The Essence of Time: Monumentally Important Clocks

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.
Above:

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.
Image: NIST