Page:Proceedings of the Royal Society of London Vol 1.djvu/217

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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 avery 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 occasioning 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 sites.

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 affected 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.