon, dusk from dawn. Yet only humans "keep time" and are concerned about being "on time." How have the standards for measuring time changed throughout recorded history?

Since ancient times, humans have attempted to organize the cycle of the seasons. Without even understanding that Earth revolves around the Sun, they built calendars based on a solar year. Over 5,000 years ago civilizations had also started the quest for accurate ways to tell the time of day as Earth rotates on its axis. They used observations of the Sun and other heavenly bodies to indicate times like noon. But the challenge facing developing cultures was how to divide the day into regular units that could be synchronized, even in cloudy weather.

The ancient Egyptians made several contributions to horology, the science of measuring time. Around 1500 B.C., they developed a sundial, onto which they divided the daylight hours into 10 equal parts. They also defined two hours as "twilight hours," one in the morning and one in the evening. Historians believe that the Egyptians used an early astronomical tool called a merkhet at night to mark the passage of "clock stars," specific stars that were equally spread across the sky. During the summer night, 12 clock stars passed the merkhet.

With a 10 hour day, 2 twilight hours and 12 hours of night, the Egyptians arrived at a 24-hour day. Since an hour was always 1/12 of the period of light or darkness, it was not a fixed quantity. In the summer, for example, a daylight hour was longer than a nighttime hour.

The next major step forward came from the Babylonians, between approximately 300 and 100 BC They used the sexagesimal—or base-60—system for their astronomical calculations. Although no one knows why they chose 60, one reason may be because base-60 makes divisional operations easy since 60 is divisible by 2, 3, 4, 5, 6, 10, etc. Just as base-10 can be divided into decimal places, base-60 can be divided into fractional places. The first fractional place is called a minute, the second place is called a second. These fractional place names were applied to hours, as well as to degrees for measuring angles.

Time Units

Given that a "day" is a unit equaling one rotation of Earth on its axis, suppose that there are no accepted subdivisions of a day into smaller units, i.e., no hours, minutes, or seconds. After all, these units are arbitrary without any physical basis.

Create your own system for dividing a day into smaller units.

  1. Decide how many types of units and sub-units you need. Name them.

  2. Decide how many of each type of unit and sub-unit constitute a day (e.g., 60 seconds = 1 minute; 60 minutes = 1 hour; 24 hours = 1 day).

  3. Design the face of a clock that measures time in your units.

When you are done, you can see what one university professor suggests. But remember, his proposal is no more "correct" than yours.

Pendulum Clocks  

As long as clocks were dependent on observation of the heavens and on days that varied slightly in length over a year, time measurement could not be precise. In 1656, Dutchman Christiaan Huygens built a pendulum clock, the first accurate mechanical clock, which worked on naturally cycling periods instead of astronomical cycles. Huygens's clock actually measured the beat of its pendulum, which was synchronized to the average length of a day, called mean solar time.

A pendulum is a body suspended from a fixed point so that it can swing back and forth. Galileo had noted half a century earlier that the time for each complete oscillation—called the period—is constant. The period is affected by the length of the material that connects the body from the fixed point, and by the pendulum's position in relation to Earth. If a pendulum clock's period is synchronized with mean solar time, it will keep reliable time. Huygens's first pendulum clock was accurate to within one minute a day.

For added precision, Huygens used escapements, a system that controls the transfer of energy of the gravitational force acting on the weights to the clock's counting mechanism. Over the centuries additional improvements were made to pendulum clocks, until in 1906, they were driven by self-contained batteries for the first time. These clocks were accurate to within one second a day.

The Quest
for an Accurate Clock

According to the National Institute of Standards and Technology, a clock must have two basic components:

  1. A regular, constant, or repetitive process or action to mark off equal increments of time.

  2. A means of keeping track of the increments of time and of displaying the result.

Timekeeping through the ages has been an ongoing "search for ever more consistent actions or processes to regulate the rate of a clock." Read about the history of clocks at the following two sites:

Quartz Crystal Clocks  

The next major leap forward in timekeeping came in 1927, when W. A. Marrison introduced the quartz crystal clock. A crystal changes shape when exposed to an electric field and it generates an electric field when under mechanical stress. This phenomenon, called the piezoelectric effect, leads to vibrations at extremely high frequencies. The vibrations have a constant period, making them a reliable counting mechanism in clocks. Quartz crystal clocks are accurate to within thousandths of a second.

Yet quartz crystal clocks have disadvantages. Temperature changes and impurities in the quartz cause a drifting in the vibrations over time. Also, the period of the vibrations is dependent on the crystal's size and shape, so two quartz crystal clocks may not have identical frequency.

The quartz crystal clocks were so accurate that their precision emphasized the problematic nature of setting a standard for the length of a second. Throughout the first half of the twentieth century, a second was defined as 1/86,400 of a mean solar day. Scientists realized that there were such irregularities in Earth's rotation, that using it as a standard caused variations in the length of a second. In 1956, the International Committee on Weights and Measures redefined the second as 1/31,556,925.9747 of the length of the seasonal year 1900. This new definition was also problematic, since the year 1900 had already passed and no time measurements could be made against it.

  • Suppose you are a sprinter trying to break a world record that is measured down to thousandths of a second. Why is it important to you that the official clock measures the length of a second as precisely as possible?

Atomic Clocks
As human knowledge advanced, the need for even more accurate timekeeping arose. Space travel, satellite communications, and electric power all depend on extreme precision in time measurement.

  • Many of the terms in the following explanation may be new to you. You can learn more about an atom's ground state, excited state, and the energy changes as an electron makes the transition from one level to another in the Chemistry Gateways activity, Emission Spectra and the Bohr Atom.

Scientists identified a "natural pendulum" with a period so reproducible that it has become the basis for today's most accurate clocks: the transition frequency of atoms. When an atom absorbs or releases energy, the resulting radiation has frequency. All atoms of an element are identical as they have been for millions of years, and they all share identical transition frequency.

For atomic clocks, scientists chose the cesium-133 atom. The first cesium atomic clock was built in 1955 by Britain's National Physical Laboratory. Cesium atoms are exposed to microwave radiation, which is emitted or absorbed by the energy change—or quantum transition—of the atom. These transitions produce extremely regular oscillations of electromagnetic radiation. This frequency controls the electronic display of the clock.


In 1964 the International Committee on Weights and Measures redefined a second as "9,129,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom." For the first time, the definition of a second was no longer dependent on solar time. The International System of Units continues to use this definition today.

The atomic clock is so accurate that if left unadjusted, it would eventually drift away from the less precise solar time, just as the seasons would drift away from the solar year if there were no leap year. As a corrective, a "leap second" is added when needed to keep solar time and atomic time within 0.9 seconds of each other.

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