An hour is a unit of time based on the second. For everyday use it is defined as 60 minutes, or 3600 seconds. Its definition has changed over time and for a while was defined as of the mean solar day, which is based on the position of the Sun in the sky.

As of 2019, the hour is defined based on the atomic second, which uses a very specific property of a particular atom that is highly exact. Technically, an hour is defined scientifically as 3599-3601 seconds, because in some cases a leap second, +1 or -1 is necessary to ensure that the measured time is exact.

Like most SI units, there are numerous measurements that are derived from an hour. Some examples you may have encountered include:

Hours on a clock

The time of day in English is usually expressed in terms of hours. For example "10 o'clock" refers to both 10 AM and 10 PM. All whole hours can be expressed by adding "o'clock" after the number, though it can be unclear whether you mean the day or nighttime hour. This is why the system that uses the 24-hour clock, sometimes referred to as "military time," is the standard in most parts of the world and is an international standard.

In the 24-hour clock, the hours start with at 0 for midnight and count up to 23, for 11 PM. When using this system, there are a number of different conventions for saying the time out loud, depending on the context and location. In the US, which uses a combination of the 12-hour and 24-hour clock, the hours 12 and below are often still said as they would be in the 12-hour system. In some cases, people may still say "fifteen o'clock" for times above 12, but this is less common in the US.

Pronouncing military time

Since the 24-hour clock is often referred to as military time, it is common to pronounce times in the 24-hour system as the military does. In this method, the hour is always said first, including the zero if the hour is less than 10. The zero is pronounced as "oh." Whole hours are usually pronounced by saying the hour, followed by "hundred hours." For example, 08:00 would be pronounced "oh eight hundred hours," and 23:00 would be pronounced "twenty-three hundred hours." The "hours" is not mandatory though, and in some cases people may just say "oh eight hundred." Hundred is always used though, even for times such as 10:00 and 20:00, we would not say "one-thousand hours" or "two-thousand."

Although this may seem confusing or difficult to navigate, in most cases when speaking in the context of time, most people will understand the time you are trying to communicate, regardless how you say it.

Examples of how a person may pronounce time in the 24-hour system:

As you can see, there are many different variations on how to pronounce time on the 24-hour clock, but if you want to make it as clear as possible, within the context of talking about the time, you can say each individual number in the time.

Did you know?

Hours were not always counted starting at midnight (00:00). Back when artificial light didn't exist, a lot of societies based their time systems around the daylight and night hours. Many would count hours starting from sunrise with the middle of the day being the end of the sixth hour. Because of this, hours also used to vary in length because of changes in the number of daylight and night hours caused by changes in the season, as well as differences in location. These hours were called temporal, seasonal, or unequal hours. In some cases, people counted hours starting from sunset, or even midday.

It wasn't until mechanical clocks and the measurement of time became more exact that it became common to break days up based on their number of equal hours, rather than using easily identifiable points in the day (sunrise, noon, sunset) and unequal hours to segment the day.