“Here the law of series is manifest, and the approximation can easily be carried as far as we wish. But the longitude and latitude, variables employed by nearly all lunar theorists, are far from having such simple expressions; in fact their coefficients cannot be finitely expressed in terms of Besselian functions. And if this is true in the elliptic theory how much more likely is a similar thing to be true when the complexity of the problem is increased by the consideration of disturbing forces?…There is also another advantage in employing coordinates of the former kind (rectangular): the differential equations are expressed in purely algebraic functions, while with the latter (polar) circular functions immediately present themselves.”