Keeping Time: From Sundials to Atomic Clocks

Sundial is a type of clockThe interest in keeping a record of time has progressed over thousands of years with timepieces ranging from ancient sundials to modern atomic clocks. The earliest examples of measuring time were based on observing the position of the sun, moon, and constellations. This practice was followed by man-made devices, traced to early civilizations, using the shadow of the sun to determine the time of day as well as those that could function on cloudy days or at night. While some of this information has been lost to the ages, inventions of some timekeeping devices have been recorded dating back to Egyptians of the 16th century B.C.

One form of keeping time was a portable shadow clock or sundial developed by the Egyptians around 1500 B.C. This was a long stem with an elevated crossbar at one end that would cast a shadow that lengthened or shortened, depending on the location of the sun. In the mornings, the stem was oriented in an east-west direction and the crossbar was on the east end. The long stem had five marks of variable spacing. Since the day was divided into 10 parts with two additional parts representing morning and evening twilight, the device was turned around to measure the afternoon shadows.

Keeping Time with Water Clock
Water Clock

Another type of clock was in use around 1500 B.C. that did not rely on observation of planets or stars or sunlight. This was the water clock which worked in one of two ways: Either the water dripped at a consistent rate from a small hole at the bottom of a stone vessel with sloping sides, or bowl-shaped containers were designed to fill as water came in at a steady rate. The inside of the containers had markings that determined the time based on the water level. A water clock was found buried in the tomb of the Egyptian pharaoh, Amenhotep I, around 1500 B.C.

The Egyptians developed the oldest known astronomical tool, the “merkhet,” around 600 B.C. Merkhets were used in pairs and aligned with the Pole Star to establish a north-south line or meridian. Astronomers could use the meridian to determine the night hours based on when stars crossed it.

Greek and Roman astronomers and clock makers elaborated on the Egyptian water clocks, making them more mechanically complex. The water flow became more consistent as the result of better regulated pressure. The clocks also took on a more intricate design that included bells, gongs, doors and windows that opened to reveal miniature figurines and designs that reflected the astrological constellations.

Throughout most of the Middle Ages, Europe continued to rely on sundials. Some were designed to place above doorways and, by the 10th century, pocket sundials were in use. During the early 14th century, towers in big European cities, particularly in Italy, began to display large mechanical clocks that were weight-driven and operated by advancing a gear train at regular ticks or intervals. This type of movement is called “verge and foliot” escapement and was the basis for large public clocks as well as pocket watches through the mid-19th century.

Hourglasses also became popular in the 14th century. They were used aboard ships to measure time since the motion at sea did not affect their accuracy. In addition, seamen used it to determine longitude, the distance east or west from a fixed point. Since the hourglass used granular material like sand or salt, instead of water, it was not subject to corrosion or condensation.

Spring-powered clocks were invented between 1500 to 1510. The heavy weights in mechanical clocks were replaced with a mainspring. These clocks were very popular with wealthy people because they were smaller and could fit on a shelf or a table. Their disadvantage was that the clocks slowed down as the mainspring unwound.

Keeping Time With a Pendulum Clock
Pendulum clock

Pendulum clocks, such as the grandfather clocks that became very popular in the United States during the 18th and 19th centuries, were invented in 1656 by a Dutch scientist, Christiaan Huygens. His clock had an error rate of less than one minute per day. It was the first time any clock had achieved this level of accuracy. After additional refinements, his clock’s error rate was less than 10 seconds per day.

The next 100 years, from the mid-1600s to the mid-1700s, saw several major developments in clock making. Huygens, the same scientist who had developed the pendulum clock, also developed a balance wheel and spring assembly in 1675. This allowed for the portable watches of that period to keep time with an error rate of only about 10 minutes a day. Some of today’s wristwatches still use this type of assembly.

During this same time, London clock makers were making improvements on the pendulum clocks with a new mechanism that did not interfere with the pendulum’s motion. Within the next 50 years, the pendulum clock’s accuracy rate was down to within one second per day.

Clock makers were also striving for improvements in time keeping devices used at sea. John Harrison built a marine chronometer using a spring and balance wheel escapement. This was around 1761. His device was so accurate that it could determine longitude to within one-half degree after being on a ship that had sailed to the West Indies. It kept time to within a fifth of a second per day. It won a top prize from the British government which, in today’s U.S. currency, would be worth several million dollars.

The late 19th and early 20th centuries saw a new kind of pendulum clock. These had two pendulums, one larger than the other. The smaller one did the required mechanical tasks such as giving the larger one the gentle pushes needed to stay in motion while the larger one was free of any duties that could interfere with regularity. No sooner than these clocks had been perfected, quartz crystal clocks and oscillators were developed in the 1920s.

Clocks with quartz crystals did not have any gears, springs or other types of escapements to get in the way. Quartz is one of those materials that accumulates an electric charge as a result of mechanical stress. This piezoelectric property is what makes the crystal vibrate with constant frequency. Despite their accuracy, there is one other type of clock that has surpassed them: the atomic clock.

Keeping Time With Atomic Clocks
NIST Atomic Clock (1999)

The first atomic clock was built in 1949 by the National Institute of Standards and Technology (NIST). It was based on ammonia but didn’t exceed the existing standards. Scientists focused on cesium which was more promising. The National Physical Laboratory in England built the first cesium atomic frequency standard in 1955. Their collaborated with the U.S. Naval Observatory (USNO) to establish the necessary frequency of the cesium reference to astronomical time. The NIST was using these standards in their official timekeeping system by 1960. By 2002, the NIST’s latest cesium standard has an accuracy level of 30 billionths of a second per year.

There are new versions of time keeping standards under development. In this day of split second races, stock markets, transportation and technology, the days are long gone when time can be off by a quarter of an hour. At one time, clocks like sundials, hourglasses and pendulum clocks were modern and considered to be the latest invention for keeping time. Thousands of these items from the past are in the National Watch & Clock Museum in Columbia, Pennsylvania. Their exhibits also include today’s atomic clocks.

By Cynthia Collins

History of Telling Time

National Institute of Standards and Technology

National Watch & Clock Museum

3 Responses to "Keeping Time: From Sundials to Atomic Clocks"