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the stars’ proper motions, that these orbs are not fixed in space. 
Because we are quite certain that no motion which could pos- 
sibly be assigned to the sun by astronomers would account for 
all the stellar motions, whatever assumption we might form 
respecting the stellar distances. That this is so will be evident 
from the simple consideration that there are cases where two 
stars near each other (in appearance) are moving in exactly 
opposite directions. We cannot possibly account for such 
motions as these by any assumption involving fixity for both 
stars and motion in the case of our sun alone. 
Furthermore, the motions of those double stars which form 
binary systems suffice to show that motion is an attribute ap- 
pertaining to some stars ; and if to some stars as well as to our 
sun (which is but a star), why not to all ? 
But now notice a point of great importance in connection 
with the question of movements of recess or approach. 
The apparent proper motions of the stars give us the means 
of estimating their probable motions of recess or approach, 
and thence of estimating our chance of determining such reces- 
sions or approaches. 
In the first place it will be admitted that we have no reason 
whatever for believing the stellar motions to be limited to any 
special direction. If we imagine our sun for a moment set at 
rest, and that we could then tell the exact direction in which 
every star is moving, we should doubtless find that the stars 
were moving in every direction with respect to the lines of 
sight drawn to them. Here a star would be moving almost 
square to the line of sight ; here nearly along it and towards 
us ; here nearly along it and from us ; and elsewhere every 
possible variety of direction would appear without the slightest 
preference (when the whole celestial sphere was considered) for 
one direction rather than another. 
Amongst the immense number of stars, then, whose proper 
motions have been determined, there must be many whose 
motion is very nearly square to the line of sight from the 
observer on earth. And we have no reason for supposing that 
the stars which are thus moving have less or greater motions, 
on the average, than their fellows which are moving in other 
directions. Hence the motions of the former set of stars afford 
us a measure of the motions which those stars possess which are 
moving directly from or towards us, and so appear at rest ; and 
tliough we can thus learn little respecting the real rate at which 
those stars are moving, for we know little respecting the real 
rate at which the other stars are moving square to the line of 
sight, we liave a very satisfactory measure of the general rate at 
whicli the relative distances of the stars are diminishing or in- 
creasing. 
