From humanity's earliest days we've kept a close eye on time. The way we recorded its passage revealed what was important to us: To stay on schedule for planting and harvesting, the ancients tracked the seasons; for rituals and divination, they kept watch on the movements of the stars and planets; clocks allowed sailors to calculate longitude as they explored the world. Today, ubiquitous, accurate and synchronized, clocks keep civilization aligned.
The world’s highest-tech clocks, advanced atomic clocks are now so accurate they vary less than the orbit of the Earth. At the lab responsible for keeping official time in America, the National Institute of Standards and Technology in Boulder, Colorado, moving an atomic clock from one floor to another can require recalibration to correct for the relativistic effects of a change in altitude. The greater the pull of gravity, the slower a clock runs.
We weren't always such obsessive second-splicers. Hours were standardized only a few hundred years ago. Before that, sundials marked 12 hours of day and 12 of night, year-round: A daylight hour expanded in length in summer and contracted in winter.
Time-keeping devices like Stonehenge, the Aztec calendar, and the Antikythera Mechanism were more about keeping people on time for seasonal and celestial appointments, not daily ones. Now, an entirely new type of clock is being built in west Texas. Designed to last 10,000 years, it trumps all other clocks in its long-term view. To remind us of its roots, we've gathered some of the most important time-keeping devices throughout the ages.
NIST-F1 Cesium clock
The NIST-F1 cesium fountain clock keeps official U.S. time at the National Institute of Standards and Technology. Once every 60 million years, the clock might gain or lose a second. The lab operates even more advanced clocks, accurate out to 3.7 billion years.
Iconic and much-studied, Stonehenge stands on the plains of Wiltshire, England, and remains a bit of a mystery. The builders left no written records, but archaeologists currently estimate that construction began about 5,000 years ago, around the time Troy was being founded in the Mediterranean. Construction continued in three major stages, over many centuries.
However the site was used — in burial, sacrifice, worship or healing — it seems designed to mark the solstices, equinoxes and eclipses of the sun and the moon.
Two primary stone types make the monument. The enormous "pi"-shaped structures forming the outer circle are sandstone blocks, known as sarsen stones, from about 20 miles north of the site. These were possibly floated down the river Avon to be close to the site. The smaller bluestones were originally from Wales, nearly 250 miles away.
Either manpower or glacial ice could have brought them to the site. These stones, which turn blue-grey in the rain, are arranged inside the circle of sarsen stones.
Stonehenge was reconstructed in the first half of the 20th century to look like it did thousands of years ago. Many of the stones standing now were re-erected or re-positioned with cranes and set in concrete to prevent them from falling.
Image: Mark Whatmough/Flickr
The Aztec calendar is one of several calendars from Mesoamerica. It monitors both the 365-day agricultural cycle and the 260-day ritual cycle. When these cycles align every 52 years, a new century begins, celebrated by a 12-day festival.
In the ritual cycle, days are allotted to deities, maintaining the spirit-world power balance. The age-lined face of the sun god, or Lord of Heaven, is the center of this stone calendar. This stone was found lying facedown in the main square of Mexico City, formerly the Aztec capital. It was incorporated into a tower wall of the city's cathedral, where it stayed for nearly a century. In 1885, it was taken to the National Museum of Anthropology and History in Mexico City.
The earliest sundials still existing today are from Egypt, dating from about 3,500 years ago. Sundials come in dozens of varieties: sky-poking obelisks, tilted sticks in the ground and handheld models — the equivalent of a modern PDA. (Download a cut-out kit for a sundial dating from 1596.)
To be accurate, beyond needing a sunny day, sundials must be corrected based on latitude and time of year. Despite their drawbacks, sundials continued to be used for many years after the invention of mechanical clocks. Prior to World War I, France used sundials to synchronize their train travel.
The Antikythera Mechanism
This mechanical wonder was found by a diver exploring a shipwreck off the coast of Greece in 1900. It is an ancient analog computer, built around 150 BC, that calculated astronomical positions and the dates of upcoming Olympic games.
The machine is built as an interlocking system of gears and rivals 19th-century Swiss clocks for accuracy and complexity. Recently, Apple software engineer Andrew Carol built a 1,500-part LEGO replica. With it he accurately predicted the upcoming solar eclipse of April 8, 2024.
Mechanical clocks, driven by weights, began to be built around 1300 AD. Their purpose wasn't to tell time on a dial, but to describe the locations of astronomical bodies and gong hourly. The earliest surviving example, dating from 1386, is a cathedral in Salisbury, England. The clock pictured above, of the same era, is mounted on Old Town City Hall wall in Prague's Old Town Square and has some parts that date from 1410.
Galileo designed a clock using a pendulum as the time keeping element in 1635. Dutch scientist Christiaan Huygens designed and commissioned the first working model, in 1656 (shown in the book above). A pendulum dramatically improved a clock's ability to keep time and became the standard for precise clocks.
Well-run pendulum clocks in the 1660s differed by only about 15 seconds a day. Non-pendulum clocks of the same era (ones that used rotating wheels) varied by about 15 minutes a day. The National Institute of Standards and Technology used pendulum clocks to keep official time until the 1930s.
Image: Christiaan Huygens, clock and model displayed at the Museum Boerhaave in Leiden, Netherlands. Rob Koopman/Flickr
Accurate, portable clocks, like those designed by John Harrison, were a huge boon to mariners.
Previously, navigators at sea used the sun to calculate how far north or south they were, but until the mid-1750s, it was impossible to calculate longitude.
Pendulum clocks were rendered useless on a ship by humidity and movement of the waves. Consequently, it was impossible for navigators to keep an accurate record of time at their starting point, which they could compare to local time and calculate the distance they had traveled east or west.
To solve the problem of finding longitude at sea, England's King Charles II founded the Royal Observatory in 1675. He offered an enormous cash prize to anyone who could build a clock that would remain accurate at sea.
John Harrison, a working-class joiner with relatively little education eventually won the prize after building a series of clocks, each improving on the previous one. Above, Harrison Number One or H1 took Harrison five years to build. He went on to build other models H2 and H3, and won the prize with H4. His models are now on display at the National Maritime Museum in Greenwich, London.
Image: © National Maritime Museum, London
Great Clock of Westminster
Most people think of this London landmark as Big Ben, but Big Ben is actually the name of the largest bell inside this pendulum clock. Completed in 1849, the faces on the sedately named Great Clock of Westminster measure 23 feet square.
The Doomsday Clock
The Doomsday Clock isn't a clock and it doesn't keep time. It's a metaphor introduced by The Bulletin of the Atomic Scientists to gauge public safety. The "clock" debuted in 1947 at seven minutes to midnight because of worries about nuclear proliferation.
Over the years the hand has nudged nearer to and further from midnight — representing utter self-destruction — as Cold War tensions have risen and fallen, treaties have been signed, and new threats have appeared. It stands currently at six minutes to midnight.
The Bulletin cites concerns over North Korean and Iranian nuclear ambitions, the arsenal retained by the United States and Russia, and wide-spread ecological damage.
Image: The Bulletin of the Atomic Scientists
NASA Countdown Clocks
When the last space shuttle readies for launch from the Kennedy Space Center, scheduled for July 8, NASA's giant digital countdown clock will be one of the most watched time pieces in the world. Shuttle countdown clocks begin at T-43 hours and counting, which begins the call to stations and a series of checks and inspections leading up to lift off.
Image: NASA/Jim Grossmann
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