884 
MR. G. H. DARWIN ON THE SECULAR CHANGES IN 
The periodic time of the satellite Deimos is 30 hrs. 18 m.,* and as the period of rota¬ 
tion of Mars is 24hrs. 37m.,+ Deimos must be still receding from Mars, but very slowly. 
The periodic time of the satellite Phobos in 7 hrs. 39 m. ; therefore Phobos must be 
approaching Mars. It does not seem likely that it has ever been remote from the 
planet. 
The eccentricities of the orbits of both satellites are small, thouedi somewhat 
uncertain. The eccentricity of the orbit of Phobos appears however to be the larger 
of the two. 
If the viscosity of the planet be small, or if oceanic tidal friction be the principal 
cause of change, both eccentricities are diminishing ; but if the viscosity be large, 
both are increasing. In any case the rate of change must be excessively slow. As we 
have no means of knowing whether the eccentricities are increasing or diminishing 
this larger eccentricity of the orbit of Phobos cannot be a fact of much importance 
either for or against the present views. But it must be admitted that it is a slightly 
unfavourable indication. 
The position of the proper plane of a satellite is determined by the periodic time of 
the satellite, the oblateness of the planet, and the sun’s distance. The inclination of 
the orbit of a satellite to its proper plane is not determined by anything in the system. 
Hence it is only the inclination of the orbit which can afford any argument for or 
against the theory. 
The proper planes of both satellites are necessarily nearly coincident with the 
equator of the planet; but it is in accordance with the theory that the inclinations of 
the orbits to their respective proper planes should be small.J 
Any change in the obliquity of the equator of Mars to the plane of his orbit must 
be entirely due to solar tides. The present obliquity is about 27°, and this points also 
to an advanced stage of evolution—-at least if the axis of the planet was primitively 
at all nearly perpendicular to the ecliptic. 
W e now come to the system of Jupiter. 
This enormous planet is still rotating in about 10 hours, its axis is nearly perpen¬ 
dicular to the ecliptic, and three of its satellites revolve in 7 days or less, whilst the 
fourth has a period of 16 days 16 hrs. This system is obviously far less advanced 
than our own. 
The inclinations of the proper planes to Jupiter’s equator are necessarily small, but 
* ‘Observations and Orbits of the Satellites of Mars,’ by Asaph Hall. Washington Government 
Printing Office, 1878. 
t According to EAiser, as quoted by Schmidt. ‘Ast. Naeh.,’ vol. 82, p. 333. 
j For the details of the Martian system, see the paper by Professor Asaph Hall, above quoted. 
With regard to the proper planes, see a paper by Prof. J. C. Adams read before the R. Ast. Soc. on 
Nov. 14, 1879, R. A. S. Month. Not. There is also a paper by Mr. Marth, ‘Ast. Nach.,’ No. 2280, vol. 95, 
Oct., 1879. 
