Time is defined as what a clock reads. Modern physics views time as part of spacetime rather than a separate entity. The international standard unit of time is the second, which is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two energy levels of the caesium-133 atom in a caesium clock, the most accurate timekeeping device. Caesium clocks serve as the primary standard for the definition of the second.

2. INTRODUCTION
 Time in physics is defined by its measurement: time is
what a clock reads.
 In classical, non-relativistic physics it is a scalar quantity
and, like length, mass, and charge, is usually described
as a fundamental quantity.
 Time can be combined mathematically with other physical
quantities to derive other concepts such as motion, kinetic
energy and time-dependent fields.
 Timekeeping is a complex of technological and scientific
issues, and part of the foundation of recordkeeping.

3. Foucault's pendulum in the
Panthéon of Paris can measure time as well
as demonstrate the rotation of Earth.

4.  In the International System of Units (SI), the
unit of time is the second.
 It is a SI base unit, and it is currently defined
as "the duration of 9 192 631 770 periods of
the radiation corresponding to the transition
between the two hyperfine levels of the
ground state of the caesium 133 atom".
 This definition is based on the operation of a
caesium atomic clock.

5. Conceptions of time
• Both Galileo and Newton and most people up until the 20th
century thought that time was the same for everyone
everywhere.
• Our modern conception of time is based on Einstein's theory
of relativity, in which rates of time run differently depending
on relative motion, and space and time are merged
into space time, where we live on a world line rather than a
timeline.
• Thus time is part of a coordinate, in this view.
• Physicists believe the entire Universe and therefore time
itselfbegan about 13.8 billion years ago in the big bang; see.
• Whether it will ever come to an end is an open question.

6. Markers of time
• The first appearance of Sirius to mark the flooding
of the Nile each year.
• The periodic succession of night and day,
seemingly eternally.
• The position on the horizon of the first appearance
of the sun at dawn.
• The position of the sun in the sky.
• The marking of the moment of noontime during
the day.
• The length of the shadow cast by a gnomon.

7. Mechanical clocks
 Richard of Wallingford(1292–1336), abbot of St.
Alban's abbey, famously built a mechanical clock as an
astronomical orrery about 1330.
 By the time of Richard of Wallingford, the use
of ratchets and gears allowed the towns of Europe to
create mechanisms to display the time on their
respective town clocks; by the time of the scientific
revolution, the clocks became miniaturized enough for
families to share a personal clock, or perhaps a pocket
watch.

9.  At first, only kings could afford them.
 Pendulum clocks were widely used in the
18th and 19th century.
 They have largely been replaced in
general use by quartz and digital clocks.
 Atomic clocks can theoretically keep
accurate time for millions of years.
 They are appropriate for standards and
scientific use.

10. caesium atomic clock
 A caesium standard or caesium atomic clock is a
primary frequency standard in which electronic
transitions between the two hyperfine ground
states of caesium-133 atoms are used to control the output
frequency.
 The first caesium clock was built by Louis Essen in 1955 at
the National Physical Laboratory in the UK.
 Caesium clocks are the most accurate commercially
produced time and frequency standards, and serve as
the primary standard for the definition of
the second in SI (the metric system).

12. • By definition, radiation produced by the transition
between the two hyperfine ground states of
caesium (in the absence of external influences such
as the Earth's magnetic field) has a frequency of
exactly 9,192,631,770 Hz.
• That value was chosen so that the caesium second
equaled, to the limit of human measuring ability in
1960 when it was adopted, the existing
standard ephemeris second based on the Earth's
orbit around the Sun.
• Because no other measurement involving time had
been as precise, the effect of the change was less
than the experimental uncertainty of all existing
measurements.