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The two-body orbit theory described in Chapter 4 neglects
the direct gravitational interactions between the planets, while retaining those between each individual planet
and the Sun. This is an excellent first approximation, because the former interactions
are
much weaker than the latter, as a consequence of the small masses of the planets relative to the Sun. (See Table 4.1.)
Nevertheless, interplanetary gravitational interactions
do have a profound influence on planetary orbits when integrated over long periods of time. In this chapter, a branch of celestial mechanics known as *orbital perturbation theory* is used to examine the *secular* (i.e., long-term) influence of interplanetary gravitational
perturbations on planetary orbits.
Orbital perturbation theory is also used to investigate the secular influence of planetary perturbations on the orbits
of asteroids, the secular influence of the Earth's oblateness and atmospheric drag on the orbits of artificial satellites, and
the secular influence of solar radiation pressure on the orbits of interplanetary dust particles.

Richard Fitzpatrick
2016-03-31