Unit of time

A unit of time is a way to measure how long something lasts. The most common unit scientists use is the second. One second is based on how atoms of a special material, caesium, vibrate. This helps everyone measure time the same way.

Long ago, people used movements in space to measure time. For example, a year is based on how long it takes the Earth to go around the Sun. A month comes from how long it takes the Moon to go around the Earth. And a day is how long it takes the Earth to turn once.

Because these space-based units don’t always fit together perfectly, people made adjustments. For instance, a year isn’t exactly twelve months of 28 days each. To keep our calendars right, we add leap days and sometimes even leap seconds. Today, all these units connect back to the second for science.

Historical

Main article: History of calendars

Many ancient cultures used natural patterns to measure time, like the length of a day, the sun's path through the year, and the moon's phases. Examples include the calendars of the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican peoples.

Today’s calendar comes from the Roman calendar, which changed over time into the Julian calendar and finally the Gregorian calendar we use now.

Scientific

The Planck time is the time it takes for light to travel a very small distance called a Planck length. The Jiffy is the time light takes to travel the width of a tiny part of an atom, called a femtometre. Atomic time relates to how long it takes an electron to move around a hydrogen atom. The svedberg is a unit used to measure how quickly tiny particles settle down, and it equals 10⁻¹³ seconds. The TU stands for time unit and is used in engineering, equal to 1024 microseconds. The galactic year is the time it takes for our solar system to orbit the center of our galaxy. The geological time scale divides Earth's long history into sections based on important events, such as the Cretaceous–Paleogene extinction event.

Note: The light-year is a measure of distance, not time.
Note: The parsec is also a measure of distance, not time, despite some movie references.

List

Interrelation

All the official units of time are related to each other by size. The most common units are the second, which is based on an atomic process; the day, which is made up of seconds; and the year, which is usually 365 days. Other units are bigger or smaller versions of these three.