MOTION OF THE SOLAR SYSTEM. 
81 
the heavens, it is manifest they do not proceed from this or any similar common 
cause, but belong to the stars themselves, though the true and genuine cause may 
remain unknown for ages. 
This conclusion, if understood as applying to the whole of the changes of position 
indicated by the comparison of the catalogues, was no doubt correct ; but it is evi- 
dent that, although the apparent displacements may not be capable of being com- 
pletely explained on the hypothesis of a solar motion, it by no means follows that 
they do not in part depend upon this cause, and that, widely as the observed motions 
may differ in their relative directions, there may not still be a preponderance of mo- 
tion, or a general tendency to move, towards some determinate point. Lambert, 
writing in 1761, remarked that the apparent changes in the positions of the fixed stars 
depend on the motion of the sun as well as on the motions of the stars themselves, 
whence, he says, “ we may perhaps in time arrive at the means of determining towards 
what region of space the sun holds its course.” The same philosopher also noticed 
that the rotation of the sun on its axis gives rise to a probability of its translation in 
space, although no proof can be given that the latter motion is a necessary conse- 
quence of the former. 
The probability of the opinion that the observed proper motions of the stars are 
compounded of a real and an apparent motion was also noticed by Michell, who, in 
a note to a paper published in the Philosophical Transactions for 1767, remarked 
that the apparent change of situation which has been observed in a few of the stars, 
is a strong circumstance in favour of the opinion that those stars are among the 
nearest to us ; and that the apparent displacement may be owing either to a real mo- 
tion of the stars themselves, or to that of the sun, or partly to the one and partly to 
the other. And, he adds, “as far as it is owing to the sun’s motion it may be re- 
garded as a kind of secular parallax, which, if the annual parallax of a few of the 
stars should some time or other be discovered, and the quantity and direction of the 
sun’s motion should be discovered also, might serve to inform us of the distance of 
many of them, which it would be utterly impossible to find out by any other means.” 
Lambert’s argument, that the fact of the sun’s rotatory motion about its axis affords 
a presumption of its translation in space, was adopted by Lalande, who, in a memoir 
presented to the Academy of Sciences of Paris in 1776, concludes that inasmuch as 
the application of any force causing a body to turn about its centre cannot fail to 
displace the centre, the sun must necessarily have a real motion in absolute space. 
This argument will not be allowed to have much weight when it is considered that 
the sun’s rotatory motion may, and probably does, proceed from causes wholly dif- 
ferent from an eccentric impulsion. 
Whatever degree of probability such a priori considerations may be supposed to 
give to the hypothesis of the sun’s proper motion, it is evident that something more 
is necessary to render the hypothesis of any practical importance. The first astro- 
nomer who attempted to prove the existence of the sun’s motion from observations, 
MDCCCXLVII. M 
