ON MAPPING THE SURFACE OF THE MOON. 3 
indistinctness is not occasioned by the state of the earth’s atmosphere, but 
by some other agency. 
ParricutaR PHEenomEnA.—Under this head may be classed observations of 
Linné, the spot IV A*!7, IV A$ *9, Alpetragius d, and others of the same kind 
(see post, pp. 29 and41). From my own observations, and from several others 
which have come to hand, it appears that those phenomena that have been 
considered indicative of ‘‘ change” have been mostly characterized by occa- 
sional indistinctness of the objects observed, contributing to the suspicion 
that such objects have either disappeared, or that new craters have been 
formed. Without a very careful discussion in connexion with solar alti- 
tudes and azimuths, and the difference between the angle of incidence on the 
moon’s surface and the angle of reflexion from the moon’s surface, which is 
equal to the supplement of the angle ( —©, itis very difficult to refer 
such phenomena to their legitimate sources. 
Cuaner.—tThe question of “‘ change” on the moon’s surface still remains 
undecided. Although the circumstances associated with the earlier records 
and delineations fail to invest them with that authority which is necessary 
to a decision of the question, they are nevertheless exceedingly important, 
and a careful study of the works of the four leading selenographers, Schroter, 
Lohrmann, Beer and Miidler, and Schmidt, is essential to a competent know- 
ledge of the surface of our satellite. The “facts” recorded by them are 
very numerous. These facts, compared with the results obtained by the aid 
of photography, and with those of recent observation, must tend in no small 
degree to advance selenographical knowledge. 
In the course of such a comparison many differences will be found. Lar- 
lier delineations and photograms will not agree ; and in seeking for an expla- 
tion of such differences, we are naturally led to regard variations of distance, 
hbration, and illumination as fruitful sources of apparent change. It has 
been a matter of solicitude in preparing the areas and catalogue already issued, 
to define the extent of apparent change produced by alterations of distance 
and by libration, which near the middle of the moon is but small. Appa- 
rent changes occasioned by differences in the angles of illumination are not 
so easily dealt with. Before we can venture to express an opinion on a sup- 
posed apparent change as dependent upon the sun’s altitude above and his 
azimuth at any particular spot, it is manifestly necessary to know all the 
changes of appearance which the object undergoes as the sun rises higher 
above it, culminates, and declines. This necessarily involves a considerable 
amount of calculation, especially if the three coordinates are employed, viz. 
the latitude of the spot, the sun’s declination, and his hour-angle. Some 
approximation, however, may be made to the sun’s altitude at intervals of 
12 hours, from his rising at the moon’s equator, on a point at which the lon- 
gitudes of the terminator and the spot agree, to his meridian passage at the 
same point. I am accordingly preparing a set of Tables of Solar Altitudes 
at intervals of twelve hours (nearly) for every five degrees of lunar latitude. 
and also for the solstices and equinoxes at each. The Tables for the Equator, 
5°, and 10° of latitude will be found on pp. 10 and 11. 
Before a correct judgment can be formed on the gradations of appearance 
presented by any one spot as the sun’s altitude increases and declines, it is 
necessary to obtain observations of that spot at intervals of at least 12 hours, 
The greatest change of altitude during this interval is a little more than 6° 
on the equator. This change in 12 hours decreases as a spot is situated 
N. or S. of the equator. To obtain a sufficient number of observations for 
this purpose, observers must confine in some degree their attention to parti- 
B2 
