Time

Time is a means of measuring change and the order of events. It is conventionally divided into three parts: past, present, and future. Anything that moves—the Earth's rotation, a heartbeat, or the propagation of light—can be measured in time.

Humans measure time using regular intervals such as seconds, minutes, hours, days, months, and years. The larger intervals mostly derive from celestial motion: a day is based on the Earth's rotation on its axis, and a year on the Earth's orbit around the sun. Modern smaller intervals are defined by the oscillation of atoms.

Measurement and units

In the International System of Units (SI), the second is the fundamental unit of time. It is defined as a specific number of oscillations of radiation emitted by a cesium atom, which ensures that atomic clocks are highly accurate. These clocks underpin Coordinated Universal Time (UTC), which is used worldwide for synchronization.

Physics perspective

In classical Newtonian physics, time was universal and identical for everyone. Einstein's theory of relativity changed this conception: time is dependent on motion and gravity. A person moving very fast or near a massive gravitational field experiences time passing more slowly than someone else. This has been tested with atomic clocks carried on aircraft and satellites, and is accounted for in GPS systems.

The arrow of time

A major question in physics concerns why time moves in only one direction. The fundamental laws of motion are generally reversible, yet observation shows that events do not reverse. The widely accepted explanation rests on the second law of thermodynamics: isolated systems have increasing entropy (disorder), which creates an observable direction of time.

Society and calendars

Societies have developed various calendars based on the moon, the sun, or both. The Gregorian calendar is now used worldwide for official purposes, while the Islamic lunar calendar marks events in Islamic tradition. In Somali culture, the day is typically counted from sunrise, so what Somali speakers call 'the sixth hour' corresponds to ten in the morning in the international system.