Precession equinoxes

Equinox precession affecting the time of onset of equinoxes and solstices, which is displayed mainly on low-latitude insolations (precession is characterized by a dual periodicity of 19,000 and 23,000 years). 

Book II investigates the dynamical conditions of fluid motion. Book III displays the law of gi avitatioii at work in the solar system. It is demonstrated from the revolutions of the six known planets, including Earth, and their satellites, though Newton could never quite perfect the difficult theory of the Moon s motion. It is also demonstrated from the motions of comets. The gravitational forces of the heavenly bodies are used to calculate their relative masses. The tidal ebb and flow and the precession of the equinoxes is explained m terms of the forces exerted by the Sun and Moon. These demonstrations are carried out with precise calculations. 

There is no reason why a successful accommodation] should not count just as highly for a theory as a predictive success—nothing prevents the former being just as severe a test for the theory as the latter. So planetary stations and retrogressions and the precession of the equinoxes—which Copemican theory and Newtonian theory, respectively, accommodated —fully supported those theories. Or, to take another example, the precession of Mercury s perihelion, which had been known about for decades beforehand, fully supported the general theory of relativity. 

A final variation involves the direction at which the Earth s axis points. Today, the axis points at the star known as Polaris, the North Star. But very slowly over time, the orientation of the axis changes, pointing in a slightly different direction. After about 12,000 years, the axis will be pointing toward the star known as Vega, which will then become the new "North Star." This 23,000-year variation is known as axial precession, or precession of the equinoxes, and because of it Earth s surface receives different amounts of solar radiation over the 23,000-year period. 

The discovery that shows, beyond all others, that Hipparchus possessed one of the masterminds of all time was the detection of that remarkable celestial movement known as the precession of the equinoxes. The inquiry that led to this discovery involved a most profound investigation, especially when it is remembered that in the days of Hipparchus, the means of (5) observation of the heavenly bodies were only of the rudest description, and the available observations of earlier dates were extremely scanty. We can but look with astonishment on the genius of the man who, in spite of such difficulties, was able to detect such a phenomenon as the precession, and to exhibit its actual magnitude. I shall endeavor to explain the nature of this singular celestial movement, for it may be said to offer the first instance in the (10) history of science in which we find that combination of accurate observation with skillful interpretation, of which, in the subsequent development of astronomy, we have so many splendid examples. 

Hipparchus traced out this phenomenon, and established it on an impregnable basis, so that all astronomers have ever since recognized the precession of the equinoxes as one of the fundamental facts of astronomy. Not until nearly 2,000 years after Hipparchus had made this splendid discovery was the explanation of its cause given by Newton. 

According to the passage, Hipparchus used which of the following methods to discover the precession of the equinoxes ... 

In addition to the correlations to lunations and sunspot cycles cited, there is yet another cycle of celestial movement that appears to be built into the hierarchical / Ching calendar. This cycle is the zodiacal world year generated by the slow precession of the equinoxes. 

The reason for this shift is that the Earth wobbles on its axis, which traces a circle in space like the spindle of a spinning top. As the axis shifts, the constellations seem to slip backwards. The amount of slippage over a human lifetime is minuscule, but over generations it adds up. As a result, every equinox takes place a little earlier in the zodiac than the one before. This process is called the precession of the equinoxes. It explains why the vernal equinox. 

A few years later, the French mathematician Joseph Alphonse Adhemar was the first to suggest that the observed ice ages were controlled by variations in the Earth s orbit around the Sun. At this point it was known that there had been multiple glaciations, and Adhemar proposed that there had been alternating ice ages between the North and the South Pole following the precession of the equinoxes. 

Indeed, the winter is warmer when the Earth is at the point on its orbit closest to the Sun, and colder when the Earth is furthest from the Sun. Adhemar correctly deduced that the precession of the equinoxes had a period of approximately 21000 years, giving alternating cold and warm winters in the two hemispheres every 10500 years. 

Figure 3 The three most important cycles regulating insolation on Earth are obliquity, eccentricity, and precession (a) obliquity, or tilt of the Earth s axis varies with a period of 41000 years (b) eccentricity of the Earth s orbit varies with periods of 100000 years and 400000 years and (c) precession of the equinoxes has a dominant period of 21 000 years and is modulated by eccentricity.

The discovery of precession of the equinoxes by Hip-parchos (second century b.c.) provoked little theoretical activity until the eleventh century. A1 Bitruji introduced the trepidation, a mechanism that permitted the multiple periodicity that appeared to be required to explain the variation in the rate of precession. The mechanism, repeated in Copernicus, derives from the erroneous determination of the period of precession in which the rate of precession of the poles varied with time from about 1° per century to 0.75° per century. The mechanism demanded something like an equant in the polar motion. As retained by Copernicus, this introduced a substantial complication into the theory of rotation of the earth and also the calculation of celestial motions and the correction of star catalogs. It was not until the seventeenth century that the error was recognized and quietly suppressed, but the trepidation represents one of the few innovations in the basic dynamical theory of the heavens in the period between the Alexandrian school of astronomy and the early Renaissance. 

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