months, others in years, or in hundreds of years, are denominated Periodic Inequalities.
The inequalities of the other kind, though occasioned likewise by the disturbing energy of the planets, are entirely independent of their relative positions: they depend upon the relative positions of the orbits alone, whose forms and places in space are altered by very minute quantities in immense periods of time, and are, therefore, called Secular Inequalities.
In consequence of the latter kind of disturbances, the apsides, or extremities of the major axes of all the orbits, have a direct but variable motion in space, excepting those of the orbit of Venus, which are retrograde; and the lines of the nodes move with a variable velocity in a contrary direction. The motions of both are extremely slow; it requires more than 114755 years for the major axis of the earth's orbit to accomplish a sidereal revolution, that is, to return to the same stars; and 21067 years to complete its tropical motion, or to return to the same equinox. The major axis of Jupiter's orbit requires no less than 200610 years to perform its sidereal revolution, and 22748 years to accomplish its tropical revolution, from the disturbing action of Saturn alone. The periods in which the nodes revolve are also very great. Besides these, the inclination and eccentricity of every orbit are in a
