Imagine living in a world without clocks, calendars, or even a clear idea of what time it is. There would be no rushing to work, checking the time before bed, or worrying about being late because, for most of human history, people simply lived according to the natural rhythms of the Sun.That began to change thousands of years ago.Around 6,000 years ago, one of the world’s earliest civilizations emerged in Sumer, in what is now Iraq. The Sumerians developed writing, laws, trade, schools, and many other systems that shaped civilization. They also helped create one of the most important systems people still use every day: the measurement of time.Before standardized time existed, people generally organized their lives around sunrise and sunset. The Sumerians began dividing the day and night into periods, creating a system based around 12 parts for daylight and 12 for darkness.Around the same period, ancient Egyptians were developing their own methods of tracking the passing hours. They created some of the earliest known sundials, using the position and movement of a shadow to estimate the time of day.Eventually, the Egyptians developed a way to measure time after the Sun disappeared. Their solution was the water clock, also known as a clepsydra. Water would slowly flow from a container, and markings on the vessel indicated how much time had passed.There was still a problem. The length of daylight and darkness changes throughout the year, meaning an “hour” did not always represent the same amount of time.The ancient Greek astronomer Hipparchus proposed a more consistent approach. He suggested dividing the entire day into 24 equal hours, although fixed length hours would not become common until many centuries later.Mechanical clocks eventually appeared in Europe during the Middle Ages, bringing a major improvement in precision. But even with clocks, telling time remained surprisingly complicated because different towns and regions often followed their own local time.The problem became much more serious during the 19th century as railways, telephones, steamships, and other technologies connected distant parts of the world. In the United States alone, railways recognized dozens of different local times, making accurate scheduling extremely difficult.A solution came from Scottish Canadian engineer Sandford Fleming. In 1884, the world was divided into 24 time zones, creating a standardized system that allowed different regions to coordinate their clocks.Timekeeping continued becoming more precise as technology advanced. Eventually, scientists moved beyond measuring seconds according to the movement of the Earth and developed atomic clocks.Today, the second is defined using the behavior of cesium atoms. An atomic clock measures a second based on 9,192,631,770 cycles of radiation associated with the cesium atom.This extraordinary precision is essential to modern technology. Atomic time helps coordinate systems ranging from telecommunications and broadcasting to global navigation systems such as GPS.After thousands of years of progress, humans have gone from watching shadows move across the ground to measuring time through the behavior of atoms.And considering how much modern life depends on knowing the exact time, it is hard to imagine going back to a world where the only clock was the Sun.