5.3 Space-Time Reference Based on Newtonian Mechanics
289
time here should mathematically be an evenly independent variable, known as the
Newtonian time. When the position of the celestial body at a certain moment is
gotten from astronomical observation, then Newton’s time can easily be obtained
from the ephemeris. However, as solving the differential equations of motion, due to
the errors of the integral constants determined by the actual measurements, as well
as those of the theoretical models of celestial motions, only the approximate Newton
time can be given by any celestial position ephemeris. Generally, the time given by
the celestial position catalogue is called Ephemeris Time (ET). The ET could be
obtained by observing the orbital position of any planet or satellite and then using
an ephemeris, which lists the calculated orbital positions as a function of time. Due
to the small difference between the ETs given by the position ephemerides of the
Moon, Venus, Mars and Sun (actually the Earth), a uniform time system, Newcomb’s
tables of the Sun, is selected as the standard to define the numerical measure of the
ET. In fact, Newcomb’s tables are the orbital positions of the Earth about the Sun,
developed mathematically by Simon Newcomb in 1898. That is to say, the ET is
the time variable of Newcomb’s tables of the Sun. The timescale of the ET is the
tropical year, the length of which is equal to the time interval between two successive
passages of the mean sun through the vernal equinox, that is, every 360° increase of
the geometric mean of the Sun’s ecliptic longitude is a tropical year. In 1960, the IAU
decided that the position ephemerides of the celestial bodies in the solar system are
reckoned by the ET, and the ephemeris second is used as the definition of a second in
the International System of Units (SI). The ET based on the Earth’s revolution around
the Sun is not perfect either in theory or in practice. The annual apparent motion of
the Sun on the celestial sphere is relatively slow. When the error of observing the
position of the Sun is 0.1 arc-seconds, then the error of the reckoned ET will reach
2.5 s. In fact, the ET is determined by observing the Moon, but its accuracy is not
better, and the needed observation time is so long that the results cannot be obtained
in time. Furthermore, there are some astronomical constants in the definition of the
ET. Once the astronomical constant system is redefined or modified, the discontinuity
of the ET will be caused. So, another new time metering system, the atomic time
system, has been adopted since 1967, and the ET has completely been replaced by
the atomic time since 1984.
5.3.2.6 International Atomic Time
The atomic time is a timescale generated by atomic clocks, which furnish time more
accurately than was possible with previous astronomical means, such as the sidereal
time, mean solar time and ephemeris time. In 1967, at the 13th General Conference
on Weights and Measures, a new definition of second length in the SI units was
adopted: an atomic second refers to the duration of 9,192,631,770 periods of the
oscillating radiation, based on the transition between two hyperfine levels of the
ground state of Cesium 133 atom on the sea level and in zero magnetic field. The
atomic time has high precision and stability, so it has been developed and widely
used. Many countries have established their own local atomic time systems, but
289
time here should mathematically be an evenly independent variable, known as the
Newtonian time. When the position of the celestial body at a certain moment is
gotten from astronomical observation, then Newton’s time can easily be obtained
from the ephemeris. However, as solving the differential equations of motion, due to
the errors of the integral constants determined by the actual measurements, as well
as those of the theoretical models of celestial motions, only the approximate Newton
time can be given by any celestial position ephemeris. Generally, the time given by
the celestial position catalogue is called Ephemeris Time (ET). The ET could be
obtained by observing the orbital position of any planet or satellite and then using
an ephemeris, which lists the calculated orbital positions as a function of time. Due
to the small difference between the ETs given by the position ephemerides of the
Moon, Venus, Mars and Sun (actually the Earth), a uniform time system, Newcomb’s
tables of the Sun, is selected as the standard to define the numerical measure of the
ET. In fact, Newcomb’s tables are the orbital positions of the Earth about the Sun,
developed mathematically by Simon Newcomb in 1898. That is to say, the ET is
the time variable of Newcomb’s tables of the Sun. The timescale of the ET is the
tropical year, the length of which is equal to the time interval between two successive
passages of the mean sun through the vernal equinox, that is, every 360° increase of
the geometric mean of the Sun’s ecliptic longitude is a tropical year. In 1960, the IAU
decided that the position ephemerides of the celestial bodies in the solar system are
reckoned by the ET, and the ephemeris second is used as the definition of a second in
the International System of Units (SI). The ET based on the Earth’s revolution around
the Sun is not perfect either in theory or in practice. The annual apparent motion of
the Sun on the celestial sphere is relatively slow. When the error of observing the
position of the Sun is 0.1 arc-seconds, then the error of the reckoned ET will reach
2.5 s. In fact, the ET is determined by observing the Moon, but its accuracy is not
better, and the needed observation time is so long that the results cannot be obtained
in time. Furthermore, there are some astronomical constants in the definition of the
ET. Once the astronomical constant system is redefined or modified, the discontinuity
of the ET will be caused. So, another new time metering system, the atomic time
system, has been adopted since 1967, and the ET has completely been replaced by
the atomic time since 1984.
5.3.2.6 International Atomic Time
The atomic time is a timescale generated by atomic clocks, which furnish time more
accurately than was possible with previous astronomical means, such as the sidereal
time, mean solar time and ephemeris time. In 1967, at the 13th General Conference
on Weights and Measures, a new definition of second length in the SI units was
adopted: an atomic second refers to the duration of 9,192,631,770 periods of the
oscillating radiation, based on the transition between two hyperfine levels of the
ground state of Cesium 133 atom on the sea level and in zero magnetic field. The
atomic time has high precision and stability, so it has been developed and widely
used. Many countries have established their own local atomic time systems, but