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Fundamental ephemeris
A fundamental ephemeris of the Solar System is a model of the objects of the system in space, with all of their positions and motions accurately represented. It is intended to be a high-precision primary reference for prediction and observation of those positions and motions, and which provides a basis for further refinement of the model. It is generally not intended to cover the entire life of the Solar System; usually a short-duration time span, perhaps a few centuries, is represented to high accuracy. Some long ephemerides cover several millennia to medium accuracy.
==Description==
The set of physical laws and numerical constants used in the calculation of the ephemeris must be self-consistent and precisely specified. The ephemeris must be calculated strictly in accordance with this set, which represents the most current knowledge of all relevant physical forces and effects. Current fundamental ephemerides are typically released with exact descriptions of all mathematical models, methods of computation, observational data, and adjustment to the observations at the time of their announcement.〔See, for instance, (【引用サイトリンク】title=JPL Planetary and Lunar Ephemerides DE405/LE405 ); ; (【引用サイトリンク】title=High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants )〕 This may not have been the case in the past, as fundamental ephemerides were then computed from a collection of methods derived over a span of decades by many researchers.〔(【引用サイトリンク】author=Standish and Williams ) A chapter from an as-yet-unpublished (Feb 2012) version of the ''Explanatory Supplement'' (see Sources)〕
The independent variable of the ephemeris is always time. In the case of the most current ephemerides, it is a relativistic coordinate time scale equivalent to the IAU definition of TCB.〔 In the past, mean solar time (before the discovery of the non-uniform rotation of the Earth) and ephemeris time (before the implementation of relativistic gravitational equations) were used. The remainder of the ephemeris can consist of either the mathematical equations and initial conditions which describe the motions of the bodies of the Solar System, of tabulated data calculated from those equations and conditions, or of condensed mathematical representations of the tabulated data.
A fundamental ephemeris is the basis from which apparent ephemerides, phenomena, and orbital elements are computed for astronomical, nautical, and surveyors' almanacs. Apparent ephemerides give positions and motions of Solar System bodies as seen by observers from the surface of Earth, and are useful for astronomers, navigators, and surveyors in planning observations and in reducing the data acquired, although much of the work of latter two has been supplanted by GPS technology. Phenomena are events related to the configurations of Solar System bodies, for instance rise and set times, phases, eclipses and occultations, and have many civil and scientific applications. Orbital elements are descriptions of the motion of a body at a particular instant, used for further short-time-span calculation of the body's position when high accuracy is not required.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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