Humans have been using clocks to track the passage of time for thousands of years. With such a rich history of timekeeping, there are many interesting little-known facts about clocks out there. To kick off this discussion, we bring you 7 clock facts for the curious clock lover. If you’ve got any clock facts you’d like to share, be sure to let us know in the comments section.

1. The oldest working clock in the world is the Salisbury Cathedral Clock in Salisbury, England, and dates all the way back to 1386.

2. The first American alarm clock was invented in Concord, New Hampshire in 1787 by Levi Hutchins. It was never patented and rang only at one time — 4 a.m. — to wake him for work.

3. The National Institute of Standards and Technology in Colorado created an atomic clock so accurate they claim will neither gain nor lose a second in 20 million years, called the NIST-F1 Cesium Fountain Clock.

4. The term “o’clock” is a contraction of “of the clock” and comes from 15th-century references to medieval mechanical clocks. At the time, sundials were also common timekeepers, so to make clear one was referencing clock’s time they would say something like, “It is six of the clock” — which overtime was shortened to “six o’clock.”

5. Due to changes in local gravity, a pendulum clock that is accurate at sea level will lose nearly 16 seconds per day if moved to an altitude of 4,000 feet or more.

6. In the United States, standardized time zones as we know them were first established by the railroad industry in 1883. Before that, every town kept its own time based on the sunrise and a central clock, which made scheduling trains nearly impossible.

7. Pennies are used to adjust the time in London’s Big Ben clock tower. Adding or removing a penny to the pendulum changes its center of mass and can alter the clock’s speed by 0.4 seconds per day.