Progress of Astronomy during the Year 1879. 445 
The preceding statements may be subject to varieties of detail, 
according to the nature of the tides raised in the Earth, but the 
above is a summary of what appears to be the most probable 
course of evolution. The hypothesis which is suggested as most 
probable is, that the more recent changes in the system have been 
principally due to oceanic tidal friction, and that the more ancient 
changes were produced by bodily tidal friction. The initial state 
of the Moon, nearly in contact with the earth, and always opposite 
the same face of it, suggests that the Moon was produced by the 
rupture in consequence of rapid rotation or other causes of a 
primeval planet, whose mass was made up of the present Moon 
and Earth. It is a remarkable coincidence, that the shortest 
period of revolution of a fluid mass of the same mean density as 
the earth, which is consistent with an ellipsoidal form of equili- 
brium, is two hours and twenty-four minutes; and that if the 
Moon were to revolve about the Earth with this periodic time, 
the surfaces of the two bodies would be almost in contact with 
one another. The theory gives an interesting explanation of the 
rapid movement of the inner satellite of Mars. 
4. The Sun. 
Sun-spots were during the greater part of the year either 
totally absent or were only represented by very small points, 
until on October 18th a group of three large spots showed that a 
new period of activity had commenced. 
Carrington remarked that before the Sun-spot minimum of 
1855-56 the spots had moved nearer to the solar equator, and 
that after the minimum the new spots began to appear in higher 
latitudes. Prof. Sporer (A. N. 2282) has seen this phenomenon 
repeated in 1867. The mean heliographic latitude at the time of 
a maximum appears to be 17°, while the spots at the time of a 
minimum do not go nearer the equator than to latitude 8°7. 
Time will show whether this phenomenon is a regularly occurring 
one. 
Several communications have appeared in Natwre (xx., 131, 
146, and 189) on observations of sun-spots made with the naked 
eye before the invention of the telescope. The last one, by Mr. 
Hind, is the most interesting one; it contains a number of 
