240 
POPULAR SCIENCE REVIEW. 
respecting the physical condition of the Moon, Mars, and 
Jupiter, the information would largely surpass in real interest 
the most correct determination of the Sun’s distance. If any 
proof of this were needed, it would be found, I conceive, in 
the fact that every piece of information obtained respecting 
the physical condition of the planets is at once recognized as 
full of significance, whereas when a change has been made in 
our estimate of the Sun’s distance, corresponding changes are 
made in the numerical relations indicated in our text-books, and 
that is all. No suggestions are made, and indeed none seem 
needed, to the effect that we must modify views formerly enter- 
tained respecting either the Sun or the members of his family. 
The determination of the Sun’s distance, in fact, is a problem 
of surveying and statistical astronomy, not of that living as- 
tronomy, which alone has any interest for us as reasoning in- 
habitants of one of the worlds which fill God’s universe. 
The aspect which will be presented by Mars as seen in the 
telescope will not differ greatly at any time during the approach- 
ing opposition from that indicated in the six illustrative projec- 
tions (PI. VII.). These represent six stages of Martian rotation, 
separated by 60°, or by 4 hours of Martian time. On Sept. 5, 
Mars comes into opposition at midnight. At this hour the Mar- 
tian meridian crossing the centre of the disc of Mars will be 
very nearly in 26° east Martian longitude, or the meridian passing 
some 15° east of Dawes’ Forked Bay in the accompanying 
chart (p. 245). The view nearest to this in the series of six is 
No. 2, in which the central meridian is in 30° Martian longitude 
east. The rotation of Mars occurring in the direction shown 
by the arrow, and one degree of rotation being completed in 
about 4m. 6^s., it follows that the aspect shown in No. 2 will 
be presented at about 16^- minutes before midnight Sept. 5. 
The observer will have no difficulty in determining when the 
other views may be looked for. On any the same night, the 
interval between one view and the next amounts to about 4h. 
6m. 14s. (the rotation period of Mars being 24h. 37m. 22*7s.); 
and neglecting the angular motion of Mars about the earth, 
which in such a case we may do for short intervals of time, any 
view changes nearly into the 'preceding for the same hour of 
the night in the course of six nights. For, in each terrestrial 
day the planet completes one rotation, less the amount of rota- 
tion corresponding to 37m. 22*7s., and six times this daily 
loss of rotation gives the rotation corresponding to about 3h. 
44m. 16s., or only 22m. short of the amount corresponding to 
one-sixth of a Martian day. In seven days, the loss corre- 
sponds to 4h. 21m. 39s., or only about 15|m. more than the 
amount corresponding to one-sixth of a Martian day. Thus, 
neglecting his angular motion round the earth, Mars presents 
