Hour

An hour (symbol: h; also abbreviated hr) is a unit of time. It is exactly equal to 3,600 seconds. We have 60 minutes in an hour, and there are 24 hours in a day.

The idea of an hour began a long time ago in the ancient Near East. At first, an hour was a changing amount of time, equal to 1⁄12 of either the night or the daytime. These early hours changed with the season and place.

Later, people made hours equal for everyone, defining them as 1⁄24 of the day from noon to noon. As the Earth's rotation was not always the same, the hour was finally set not by the Earth, but by the atomic or physical second.

Today, the hour is a unit that can be used with the metric system. Sometimes, an hour can be a little shorter or longer by one second — this is called a leap second and helps keep our time very close to the Earth's natural day.

Etymology

The word "hour" comes from old languages like Anglo-Norman and Middle English. It started from the Old French word ure, which came from Latin hōra and Greek hṓrā. These words were used to talk about any amount of time, like seasons or years.

We often talk about the time of day using hours. On a 12-hour clock, we say things like "ten o'clock" for both 10 am and 10 pm. On a 24-hour clock, sometimes called "military time," we might say "ten hundred" for 10:00 am. We also use phrases like "a quarter past" for 15 minutes after the hour and "half past" for 30 minutes after the hour.

Time using a 48-hour day moves twice as fast to match the regular 24-hour day.

History

Antiquity

Ancient Egypt

In ancient Egypt, the flooding of the Nile River was very important for farming. This flooding happened when the star Sirius appeared in the sky before sunrise. The Egyptians noticed that 12 groups of stars appeared during the night. Because of this, they split the night into 12 equal parts. These parts were called "seasonal hours" and were shorter in summer and longer in winter. Later, they also split the day into 12 parts, which became more important than the night parts. These ideas spread to Greece and then to Rome.

Ancient Greece

The ancient Greeks had a different way of telling time. Instead of splitting the day into 24 equal parts, they split the time from sunrise to sunset into 12 "seasonal hours." The length of these hours changed with the seasons. At first, they only split the day into 12 hours and the night into three or four watches.

By a certain period, the night was also split into 12 hours. One person possibly first divided the whole day and night into 24 hours. Another person built a time-telling structure called the Tower of the Winds in Athens. This structure showed the time using sundials and moving parts.

The idea of special times for prayers came from older traditions. By the year 60, followers of a certain belief were told to say a special prayer three times a day. This idea continued, and by the second and third centuries, leaders wrote about praying in the morning, evening, and at the third, sixth, and ninth hours. During special nights before celebrations, people stayed awake and prayed.

There were also 12 representations of the hours of the day, describing different parts of the day and night.

Middle Ages

Medieval astronomers split the hour into 60 minutes, and each minute into 60 seconds. This idea came from ancient Babylonian astronomy.

In medieval Europe, people still used the old Roman way of marking hours on sundials. But the most important times were set by the church. Bells rang at certain times during the day: about 6 am, 9 am, noon, 3 pm, and either 6 pm or sunset. Other bell times happened irregularly during the morning, evening, and night. In 1963, changes were made for one church, but another church still follows these old times.

When mechanical clocks were invented, they needed to be adjusted every day to show the correct time. Using 24 hours in a day made the hours more even, so the clocks needed adjusting only a few times a month.

Modernity

The way we measure time today smooths out small changes in the Sun's motion. Scientists noticed that the Earth's rotation is very slowly changing because of the Moon's pull, which makes days get a tiny bit longer over time.

During a big change in France, they tried a new way of measuring time called decimal time. In this system, one hour was one-tenth of a day, split into 100 decimal minutes, and sometimes into 10 tenths. People had to start using this in 1794, but it was stopped six months later. Some places kept using it until 1800.

Today, time is measured using the metric system, based on the second. An hour is exactly 3,600 seconds. However, sometimes an extra second is added to keep our time very close to the Earth's actual rotation.

Now, clocks and timers are everywhere, and most jobs measure how many hours people work. Many people around the world worked to have an eight-hour day. We also have rush hours when many people are traveling, and happy hours when drinks cost less. There are also special records, like the farthest distance someone has cycled in one hour, which is a big honor in cycling.

Counting hours

Many different ways of counting the hours have been used. Because sunrise, sunset, and noon are easy to notice, most early societies started counting time from these points rather than from midnight. Today, with accurate clocks and modern tools, this is less of an issue.

Astrolabes, sundials, and astronomical clocks sometimes show the hour using older methods and definitions.

Counting from dawn

In ancient and medieval cultures, people often started counting hours with sunrise. Before electric lights, daily life revolved around daylight, so routines began when there was enough light.

"Babylonian hours" split the day and night into 24 equal parts, starting at sunrise. They were named this way because people long ago thought the Babylonians did this, though they actually split the day into 12 parts or 60 smaller parts.

Unequal hours

Main article: Unequal hours

Sunrise began the first hour, midday ended the sixth hour, and sunset ended the twelfth hour. This meant hours changed length with the seasons. In places far from the equator, summer hours were longer than winter hours, each being one twelfth of the time from sunrise to sunset. These changing hours were called temporal, unequal, or seasonal hours. They were used until mechanical clocks helped people adopt hours of equal length.

This system was also used in Jewish law and is often called "Talmudic hour" (Sha'a Zemanit). A Talmudic hour is one twelfth of the time from sunrise to sunset, so day hours are longer than night hours in summer and the opposite in winter.

The Indic day also began at sunrise. The term hora meant an hour. Time was measured by the length of shadows during the day. A hora equaled 2.5 pe. There were 60 pe in a day, 60 minutes in a pe, and 60 kshana (a snap of a finger or instant) in a minute. Pe was measured using a bowl with a hole placed in still water; the time it took for the water level to change was one pe. Kings often had an officer in charge of this timekeeping.

Counting from sunset

In what is called "Italian time," "Italian hours," or "old Czech time," the first hour began at sunset, marked by the Angelus bell or half an hour after sunset, depending on local customs and location. Hours were numbered from 1 to 24. For example, in Lugano, the sun rose during the 14th hour in December and at noon during the 19th hour; in June, the sun rose during the 7th hour and noon during the 15th hour. Sunset always ended the 24th hour. Church clocks only struck from 1 to 12, so only during night or early morning hours.

This way of counting helped people know how much time they had left to finish their work without using artificial light. It was common in Italy by the 14th century and lasted until the mid-18th century. It was officially stopped in 1755, though some places kept the custom until the mid-19th century.

You can still see this system on clocks in Europe, where the dial is numbered from 1 to 24 in Roman or Arabic numbers. The St Mark's Clock in Venice and the Orloj in Prague are famous examples. It was also used in Poland, Silesia, and Bohemia until the 17th century.

It was replaced by dividing the day into twice twelve (equinoctial) hours, also called small clock or civic hours, as early as the 16th century.

Until 1968, Saudi Arabia counted 24 equal hours, but the first hour began at sunset.

Counting from noon

For many centuries, up to 1925, astronomers counted hours and days from noon because it was the easiest solar event to measure precisely. An advantage of this method, used in the Julian Date system (where a new Julian Day starts at noon), is that the date doesn’t change during a single night of observation.

Counting from midnight

In the modern 12-hour clock, counting hours starts at midnight and restarts at noon. Hours are numbered 12, 1, 2, ..., 11. Solar noon is always close to 12 noon, though it can vary by up to fifteen minutes because of the equation of time. At the equinoxes, sunrise is around 6 a.m. (Latin: ante meridiem, before noon), and sunset around 6 p.m. (Latin: post meridiem, after noon).

In the modern 24-hour clock, counting starts at midnight, and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again varying because of the equation of time. At the equinoxes, sunrise is around 06:00, and sunset around 18:00.

History of timekeeping in other cultures

Further information: History of timekeeping devices

Egypt

Further information: Egyptian calendar, Decans, and History of timekeeping devices in Egypt

The ancient Egyptians began dividing the night into parts called wnwt long ago. By around 2150 BC, they noted the rising of 36 stars or groups of stars, called "decans," to separate the night into 12 parts. Each part lasted about 40 minutes. During the day, they used sundials to divide the daylight into 12 hours. The hours were linked to religious ideas, with each hour linked to a part of the sky or the world below, guarded by protective spirits.

East Asia

Main article: Traditional Chinese timekeeping

In ancient China, the day was divided into 100 parts called "marks" or kè, measured using water clocks and sundials. Later, the day was split into 12 "double hours" named after animals and ideas from their calendar system. These were used in China and later spread to Japan and other nearby lands. Over time, these ideas changed, and today, people use numbered hours instead.

Southeast Asia

Main article: Thai six-hour clock

In Thailand, Laos, and Cambodia, an old way of telling time split the day into six parts. For example, 7 am was the start of the first daytime part, and 7 pm started the first nighttime part. This way of keeping time came from old practices of announcing the time with gongs during the day and drums at night. It is mostly used only in Thailand today.

India

Main article: Hindu units of time

Ancient texts from India used different ways to measure time, such as dividing the day into 30 parts called muhūrta-s, each lasting about 48 minutes, or into 60 parts called dandas lasting about 24 minutes each. Later, the day was split into 60 parts called ghaṭikás, each broken into smaller pieces. Nearby cultures had their own names for these parts, too.

Derived measures

Here are some important ways we measure things using the word "hour":

• air changes per hour measures how often air is replaced in a room, which helps keep the air clean inside buildings.
• ampere hour is a way to measure electric charge, used in chemistry and electricity studies.
• BTU-hour is a unit to measure power, especially for heating and cooling machines.
• credit hour tells us how much classroom time a school course requires each week during a semester.
• horsepower-hour measures energy used in trains and railroads.
• hour angle is a way to measure angles in the sky, used when studying stars and planets.
• kilometres per hour and mile per hour are both ways to measure how fast something is moving on land.
• kilowatt-hour is a common way to measure energy used in homes and businesses.
• knot measures speed for ships and airplanes, using nautical miles per hour.
• man-hour counts the amount of work one person can do in an hour, used to see how efficient work is.
• metre per hour is a way to measure very slow speeds.
• passengers per hour per direction measures how many people a bus or train can carry each hour in one direction.
• pound per hour measures how much fuel an engine uses in an hour.
• work or working hour is used in rules about jobs, like how many hours people can work in a day.