Clock

A clock is a tool that measures and shows the time. Most clocks divide the day into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. Clocks are found almost everywhere people live: on walls, on phones, on towers, on wrists, and inside computers and cars.

The first clocks were not mechanical. Ancient people watched the sky. They tracked the position of the sun during the day and the stars at night. The sundial, invented thousands of years ago in Egypt, used a shadow to mark the hours. But sundials only worked when the sun was out. So people also built water clocks, which measured time by how fast water dripped from one bowl into another. Hourglasses, which use sand, came later.

Mechanical clocks appeared in Europe around the 1300s. Early ones lived in church towers and rang bells to call people to prayer. They used heavy weights and gears to keep moving. These clocks were not very accurate. They could lose 15 minutes a day, about the time it takes to eat lunch.

In 1656, a Dutch scientist named Christiaan Huygens built the first pendulum clock. A pendulum is a swinging weight, and Huygens used it to keep the gears moving at a steady pace. Pendulum clocks were 100 times more accurate than older clocks. Galileo had figured out the science of pendulums decades earlier, but he died before building one.

Better clocks changed the world. Sailors needed accurate clocks to find their position at sea. Trains needed them to run on schedule. Factories used them to organize the workday. Before clocks were common, most people did not know the exact time, and they did not need to. The Industrial Revolution made the minute hand matter.

Modern clocks use crystals or atoms instead of pendulums. A quartz clock, the kind in most watches, uses a tiny crystal that vibrates 32,768 times per second when electricity flows through it. The clock counts the vibrations to keep time. Atomic clocks, the most accurate clocks ever built, count the vibrations of cesium atoms, which buzz more than 9 billion times per second.

The GPS satellites that help phones find their location depend on atomic clocks. If those clocks were off by even a millionth of a second, your map could place you a quarter of a mile from where you actually stand.