On the Proper Motion of the Sun and Solar System

Author: William Herschel

On the Proper Motion of the Sun and Solar System

William Herschel

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 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, strictly 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 consequently 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 until all are in motion. Now as we know that several stars, in divers parts of the heavens, do actually change their places, it will follow, that the motion of our solar systems is not a mere hypothesis; and what will give additional 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 been originally at rest.

Admitting this for granted, the greatest difficulty will be, how to discern the proper motion of the sun among so many other, and variously compounded, motions of the stars. This is an arduous task indeed, which we must not hope to see accomplished in our time; but we are not to be discouraged from the attempt. Let us, at all events, says Mr. H., endeavour to lay a good foundation for those who are to come after us. I shall therefore now point out the method of detecting the direction and quantity of the supposed proper motion of the sun by a few geometrical deductions, and at the same time show by an application of them to some known facts, that we have already some reasons to guess which way the solar system is probably tending in its course.

It remains now only to make an application of this theory to some of the facts we are already acquainted with, relating to the proper motion of the stars. Astronomers have already observed what they call a proper motion in several of the fixed stars, and the same may be supposed of them all. We ought therefore to resolve that which is common to all the stars, which are found to have what has been called a proper motion, into a single real motion of the solar system, as far as that will answer the known facts; and only to attribute the proper motion of each particular star, the deviations from the general law the stars seem to follow in those movements. By Dr. Maskelyne’s account of the proper motion of some principal stars, we find that Sirius, Castor, Procyon, Pollux, Regulus, Arcturus, and a Aquilae, appear to have respectively the following proper motions in right ascension: -0".63; -0".28; -0".80; -0".93; 0".41; -1".40+0".57; and two of them, Sirius and Arcturus, in declination, viz. 1",20 and 2".01, both southward. Let figure 10 represent an equatorial zone, with the above mentioned stars referred to it, according to their respective right ascensions, having the solar system in its centre. Assume the direction ab from a point somewhere not far from the 77th degree of right ascension to its opposite 257th degree, and suppose the sun to move in that direction from s towards b; then will that one motion answer that of all the stars together: for if the supposition be true, Arcturus, Regulus, Pollux, Procyon, Castor and Sirius, should appear to increase. Again, suppose the sun to ascend at the same time in the same direction towards some point in the northern hemisphere, for instance, towards the constellation of Hercules; then will also the observed change of declination of Sirius and Arcturus be resolved into the single motion of the solar system. But lest Mr. H. should be censured for admitting so new and capital a motion on too slight a foundation, he observes, that the concurrence of those seven principal [p.349] stars cannot but give some value to an hypothesis that will simplify the celestial motions in general. We know that the sun, at the distance of the fixed star, would appear like one of them; and from analogy we conclude the stars to be suns. Now, since the apparent motions of these seven stars may be accounted for, either by supposing them to move just in the manner they appear to do, or else by supposing the sun alone to have a motion in a direction, somehow not far from that above assigned to it, we are no more authorized to suppose the sun at rest, than we are to deny the diurnal motion of the earth, except in this respect, that the proofs of the latter are very numerous, whereas the former rests only on a few though capital testimonies.

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Chicago: William Herschel, On the Proper Motion of the Sun and Solar System in The Library of Original Sources, ed. Oliver J. Thatcher (Milwaukee, WI: University Research Extension Co., 1907), 347–349. Original Sources, accessed October 19, 2019, http://originalsources.com/Document.aspx?DocID=8ISCJYAETY2PIGM.

MLA: Herschel, William. On the Proper Motion of the Sun and Solar System, in The Library of Original Sources, edited by Oliver J. Thatcher, Vol. 6, Milwaukee, WI, University Research Extension Co., 1907, pp. 347–349. Original Sources. 19 Oct. 2019. originalsources.com/Document.aspx?DocID=8ISCJYAETY2PIGM.

Harvard: Herschel, W, On the Proper Motion of the Sun and Solar System. cited in 1907, The Library of Original Sources, ed. , University Research Extension Co., Milwaukee, WI, pp.347–349. Original Sources, retrieved 19 October 2019, from http://originalsources.com/Document.aspx?DocID=8ISCJYAETY2PIGM.