Time is conscious experience of duration, the period
during which an action or event occurs. Time is also a dimension representing a
succession of such actions or events. Time is one of the fundamental quantities
of the physical world, similar to length and mass in this respect. The concept
that time is a fourth dimension—on a par with the three dimensions of space:
length, width, and depth—is one of the foundations of modern physics. Time
measurement involves the establishment of a time scale in order to refer to the
occurrence of events. The precise determination of time rests on astronomical
and atomic definitions that scientists have established with the utmost
mathematical exactness.
Physicists agree that time is one of the most difficult
properties of our universe to understand. Although scientists are able to
describe the past and the future and demarcations such as seconds and minutes,
they cannot define exactly what time is. The scientific study of time began in
the 16th century with the work of Italian physicist and astronomer Galileo
Galilei. In the 17th century English mathematician and physicist Sir Isaac
Newton continued the study of time. A comprehensive explanation of time did not
exist until the early 20th century, when German-born American physicist Albert
Einstein proposed his theories of relativity. These theories define time as the
fourth dimension of a four-dimensional world consisting not just of space but
of space and time.
Several ways to measure time are in use today.
Solar time is based on the rotation of Earth on its axis. It makes use of the
Sun’s apparent motion across the sky to measure the duration of a day. Sidereal
time is also based on Earth’s rotation, but uses the apparent motion of the
“fixed” stars across the sky as Earth rotates as the basis for time
determination. Standard time, the familiar clock time most people use in
everyday life, is based on the division of Earth’s sphere into 24 equal time
zones. Dynamical time—formerly called ephemeris time—is the timescale of
astronomy. Astronomers use the orbit of Earth around the Sun, as well as the
orbital motions of the Moon and the other planets, to determine dynamical time.
Atomic time is based on the frequency of electromagnetic waves that are emitted
or absorbed by certain atoms or molecules under particular conditions. It is
the most precise method for measuring time.
(Encarta Encyclopedia)