Description: All of us have a basic idea of what time is: A progression of events, one after the other, from the past through the present to the future. Yet time is not something that can be seen, heard, smelled, touched, or tasted. It can be measured, though, and that is the aspect of time that matters in astronomy and in physics.

We measure time by comparing the duration of whatever is happening with the duration measured on a clock or, for longer periods of time, on a calendar. Traditionally, measurements of time of this kind have been based on the Earth's rotation. The time it takes the Earth to complete one full rotation (as judged by the position of the Sun) defines the length of one day. The usual subdivisions – one day divided into 24 hours, each hour divided into 60 minutes, each minute into 60 seconds – provide additional units of time.

Since 1967, the definition of time has instead been based on the duration of a second, as measured by a Caesium-133 atomic clock ("SI second"). Several systems of time are defined on this basis, notably Universal Time (UTC) which is used in official time-keeping world-wide, and what is known as the Julian Date and its variations, a continuous counting of days used in astronomy.

Einstein's special theory of relativity and his general theory of relativity have shown that the time that passes on a clock depends both on the motion and on the influence of gravity of that clock. These relativistic effects need to be accounted for in highly accurate measurements of time, such as the ones using the satellites of the Global Positioning System (GPS).

Related Terms:

• Sidereal Day

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".