197 



On the Direction and Velocity of the Motion of the Sun, and Solar 

 System. By William Herschel, LL.D. F.R.S. Read May 16, 

 1805. \_Phil. Trans. 1805, p. 233.] 



Although, in the title to this paper, Dr. Herschel mentions both 

 the direction and velocity of the solar system, it is his intention, he 

 says, to limit his inquiries, at present, to the first of these subjects, 

 and to discuss the other at some future opportunity. He is induced 

 to enter into this inquiry, because a solar motion, if established, 

 seems to contradict the original intention for which it was intro- 

 duced ; namely, to take away many of the proper motions of stars, 

 by investing the sun with a contrary one. But as the solar motion 

 will reveal a greater number of concealed real motions than need be 

 admitted if the sun were at rest, the necessity of admitting its motion 

 ought to be well established. 



From the motion of the secondary planets round the primary ones, 

 and of these round the sun, the solar motion must be allowed to be 

 a very possible event ; and the rotatory motion of the sun, from which 

 a displacing of the solar centre has been inferred, also indicates a 

 motion of translation in space ; for it does not appear probable that 

 any mechanical impression should produce the former without occa- 

 sioning the latter. 



It would, Dr. Herschel thinks, be worth while for those who have 

 fixed instruments, to observe those stars which change their magni- 

 tudes periodically ; for, as this change is probably owing to a rotatory 

 motion, a real motion in space may be expected to attend it : and, 

 on the other hand, all those stars that have a motion in space may be 

 supposed to have also a rotation on their axes. 



Dr. Herschel now proceeds to consider the symptoms of parallactic 

 motions. If, says he, the sun be supposed to move towards a certain 

 part of the heavens, the stars will appear, to an inhabitant of the 

 earth, to move in an opposite direction. This may be called the 

 parallactic motion of a star ; and, if the star has no real motion, it 

 will also be its apparent motion ; but, if the star should have a real 

 motion, it will appear to move along the diagonal of a parallelogram, 

 which diagonal will represent its real motion. This is illustrated by 

 a diagram, to which we must refer for a fuller explanation of this 

 part of the paper. We shall only observe, that the absolute motion 

 of a star in space will still remain unknown, as well as its velocity, 

 because the inclination of that motion, on which its real velocity will 

 depend, admits the greatest variety of directions. 



In order to ascertain whether parallactic motions exist, we ought, 

 Dr. Herschel says, to examine the brightest stars ; it being probable 

 that they are most liable to be visibly aft'ected by solar motion : and 

 we should also seek for a criterion by which parallactic motions may 

 be distinguished from real motions. This we find in their directions ; 

 for, if a solar motion exists, all parallactic motions will tend to a 

 point in opposition to its direction ; whereas real motions will be 

 dispersed indiscriminately to all parts of space. 



