HISTORY OF PHYSICAL ASTRONOMY.

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When Bradley was engaged in investigating the phenomenon of aberra-tion, lie found that in the case of some stars near the equinoctial colure,the change of declination was greater than that which would have resultedfrom a mean annual precession of 50". Similar observations of starsnear the solstitial colure, indicated an effect of a contrary nature, the changeof declination being less than the quantity due to the same value of pre-cession. It was manifest, from this circumstance, that the phenomenoncould not be attributed to an error in the motion of the equinoctial points.Bradley made repeated observations with the view of discovering its realnature from the year 1727, when his instrument was first erected atWansted, until 1732, the year of his removal to Oxford. During thisinterval of time some of the stars near the solstitial colure had changedtheir declination 9" or 10" less than the quantity due to a precessionof 50" ; while others near the equinoctial colure had exhibited an equalchange of declination in the opposite direction. The north pole of theequator appeared to have approached the stars which came to the meridianwith the sun about the vernal equinox and the winter solstice, and tohave receded from those which came to the meridian with the sun aboutthe autumnal equinox and the summer solstice.

After pondering for some time on the phenomenon, Bradley began tosuspect that it might arise from the variable action of the moon on thematter accumulated round the terrestrial equator. On account of themotion of the moon’s nodes, the inclination of the lunar orbit to the earth’sequator is in a state of perpetual change, whence it might readily be in-ferred that the efficacy of the moon to disturb the earth would be subjectto a corresponding variation. During a revolution of the moon’s nodes theinclination of the lunar orbit passes through all its values, increasing duringthe half of this period to the extent of 10°, and then returning during theother half to its original state. It therefore occurred to Bradley that themotion of the equinoctial points, which partly depends upon the disturbingaction of the moon, would be subject to an oscillatory movement of sensiblemagnitude, the period of which would coincide with the time of a completerevolution of the moon’s nodes. Upon comparing the observations of starsnear the solstitial colure that were almost opposite in right ascension, hefound that the change of declination, though opposite in direction, wasequal in quantity; for while y Draconis had advanced northward, thesmall star 35 Camelopardali appeared to have moved as far towards thesouth. A nutation of the earth’s axis was, therefore, in this case, capableof explaining the phenomenon, although it was found to be inadequate tothat end, when the question referred to aberration. The same conclusionfollowed from a comparison of observations of other stars that, taken twoand two, were nearly opposite in right ascension.

In 1727, when Bradley commenced his observations, the moon’s ascend-ing node was in Aries, and the inclination of the lunar orbit was at itsmaximum. In 1736 the lunar node was in the beginning of Libra,having completed half a revolution in virtue of its retrograde motion,and the inclination of the orbit had consequently now descended to itsminimum. During the intermediate period the change in the declina-tion of stars near the solstitial colure, was less by 18" than that whichwould have resulted from an annual precession of 50"; for y Draconis,which ought to have advanced about 8" farther towards the south, was ob-served in 1736 to be situate 10" more to the north than it was in 1727. Ifthe phenomenon was supposed to arise from a change in the obliquity ofthe ecliptic, it would indicate a diminution equal to 1' in 30 years. As this