The laws of motion of the ecliptic and equator are stated in the article Precession Of The Equinoxes.
To the Greek astronomer Hipparchus belongs the credit of the discovery (c. 130 B.C.) of the theory of the precession of the equinoxes, for a knowledge of which among the Babylonians we find no definite proof; but such a signal advance in pure science did not prevent the Greeks from developing in a most elaborate manner the theory of the influence of the planets upon the fate of the individual.
The Ptolemaic catalogue embraces only those stars which were visible at Rhodes in the time of Hipparchus (c. 150 B.C.), the results being corrected for precession " by increasing the longitudes by 2° 40', and leaving the latitudes undisturbed " (Francis Baily, Mem.
In 1749 he furnished a method of applying his principles to the motion of any body of a given figure; and in 1754 he solved the problem of the precession of the equinoxes, determined its quantity and explained the phenomenon of the nutation of the earth's axis.
Owing to the precession of the equinoxes it is longer than a tropical or sidereal year by 25 minutes and 2.3 seconds.
Trepidare, to tremble), a term meaning, in general, fear or trembling, but used technically in astronomy for an imagined slow oscillation of the ecliptic, having a period of 7000 years, introduced by the Arabian astronomers to explain a supposed variation in the precession of the equinoxes.
In the steady motion under no force of such a body in medium, the centre of gravity describes a helix, while the axis escribes a cone round the direction of motion of the centre of ravity, and the couple causing precession is due to the dislacement of the medium.
It is affected by the motions of Precession and Nutation, of which the former has been known since the time of Hipparchus.
The precession of the equinoxes is due to the fact that the earth performs a motion of this kind about its centre, and the whole class of such motions has therefore been termed precessional.
In the case of the earth it is inferred from the independent phenomenon of luni-solar precession that (CA)/A = 00313.
To find the small oscillation about a state of steady precession in which the axis makes a constant angle a with the vertical, we write O=a+X, and neglect terms of the second order in x.
Southern sky left blank of stellar emblazonments was necessarily centred on the pole; and since the pole shifts among the stars through the effects of precession by a known annual amount, the ascertainment of any former place for it virtually fixes the epoch.
Nasir ud-din (1201-1274) drew up the Ilkhanic Tables, and determined the constant of precession at 51".
Gravitation was thus shown to be the sole influence governing the movements of planets and satellites; the figure of the rotating earth was successfully explained by its action on the minuter particles of matter; tides and the precession of the equinoxes proved amenable to reasonings based on the same principle; and it satisfactorily accounted as well for some of the chief lunar and planetary inequalities.
James Bradley discovered in 1728 the annual shifting of the stars due to the aberration of light, and in 1748, the complicating effects upon precession of the "nutation" of the earth's axis.
They need, after a time, to be corrected, not only systematically for precession, but also empirically for proper motion.
Amongst the most important of his works not already mentioned may be named the following: - Mathematical Tracts (1826) on the Lunar Theory, Figure of the Earth, Precession and Nutation, and Calculus of Variations, to which, in the second edition of 1828, were added tracts on the Planetary Theory and the Undulatory Theory of Light; Experiments on Iron-built Ships, instituted for the purpose of discovering a correction for the deviation of the Compass produced by the Iron of the Ships (1839); On the Theoretical Explanation of an apparent new Polarity in Light (1840); Tides and Waves (1842).
The whole series accordingly shifts westward through the effect of precession by about one degree in seventy-two years.
8 Alternately admitted into or rejected from the series, it was finally, some six or seven centuries ago, eliminated by the effects of precession in reversing the order of culmination of its limiting stars.
The absence of g from the latter expressior indicates that the circumstances of the rapid precession are verb nearly those of a free Eulerian rotation (~ 19), gravity playing only a subordinate part.
Further, on examining the small variation in i/i, it appears that in a slightly disturbed slow precession the motion of any point of the axis consists of a rapid circular vibration superposed on the steady precession, so that the resultant path has a trochoidal character.
In a slightly disturbed rapid precession the superposed vibration is elliptic-harmonic, with a period equal to that of the precession itself.
In the other letters written in 1685 and 1686 he applies to Flamsteed for information respecting the orbits of the satellites of Jupiter and Saturn, respecting the rise and fall of the spring and neap tides at the solstices and the equinoxes, respecting the flattening of Jupiter at the poles (which, if certain, he says, would conduce much to the stating the reasons of the precession of the equinoxes), and respecting the difference between the observed places of Saturn and those computed from Kepler's tables about the time of his conjunction with Jupiter.
When 1 is large we have, for the slow precession cr=p/A, and for the rapid precession o-=A/v cos a=~l, approximately.
At the international conference, which met at Paris in 1896 for the purpose of elaborating a common system of constants and fundamental stars to be employed in the various national ephemerides, Newcomb took a leading part, and at its suggestion undertook the task of determining a definite value of the constant of precession, and of 1 Lionville, t.
These two motions are defined with greater detail in the articles Precession Of The Equinoxes and Nutation.