Second
The
second (symbol:
s; abbreviation:
sec.) is the name of a
unit of
time, and today refers to the
International System of Units (SI)
base unit of time.
The duration of a beat or half period (one swing, not back and forth) of a
pendulum one
metre in length is approximately one second.[
1]
Subdivisions of the second, such as
millisecond (one thousandth of a second), and (although encountered less frequently in practice) multiples of the second, such as
kilosecond (1,000 seconds), can be indicated by adding
SI prefixes to
second.
Under the
International System of Units, the second 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 refers to a caesium atom at rest at a temperature of 0 K.
The international standard symbol for a second is
s (see
ISO 31-1).
1 international second is equal to:
* 1/60
minute (1 minute is equal to 60 seconds)
* 1/3,600
hour (1 hour is equal to 3,600 seconds)
* 1/86,400
day (1 day, in the sense of non-SI units accepted for use with the International System of Units, is equal to 86,400 seconds)
Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the
minute we know today.
The factor of 60 comes from the
Babylonians who used factors of 60 in their counting system. However, the Babylonians did not subdivide their time units sexagesimally (except for the day). The
hour had been defined by the
ancient Egyptians as either 1/12 of daytime or 1/12 of nighttime, hence both varied with the seasons.
Hellenistic astronomers, including
Hipparchus and
Ptolemy, defined the hour as 1/24 of a mean
solar day. Sexagesimally subdividing this mean solar hour made the second 1/86,400 of a mean solar day. Hellenistic time periods like the mean
synodic month were usually specified quite precisely because they were
calculated from carefully selected
eclipses separated by hundreds of yearsâ€"individual
mean synodic months and similar time periods cannot be
measured. Nevertheless, with the development of pendulum clocks keeping
mean time (as opposed to the
apparent time displayed by sundials), the second became measureable.
In 1956 the second was defined in terms of the period of revolution of the
Earth around the Sun for a particular
epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in
Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892. The second thus defined is
the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. The
tropical year in the definition was not measured, but calculated from a formula describing a tropical year which decreased linearly over time, hence the curious reference to a specific
instantaneous tropical year. Because this second was the independent variable of time used in
ephemerides of the Sun and Moon during most of the twentieth century (Newcomb's Tables of the Sun were used from 1900 through 1983, and
Brown's Tables of the Moon were used from 1920 through 1983), it was called the ephemeris second.
With the development of the
atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the revolution of the Earth around the Sun.
Following several years of work, two astronomers at the
United States Naval Observatory (USNO) and two astronomers at the
National Physical Laboratory (Teddington, England) determined the relationship between the hyperfine transition frequency of the
caesium atom and the ephemeris second. Using a common-view measurement method based on the received signals from
radio station WWV, they determined the orbital motion of the
Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth
General Conference on Weights and Measures defined the second of
atomic time in the 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.The ground state is defined at zero
magnetic field. The second thus defined is equivalent to the ephemeris second.
The definition of the second was later refined at the 1997 meeting of the
BIPM to include the statement
This definition refers to a caesium atom at rest at a temperature of 0 K.In practice, this means that high-precision realizations of the second should compensate for the effects of the ambient temperature (
black-body radiation) within which atomic clocks operate to extrapolate to the value of the second as defined above. Furthermore, it indicates that the ultimate atomic clock would contain a single caesium atom at rest emitting a single frequency.
*
Leap second*
Orders of magnitude (time)*
UTC*
International System of Units*
Official BIPM definition of the second*
Seconds and leap seconds by the USNO
