Timekeeping Devices Throughout History

by Jennifer Wang

Since the beginning of civilization, humans have created systems to track change and development in the world around them. The concept of time has allowed people of all ages to meet at a set time to exchange ideas, objects, and build civilizations.

The first time tracking devices were based off of the cycles of celestial objects. Sunrises and sunsets marked the changing of days. When they were combined with the cycles of the moon, these days became months. More than 5,000 years ago, Egyptians used the star Canis Major to create 365 day cycles to predict the annual flood of the Nile. 4,000 years ago, Stonehenge was built in England to mark solar and lunar eclipses, winter and summer solstices, and other annual astronomical events. Before 2000 BCE, Babylonians developed twelve lunar months that had days alternating between 29 and 30. Four thousand years later, we still use the same system of tracking time.

Around 3000 BCE, Egyptians started to divide their days into smaller portions. Tall thin stone structures called obelisks stretched into the sky and cast shadows that moved with the sun. Depending on how the shadows fell next to markings on the ground, ancient Egyptians were able to split days into hours that governed the day-to-day activities of their cities. Around 2000 BCE, water clocks started to appear in Egypt, Greece, and China as a method to measure time after dark. Water clocks measured the steady drip of water through a hole in the bottom of a bowl. In Greece and China, bells and gongs were connected to the contraptions to ring at certain times of day. Candle clocks from China employed a similar system and measured time with candles that burned at a consistent rate. After a period of technological decline in the Dark Ages, mechanical clocks started to appear between the 15th and 16th centuries. Popular designs employed weight driven pendulums that measured out seconds with each swing. Others used spring driven clocks because of their smaller size. In the 1920s, quartz clocks finally made watches possibly. The piezoelectric property of quartz causes it to produce an electric field when compressed, and to change shape when exposed to an electric field. Watchmakers utilized this property to create crystals that oscillated in an electric field, producing accurate and lightweight watches.

On an atomic scale, all atoms vibrate at a constant frequency. Since the early 1940s, these frequencies have been used to measure time. In 1967, the natural oscillation of cesium atoms was chosen as the international unit of time. Factoring in the Earth’s revolution around the sun and it’s speed of rotation, 9,192,631,770 cesium oscillations came to define a second. Atomic clocks created to measure cesium oscillations are extremely accurate and take around 20 million years to lose a second of time. The International Bureau of Weights and Measures has 400 atomic clocks spread across 69 global labs that keep consistent measure of time– sometimes too consistent. As the Earth’s rotation slows down, days get longer and the cesium oscillation to seconds ratio becomes more inaccurate. This and the Earth’s imperfect revolution around the sun is why leap years and leap seconds are necessary. Though seemingly tedious and unnecessary, these seconds make a large difference in scientific experiments, aviary technology, and satellite navigation.

From ancient structures as large as Stonehenge to clocks the size of atoms, the need to keep time has persisted through thousands of years. It’s thanks to these inventions that we can decipher what has happened, record what is happening, and predict what will happen

 

Sources:

http://www.timeanddate.com/time/international-atomic-time.html

http://www.nist.gov/pml/general/time/early.cfm