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p Mj. Hence, on tlie whole, the action of the tangen-tial forces produces no effect; and, accordingly, whenthe moon’s orbit is placed as in fig. 36, the combinedaction of the radial and tangential forces producesvery little change in the position of the perigee.
It will be found that precisely similar reasoningapplies to the position of the lunar orbit illustrated infig. 37.
The perigeal advance attains its greatest rate whenthe lunar orbit has its major axis directed towardsthe sun (that is either as in fig.-34, or with theapogee nearest to the sun). The perigeal regressionattains its greatest rate when the major axis is atright angles to the line from the sun (that is whenthe lunar orbit is either as in fig. 35, or with the majoraxis directly reversed). As the orbit changes fromthe former position to the latter (through the effect ofthe earth’s motion round the sun), the advance of theperigee per lunar month gradually diminishes until itvanishes, and then changes into regression,which con-tinually increases until it attains its maximum value.Thence as the orbit changes to the former positionagain, the regression gradually diminishes until itvanishes, and then changes into advance, whichcontinually increases until it attains its maximumvalue. The maximum rate of perigeal advance isabout 11° in. a lunar revolution, the maximum rateof perigeal regression is about 9° in a lunar revolution.It is easy to see why the former exceeds the latter,for in the case illustrated by fig. 34, the advance of