Unit of time

A unit of time is a specific amount of time that we use to measure how long things last. The most common unit of time that scientists use is the second. One second is defined by the way atoms of a special material, caesium, vibrate. This helps everyone measure time in the same way.

Long ago, people used the movements of objects in space to decide how 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 on its axis.

Because these space-based units don’t always fit neatly together, people needed to make adjustments. For instance, a year isn’t exactly twelve months of 28 days each. To keep our calendars accurate, we add leap days and sometimes even leap seconds. Today, all these units are linked back to the second for scientific work.

Historical

Main article: History of calendars

Many ancient cultures used natural patterns to measure time, like the length of a day, the cycle of the sun through the year, and the phases of the moon. 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 one 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. These sections include eons, eras, periods, epochs, and ages, but each one can be a different length of time.

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 formal units of time are scaled multiples of each other. The most common units are the second, which is defined by an atomic process; the day, which is a multiple of seconds; and the year, which is usually 365 days. Other units are multiples or divisions of these three.