SCIENTIFIC SUMMARY. 
79 
observation the motion of the apse or point where the satellite approaches 
closest to the planet ; and from this motion it will be possible to accurately 
determine the ellipticity of the planet, and in this manner from the observed 
inclination of the orbit of the satellite, ascertain with accuracy the position 
of the equator of Mars and the figure of the planet. It is unnecessary to 
point out the great interest attaching to the exact determination of the 
position of the equator, and consequently of the poles, of Mars. There is 
some reason to believe from observation that the white polar caps of the 
planet, which are generally held to be due to the presence of arctic ice and 
snow, are not concentrically situated round the poles of the planet. It would 
throw material light on the physical constitution of the planet if this point 
were definitely settled. 
The Secular Acceleration of the Mean Motion of the Moon . — The gradual 
quickening of the motion of the Moon in its revolution around the Earth has 
long been one of the most interesting points in the theory of the solar 
system. The comparison of the places assigned to the Moon by theory and 
observation seems to show that the observed acceleration is much greater 
than that assigned by theory. In consequence it has been supposed that a 
portion of the apparent acceleration may be really due to a slow retardation 
of the rotation of the Earth from the friction of the tides of the ocean against 
the crust of the Earth. Lately the researches of Professor Newcomb on the 
early observations of the Moon would seem to show that when the tables of 
the Moon are properly corrected, there is far less discordance between the 
observed and theoretical values of the acceleration of the Moon’s motion 
than has hitherto been supposed. Lately Mr. J. N. Stockwell, in the 
American Journal of Science, November, 1879, has drawn attention to a 
new origin of a secular acceleration in the mean motion of the Moon. It has 
long been known that the attraction of a spheroidal body like the Earth on 
a distant body like the Moon is slightly different from that of a sphere of 
the same mass, the attraction being greatest when -the satellite is in the plane 
of the equator and smallest when in the plane passing through the poles. 
Eor this reason a satellite revolving around the Earth in an orbit inclined to 
the equator would move slower than if it were moving in an orbit which 
was not inclined to the equator. It is known that at present the inclination 
of the lunar orbit to the terrestrial equator is very slowly decreasing, and it 
follows, therefore, that the Moon is gradually quickening in its motion round 
the Earth. Hitherto, however, it has been supposed that this acceleration 
was so small that it could not attain, even in the course of many centuries, 
to an amount which could be detected by observation. According to Mr. 
Stcckwell, this is not so, as from his calculation he makes the acceleration 
from this cause amount to 0'T981 per century. The amount of the accelera- 
tion varies as the square of the number of centuries, so that if this calcula- 
tion be correct, it will be necessary to alter, by nearly five minutes, the theo- 
retical time of the occurrence of the famous early eclipses of the Moon 
observed in Babylonia, and quoted by Ptolemy in his Almagest. 
A New Planetary Nebula . — On November 14, 1879, while the Rev. T. 
W. Webb, F.R.A.S., the well-known astronomer, was sweeping the 
heavens near the constellation Cygnus, with his 9^-inch reflector and a low- 
power eye-piece, he noticed an object resembling a 9th magnitude star, which 
