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5 X-ray Pulsar-Based Navigation: Theories and Experiments
5.3.2.3 Mean Solar Time
In order to avoid the change of length of the apparent solar day, Simon Newcomb,
a Canadian-American astronomer, introduced an imaginary reference point, known
as mean sun. Firstly, supposed that there is an imaginary point moving at a constant
speed in the ecliptic, its moving speed is equal to the average speed of the apparent
motion of the Sun, and it passes through the perigee and apogee at the same time
with the Sun. Then, the imaginary point is called ecliptic mean sun. Secondly,
supposed that there is the second imaginary point moving at a constant speed in the
equator, its moving speed is equal to the speed of the first imaginary point, and its
right ascension as close as possible to the celestial longitude of the ecliptic mean
sun. Then, the second imaginary point is called equatorial mean sun. The timescale
determined with the equatorial mean sun as the reference point is called mean solar
time. The time interval between two successive passages of the equatorial mean sun
through the upper culmination is called a mean solar day, which is subdivided into
24 mean solar hours, 1,440 mean solar minutes and 86,400 mean solar seconds. As
same as the sidereal time, the mean solar time is also with locality, and sometimes it
is also known as local mean solar time.
5.3.2.4 Greenwich Mean Time
The Greenwich Mean Time (GMT) refers to the mean solar time at the Royal
Observatory in Greenwich, London, taking the mean midnight (lower culmination)
as a starting point of zero hour, also known as Universal Time (UT). It can be seen
that the universal time is based on the timescale of the mean solar time, and both
discrepancy is only due to the difference of the time origin. The mean solar time
is to take the Earth’s rotation as a reference. However, it has been found from the
modern astronomical observations that the rotation rate of the Earth is not uniform,
and the average length of a solar day caused by tidal friction increases by 0.0016 s per
hundred years. Moreover, there are various kinds of periodic, seasonal and irregular
changes for the UT. Since 1965, the seasonal correction terms of the polar wander and
rotation rate of the Earth have been introduced into the UT, and the resulting UT is
expressed as UT1 and UT2, respectively, while the uncorrected UT directly calculated
by observatories using their measurement data is expressed as UT0. Obviously, after
the correction of the polar wander and rotation seasonal changes, the resulting UT2
is still affected by the long-term and irregular changes of the Earth’s rotation rate.
Therefore, the UT2 is not a strictly uniform time system yet.
5.3.2.5 Ephemeris Time
Celestial mechanics is the theory of celestial motion in the solar system on the basis
of the classic Newtonian mechanics. Usually, the time in the motion equations is
used as an argument to calculate the celestial position ephemeris. Obviously, the
5 X-ray Pulsar-Based Navigation: Theories and Experiments
5.3.2.3 Mean Solar Time
In order to avoid the change of length of the apparent solar day, Simon Newcomb,
a Canadian-American astronomer, introduced an imaginary reference point, known
as mean sun. Firstly, supposed that there is an imaginary point moving at a constant
speed in the ecliptic, its moving speed is equal to the average speed of the apparent
motion of the Sun, and it passes through the perigee and apogee at the same time
with the Sun. Then, the imaginary point is called ecliptic mean sun. Secondly,
supposed that there is the second imaginary point moving at a constant speed in the
equator, its moving speed is equal to the speed of the first imaginary point, and its
right ascension as close as possible to the celestial longitude of the ecliptic mean
sun. Then, the second imaginary point is called equatorial mean sun. The timescale
determined with the equatorial mean sun as the reference point is called mean solar
time. The time interval between two successive passages of the equatorial mean sun
through the upper culmination is called a mean solar day, which is subdivided into
24 mean solar hours, 1,440 mean solar minutes and 86,400 mean solar seconds. As
same as the sidereal time, the mean solar time is also with locality, and sometimes it
is also known as local mean solar time.
5.3.2.4 Greenwich Mean Time
The Greenwich Mean Time (GMT) refers to the mean solar time at the Royal
Observatory in Greenwich, London, taking the mean midnight (lower culmination)
as a starting point of zero hour, also known as Universal Time (UT). It can be seen
that the universal time is based on the timescale of the mean solar time, and both
discrepancy is only due to the difference of the time origin. The mean solar time
is to take the Earth’s rotation as a reference. However, it has been found from the
modern astronomical observations that the rotation rate of the Earth is not uniform,
and the average length of a solar day caused by tidal friction increases by 0.0016 s per
hundred years. Moreover, there are various kinds of periodic, seasonal and irregular
changes for the UT. Since 1965, the seasonal correction terms of the polar wander and
rotation rate of the Earth have been introduced into the UT, and the resulting UT is
expressed as UT1 and UT2, respectively, while the uncorrected UT directly calculated
by observatories using their measurement data is expressed as UT0. Obviously, after
the correction of the polar wander and rotation seasonal changes, the resulting UT2
is still affected by the long-term and irregular changes of the Earth’s rotation rate.
Therefore, the UT2 is not a strictly uniform time system yet.
5.3.2.5 Ephemeris Time
Celestial mechanics is the theory of celestial motion in the solar system on the basis
of the classic Newtonian mechanics. Usually, the time in the motion equations is
used as an argument to calculate the celestial position ephemeris. Obviously, the