12 
ASTRONOMY: S. B. NICHOLSON 
So far as the program has gone, therefore, all the evidence points to a 
great distance for these objects. As the spirals are undoubtedly in 
revolution — any other explanation of the spiral form seems impossible — 
the failure to find any evidences of rotation would indicate that they 
must be of enormous actual size, and at enormous distances from us. 
At a distance of one- thousand light-years, supposing mass conditions 
to be the same as in our solar system, a nebular structure one minute of 
arc in apparent distance from its nucleus, or 60 times the distance of 
Neptune from the Sun, would have a yearly angular movement of only 
53", or a maximum yearly displacement in a given component with re- 
ference to the nucleus of only 0.015". A much greater time interval 
will probably be necessary before nebular rotations can be definitely 
established by measures of position, unless a nebula abnormally close 
to us be found. 
Wolf and Slipher have reported the observation of rotation in the spiral 
nebulae Messier 81 and N. G. C. 4594, respectively, by paeans of the 
velocity-displacements of lines in the spectra of these bodies. The 
spectrographic method, here as elsewhere, is independent of the distance 
or absolute size of the object. 
DISCOVERY OF THE NINTH SATELLITE OF JUPITER 
ByS. B. Nicholson 
LICK OBSERVATORY. UNIVERSITY OF CALIFORNIA 
Presented to the Academy. October 31.1914 
The Ninth Satellite of Jupiter was discovered on photographs taken 
with the Crossley Reflector of the Lick Observatory, on July 21 and 22, 
1914. The photographs were of two and one-half hours exposure, and 
were taken to secure observations of the Eighth Satellite. The new satel- 
lite is fainter than the Eighth, and is estimated as nineteenth magnitude. 
The identity of the object as a satellite was established from the obser- 
vations of July 22, 27, and 31 by Leuschner's Method of Direct Solution of 
Orbits of Disturbed Bodies. The motion is retrograde, and the first 
estimate of the period places it at about three years. The remaining 
elements of the orbit resemble those of the Eighth Satelhte. Additional 
observations of the satelhte have been secured in August and September. 
A detailed account of the discovery and observations, and of the investi- 
gation of the orbit, will be pubHshed in a Lick Observatory Bulletin. 
