Hour

An hour is a fundamental unit used to measure the passage of time. In our modern world, one hour is defined as exactly 3,600 seconds. This precise measurement is part of the International System of Units (SI). While the hour is a non-SI unit, it is widely accepted for use alongside the metric system. We use hours to organize almost every aspect of human life. They allow us to schedule work, track travel, and understand the rhythm of our planet.

The way we define an hour has changed significantly over thousands of years. In the ancient Near East, hours were not constant lengths. Instead, they were seasonal hours, also known as temporal or unequal hours. In these systems, the length of an hour changed based on the season and the latitude. For example, a daytime hour in the summer would be much longer than a daytime hour in the winter. Eventually, these variations were smoothed out by using the mean solar day to create equal or equinoctial hours.

Ancient civilizations developed different methods to divide their days. In ancient Egypt, the flooding of the Nile was vital for farming. This event was linked to the rising of the star Sirius and the appearance of 12 constellations. Because of this, Egyptians divided the night into 12 intervals. Later, they also divided the day into intervals. The ancient Greeks also used seasonal hours. They divided the time from sunrise to sunset into 12 parts. They also divided the time from sunset to sunrise into 12 parts. This total of 24 parts was likely first established by the astronomer Hipparchus of Nicaea.

History shows us many ways to track these passing hours. In the first century BCE, Andronicus of Cyrrhus oversaw the Tower of the Winds in Athens. This structure used sundials and mechanical indicators to track a 24-hour day. During the Middle Ages, astronomers like al-Biruni and Sacrobosco introduced new math. They divided the hour into 60 minutes. Each minute was then divided into 60 seconds. This system was influenced by Babylonian astronomy. In that system, these terms described the Sun's motion through the ecliptic.

Religious practices also shaped how people experienced the hour. In early Christianity, the "canonical hours" were used for prayer. These were based on practices from Second Temple Judaism. By the second and third centuries, Church Fathers wrote about prayers at specific hours. The night was sometimes divided into four "vigils" or watches. Each vigil lasted three hours. In medieval Europe, church bells signaled these hours to the community. These included times like prime at 6 a.m. and sext at noon.

Even the definition of an hour has faced radical changes. During the French Revolution, France attempted to use decimal time between 1794 and 1800. In this system, a French hour was one-quarter of a day. It was divided into 100 decimal minutes. This system was eventually suspended after Napoleon's coup in 1799. Today, we use Coordinated Universal Time (UTC) as our primary standard. To keep UTC within 0.9 seconds of UT1, which is based on the mean solar day, we sometimes use leap seconds. An hour might occasionally last 3,599 or 3,601 seconds to account for the Earth's slowing rotation.

In modern society, the hour is everywhere in our language and economy. We use 12-hour clocks with the phrase "o'clock" for whole hours. We also use 24-hour clocks, often called military time. In this system, 10 p.m. is expressed as twenty-two hundred hours. We use fractions of the hour in daily speech, such as "a quarter past" or "half past." The hour also defines social and economic concepts. Labor movements have fought for the eight-hour workday. We also see the term in "rush hour" for traffic or "happy hour" for discounted drinks. Even in sports, the hour is a major milestone, such as the distance a cyclist can travel in one hour.