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Bradley calculated that it takes light 8 min 13 s to reach the Earth from the Sun, very close to the modern value of 8 min 19 s (Debarbat and Wilson (1989)). A theoretical justification for aberration was provided by Clairaut in 1737 in his memoirDe ¡'aberration apparente des etoiles causee par ¡e mouvement progressif de ¡a ¡umiere. There is also an effect due to the daily rotation of the Earth, but the magnitude of this diurnal aberration is always less than 0.5" and was of no concern to eighteenth-century astronomers. Bradley sent a preliminary account of his findings to the Paris Academy in 1737.

the plane of the moon's orbit being at one time above ten degrees more inclined to the plane of the equator than another, it was reasonable to conclude that the part of the whole annual precession which arises from her action would in different years be varied in its quantity; whereas the plane of the ecliptic wherein the sun appears, keeping always nearly the same inclination to the equator, that part of the precession which is owing to the sun's action may be the same every year: and from hence it would follow, that although the mean annual precession, proceeding from the joint action of the sun and moon, were 50'', yet the apparent annual precession might sometimes exceed, and sometimes fall short of that mean quantity, according to the various situations of the nodes of the moon's orbit... .

As well as identifying the cause, Bradley also determined the effect. He suggested that the axis of the Earth underwent a small conical motion, tracing out an ellipse with axes measuring 18 and 16''. This oscillation, which is superimposed on the slow conical motion of precession, is known as the 'nutation' of the axis of the Earth and leads, as a consequence, to a variation in the obliquity of the ecliptic by ±9'' over an 18.6-year period. A quantitative theoretical derivation of nutation from the equations for the Earth-Sun-Moon system, by

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