VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 3Q7 



XVII. On the Proper Motion of the Sun and Solar System ; ivith an Account of 

 several Changes that have happened among the Fixed Stars since the Time of 

 Mr. Flamsteed. By Wm. Herschel, Esq., F.R.S. p. 247. 

 That several of the fixed stars have a proper motion, is now already so well 

 confirmed, that it will admit of no further doubt. From the time this was first 

 suspected by Dr. Halley we have had continued observations that show Arcturus, 

 Sirius, Aldebaran, Procyon, Castor, Rigel, Altair, and many more, to be actually 

 in motion; and considering the shortness of the time we have had observations 

 accurate enough for the purpose, it may rather be wondered that we have already 

 been able to find the motions of so many, than that we have not discovered the 

 like alterations in all the rest. Besides, we are well prepared to find numbers of 

 them apparently at rest, as, on account of their immense distance, a change of 

 place cannot be expected to become visible to us till after many ages of careful 

 attention and close observation, though every one of them should have a motion 

 of the same importance with Arcturus. This consideration alone would lead us 

 strongly to suspect, that there is not, in strictness of speaking, one fixed star in 

 the heavens; but many other reasons will render this so obvious, that there can 

 hardly remain a doubt of the general motion of all the starry systems, and con- 

 sequently of the solar one among the rest. 



We might begin with principles drawn from the theory of attraction, which 

 evidently oppose every idea of absolute rest in any one of the stars, when once 

 it is known that some of them are in motion: for the change that must arise by 

 such motion, in the value of a power which acts inversely as the squares of the 

 distances, must be felt in all the neighbouring stars; and if these be influenced 

 by the motion of the former, they will again affect those that are next to them, 

 and so on till all are jn motion. Now as we know that several stars, in divers 

 parts of the heavens, do actually change their place, it will follow, that the 

 motion of our solar system is not a mere hypothesis ; and what will give addi- 

 tional weight to this consideration is, that we have the greatest reason to suppose 

 most of those very stars, which have been observed to move, to be such as are 

 nearest to us; and therefore their influence on our situation would alone prove a 

 powerful argument in favour of the proper motion of the sun, had it actually 

 been originally at rest. 



Mr. H. first gives a short but general account of the most striking changes he 

 had found to have happened in the heavens since Flamsteed's time. He had 

 made what he calls 3 reviews. The first was made with a Newtonian telescope, 

 something less than 7 feet focal length, a power of 222, and an aperture of &± 

 inches. It extended only to the stars of the 1st, 2d, 3d, and 4th magnitudes. 

 ^Of his 2d review he gave some account, in the Philos. Trans., vols. 70, 71, 72: 

 it was made with an instrument much superior to the former, of 85.2 inches 



