Time Determination and Time Keeping

1948 ◽  
Vol 1 (2) ◽  
pp. 109-117
Author(s):  
Harold Spencer Jones
Keyword(s):  
Frequency Control ◽  
Rate Of Change ◽  
Time Interval ◽  
The Earth ◽  
Convenient Means ◽  
Time Signals ◽  
Time Determination ◽  
The Mean ◽  
Rotation Of The Earth ◽  
Definite Manner

The rotation of the Earth provides the ultimate standard of time. As the fundamental unit of time we can use either the mean solar day or the sidereal day; these two units are related in a definite manner, so that when one is determined, the other can be inferred. The purpose of any timepiece is to subdivide the day into shorter intervals, and so give the time at any instant. No timepiece will give exact time; the error of the timepiece at some definite instant and the rate of change of that error, or, briefly, the rate, must be determined in order to extrapolate for the correct time at some subsequent instant. The accuracy of the extrapolation will depend upon the uniformity of the rate of the timepiece. Radio time signals sent out from an observatory, which is responsible for the determination and distribution of time, provide the most convenient means for deriving the error and rate of a timepiece. For normal navigational purposes an accuracy of about 0·05 seconds is adequate. But for the purpose of frequency control a very much higher precision is needed—but a precision in time interval rather than in absolute time. Some of the radio-aids to navigation depend upon the accuracy in standardization of frequency, so that high accuracy in time interval has become, indirectly, a navigational requirement.

scholarly journals The Origin of Celestial Coordinates

1990 ◽  
Vol 141 ◽  
pp. 51-59
Author(s):  
C. A. Murray
Keyword(s):  
Earth Rotation ◽  
Zero Point ◽  
Rate Of Change ◽  
Reference Systems ◽  
The Earth ◽  
Private Correspondence ◽  
Rotation Of The Earth ◽  
Right Ascension

In 1978, Guinot proposed that, for studies of Earth rotation, the zero point of the apparent “right ascension” coordinate on the true equator should be so chosen that the rate of change of its hour angle is exactly proportional to the inertial rate of rotation of the Earth. It has been subsequently suggested that this concept of the “non-rotating origin” supersede the equinox quite generally as the origin of celestial coordinates. Since this proposal was first put forward, there has been much discussion, and some criticism, from Aoki and his colleagues, both published and in private correspondence. Some of the arguments for and against Guinot's proposal are discussed, as a contribution to the wider debate on reference systems now being carried out under the auspices of the IAU.

scholarly journals An Analytical Theory for a Gyrostatic Earth

1997 ◽  
Vol 165 ◽  
pp. 307-312
Author(s):  
R. Molina ◽  
A. Vigueras
Keyword(s):  
Perturbation Method ◽  
Polar Motion ◽  
Analytical Theory ◽  
Canonical Variables ◽  
The Earth ◽  
The Mean ◽  
Rotation Of The Earth

AbstractIn this paper, we consider the problem of the rotation of the Earth, using a stationary triaxial gyrostat as a model. The problem is formulated by means of dimensionless canonical variables of Serret-Andoyer, referred to the mean ecliptic of date, in a similar way to Kinoshita (1977). We choose the constant components of the gyrostatic momentum in such a way that the period of the polar motion corresponds to Chandler’s period. Finally, the problem is integrated by means of Deprit’s perturbation method.

scholarly journals ‘The Metrication of Navigation’

1968 ◽  
Vol 21 (2) ◽  
pp. 236-237
Author(s):  
D. H. Sadler
Keyword(s):  
Universal Time ◽  
Time System ◽  
Precise Position ◽  
The Earth ◽  
Time Signals ◽  
Time Argument ◽  
Orbital Periods ◽  
Rotation Of The Earth ◽  
Mean Time ◽  
Nautical Almanac

In his note (Journal 21, 81) on this subject, Ronald Turner says ‘No longer will orbital periods of the rotation of the Earth on its axis be measures of time’. This is not so, either in general or in the particular, case of navigation.Universal Time (U.T.), which is the generally accepted name for Greenwich Mean Time (G.M.T.), continues to be essential for all purposes (in astronomy, geodesy, surveying and navigation) for which are required astronomical observations related to the precise position of the observer on the Earth's surface. The Nautical Almanac must continue to tabulate the positions of the Sun, Moon, planets and stars with G.M.T. as the time-argument; and observations should be timed in a time-system related to U.T., such as the broadcast time-signals of Coordinated Universal Time (U.T.C:).

scholarly journals Elevation changes on the East Antarctic ice sheet, 1978-93, from satellite radar altimetry: a preliminary assessment

1998 ◽  
Vol 27 ◽  
pp. 7-18 ◽  
Author(s):  
Craig S. Lingle ◽  
David N. Covey
Keyword(s):  
Ice Sheet ◽  
Time Frame ◽  
Rate Of Change ◽  
Altimeter Data ◽  
Late Winter ◽  
Time Interval ◽  
Radar Altimeter ◽  
Antarctic Ice Sheet ◽  
The Mean ◽  
East Antarctic Ice Sheet

Radar altimeter data from Seasal (1978), Geosat (1985-88) and ERS-1 (1991—93) are employed to estimate multi-year mean changes of the surface height throughout a region on the East Antarctic ice sheet (EAIS) extending to 72.1° S, the southernmost limit of coverage for Seasat and Geosat altimetry, and above 1500 m elevation, using orbit crossover analysis. The changes are estimated on a same-season (austral late-winter (ALW) toALW) basis, where ALW is the 10 July 9 October time-frame of the Seasat altimetry. Altimeter data corrected for slope-induced errors are used. Altimeter data not corrected for slope-induced errors are also used, for comparison. Intersatellite orbit bias, combined with the effect of other radial errors such as instrumental bias, is estimated using crossover differences on the offshore ALW sea ice, which is employed as a geoid-parallcl reference surface. If similar intersatellite radial biases are characteristic of the continental Antarctic ice-sheet altimetry to 72.1° S, the results of all crossover analyses adjusted for this intersatellite bias — suggest that the mean rate-of-change of the surface height between Seasat and Geosat for ALWs 1978 to 1986-88 was with in the range +11 to -11 mm a−1. The bias-adjusted results of all crossover analyses between Seasat and ERS-1 suggest that the mean rate-of-change of the surface height between ALWs 1978 and 1991-93 was with in the range-17 to-55mma−1 (maximum intersatellitc bias estimate) or 0 to -40 mm a−1 (minimum bias estimate), suggesting that the surface may have lowered slightly during this time interval. The inconsistency of the adjusted Seasat to Geosat vs Seasat to ERS-1 results, however, may be an indication that orbits more accurate than JGM-2 are needed for estimation of regional multi-year mean changes of elevation on the EAIS. Alternatively, it may be a reflection of the differing orbit inclinations of Seasal and ERS-1.

scholarly journals Fluctuations in the Motion of the Mean Pole and the Rotation of the Earth

1968 ◽  
Vol 32 ◽  
pp. 98-99
Author(s):  
H.J. Abraham
Keyword(s):  
The Earth ◽  
The Mean ◽  
Rotation Of The Earth

Explanations of the progressive and librational motions of the pole are attempted.

Concurrent Astronomical Observations for Studying Continental Drift, Polar Motion, and the Rotation of the Earth

1968 ◽  
Vol 32 ◽  
pp. 25-32 ◽  
Author(s):  
Wm. Markowitz
Keyword(s):  
Polar Motion ◽  
Continental Drift ◽  
Secular Motion ◽  
Astronomical Observations ◽  
The Earth ◽  
The Mean ◽  
Rotation Of The Earth

The analysis of 66 years of concurrent latitude observations of the ILS shows that the mean pole has a secular motion which consists of a progressive component of about 0′′.0035/yr (10 cm/yr) along the meridian 65°W and a librational component (oscillation) of 24-year period along the meridian 122°W (or 58°E). Crustal displacements in latitude are not found within the errors of observation, about 1 cm/yr.Comparable, concurrent observations for time (longitude) have not been made but programs are being organized. From 30 to 50 years will be needed for detection of continental drift with PZT's and astrolabes if relative drifts in longitude of 3 cm/yr are occurring.

scholarly journals Elevation changes on the East Antarctic ice sheet, 1978-93, from satellite radar altimetry: a preliminary assessment

1998 ◽  
Vol 27 ◽  
pp. 7-18 ◽  
Author(s):  
Craig S. Lingle ◽  
David N. Covey
Keyword(s):  
Ice Sheet ◽  
Time Frame ◽  
Rate Of Change ◽  
Altimeter Data ◽  
Late Winter ◽  
Time Interval ◽  
Radar Altimeter ◽  
Antarctic Ice Sheet ◽  
The Mean ◽  
East Antarctic Ice Sheet

Radar altimeter data from Seasal (1978), Geosat (1985-88) and ERS-1 (1991—93) are employed to estimate multi-year mean changes of the surface height throughout a region on the East Antarctic ice sheet (EAIS) extending to 72.1° S, the southernmost limit of coverage for Seasat and Geosat altimetry, and above 1500 m elevation, using orbit crossover analysis. The changes are estimated on a same-season (austral late-winter (ALW) toALW) basis, where ALW is the 10 July 9 October time-frame of the Seasat altimetry. Altimeter data corrected for slope-induced errors are used. Altimeter data not corrected for slope-induced errors are also used, for comparison. Intersatellite orbit bias, combined with the effect of other radial errors such as instrumental bias, is estimated using crossover differences on the offshore ALW sea ice, which is employed as a geoid-parallcl reference surface. If similar intersatellite radial biases are characteristic of the continental Antarctic ice-sheet altimetry to 72.1° S, the results of all crossover analyses adjusted for this intersatellite bias — suggest that the mean rate-of-change of the surface height between Seasat and Geosat for ALWs 1978 to 1986-88 was with in the range +11 to -11 mm a−1. The bias-adjusted results of all crossover analyses between Seasat and ERS-1 suggest that the mean rate-of-change of the surface height between ALWs 1978 and 1991-93 was with in the range-17 to-55mma−1 (maximum intersatellitc bias estimate) or 0 to -40 mm a−1 (minimum bias estimate), suggesting that the surface may have lowered slightly during this time interval. The inconsistency of the adjusted Seasat to Geosat vs Seasat to ERS-1 results, however, may be an indication that orbits more accurate than JGM-2 are needed for estimation of regional multi-year mean changes of elevation on the EAIS. Alternatively, it may be a reflection of the differing orbit inclinations of Seasal and ERS-1.

About the Future of UTC

2014 ◽  
Vol 511-512 ◽  
pp. 209-212
Author(s):  
Shao Wu Dong
Keyword(s):  
International Organizations ◽  
Time Scale ◽  
International System ◽  
Universal Time ◽  
Current Form ◽  
Reference Time ◽  
The Earth ◽  
Time Signals ◽  
Scale Interval ◽  
Rotation Of The Earth

The Coordinated Universal Time (UTC) is the international reference time-scale, and provides the basis for broadcast time signals and for time-keeping in the large majority of countries. In its current form, UTC is computed as the average of a large number of atomic clocks and its scale interval is based on the second in the International System units (SI), but it is adjusted by the occasional insertion of one second (called as leap second), to maintain close alignment with Universal Time 1 (UT1), a time-scale determined by the rotation of the Earth. The UTC system with leap seconds is not a continuous time scale, in resent years, several countries and international organizations have started a discussion on the need to abolish the application of leap second in UTC, however, some countries oppose it. Issues on leap second worldwide are presented in this paper.

The motion of a lunar orbiter

1966 ◽  
Vol 25 ◽  
pp. 373
Author(s):  
Y. Kozai
Keyword(s):  
Degrees Of Freedom ◽  
Dimensional Space ◽  
Artificial Satellite ◽  
Equations Of Motion ◽  
Triaxial Ellipsoid ◽  
The Moon ◽  
Secular Motion ◽  
The Earth ◽  
Close Satellite ◽  
The Mean

The motion of an artificial satellite around the Moon is much more complicated than that around the Earth, since the shape of the Moon is a triaxial ellipsoid and the effect of the Earth on the motion is very important even for a very close satellite.The differential equations of motion of the satellite are written in canonical form of three degrees of freedom with time depending Hamiltonian. By eliminating short-periodic terms depending on the mean longitude of the satellite and by assuming that the Earth is moving on the lunar equator, however, the equations are reduced to those of two degrees of freedom with an energy integral.Since the mean motion of the Earth around the Moon is more rapid than the secular motion of the argument of pericentre of the satellite by a factor of one order, the terms depending on the longitude of the Earth can be eliminated, and the degree of freedom is reduced to one.Then the motion can be discussed by drawing equi-energy curves in two-dimensional space. According to these figures satellites with high inclination have large possibilities of falling down to the lunar surface even if the initial eccentricities are very small.The principal properties of the motion are not changed even if plausible values ofJ3andJ4of the Moon are included.This paper has been published in Publ. astr. Soc.Japan15, 301, 1963.

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