Biography of Percival Lowell

By A. Lawrence Lowell | Go to book overview

APPENDIX I

Professor Henry Norris Russell's later views on the size of Pluto (written to the Biographer and printed with the writer's consent).

LATER investigations have revealed a very curious situation. When once the elements of Pluto's orbit are known, the calculation of the perturbations which it produces on another planet, such as Neptune, are greatly simplified. But the problem of finding Pluto's mass from observations of Neptune is still none too easy, for the perturbations affect the calculated values of the elements of Neptune's orbit, and are thus "entangled" with them in an intricate fashion.

Nicholson and Mayall, in 1930, attacked the problem, and found that the perturbations of Neptune by Pluto, throughout the interval from its discovery to the present, were almost exactly similar to the effects which would have been produced by certain small changes in the elements of Neptune's orbit, so that, from these observations alone, it would have been quite impossible to detect Pluto's influence. Outside this interval of time, the effects of the perturbations steadily diverge from those of the spurious changes in the orbit, but we cannot go into the future to observe them, and all we have in the past is two rather inaccurate observations made in 1795 by Lalande.1 If the average of these two discordant observations is taken as it stands, Pluto's mass comes out 0.9 times that of the Earth, and this determination is entitled to very little weight.

Uranus is farther from Pluto, and its perturbations are smaller; but it has been accurately observed over one and a half revolutions, as against half a revolution for Neptune, and this greatly favors the separation of the perturbations from changes in the assumed orbital elements. Professor E. W. Brown--the most distinguished living student of the subject--concludes from a

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1
See page 181supra.

-203-

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