Page:Proceedings of the Royal Society of London Vol 1.djvu/219

Rh two, yet, in doing this, we must assign a high degree of real motion to Capella. To this it may be objected, that we have no reason t5 deprive Arcturus and Sirius of real motions, in order to give a motion of the same nature to every star that has a proper motion not tending to the same parallactic centre as the motions of Arcturus and Sirius.

It appears, therefore, that such an apex for the solar motion ought to be ﬁxed upon as is equally favourable to every star that is proper for directing our choice ; and our aim must be, to reduce the proper motions of the stars to their lowest quantities.

From a table given by Dr. Herschel, it appears, that the sum of the apparent motions of the six principal stars whose intersecting arches are given, namely, Sirius, Arcturus, Capella, Lyra, Aldebaran, and Procyon, is 5"‘353; and if we suppose the point towards which the sun moves to be A Herculis, the annual proper motions of the six stars will be reduced to real motions of no more than 2”'219.

It appears, from the inspection of a ﬁgure that represents the quantities of real motion required when A Herculis is ﬁxed upon. that, by a regular method of approximation, a situation might be found where the apparent motion of the six stars would be much re- duced. Accordingly, by ﬁxing upon a point near the following knee of Hercules, whose right ascension is 270° 15', and north polar di- stance 54" 45', the annual proper motion of the six stars was reduced to l"'459, which is 0"‘760 less than when the apex was A Hemulis.

In approximating to the above point, the line of the apparent mo- tion of Sirius was principally considered; but, as Sirius is not the star that has the greatest proper motion, it occurred to Dr. Herschel that another minimum, obtained from the line in which Arcturus seems to move, would be more accurate; and he was scon led to a point, not only in the line of the apparent motion of Arcturus, but equally favourable to Sirius and Procyon, the remaining two stars that have the greatest motion. The right ascension of this point is 245° 52' 30", and its north polar distance 40" 22’.

If the principles which have been laid down for determining the solar motion are admitted, the above apex must be very near the truth; for an alteration of a few minutes in right ascension or polar distance, either way, will increase the required real motion of these stars. The sum of the real motions with the before-mentioned apex is only '859. being less than that of the former calculation by '599.

Dr. Herschel does not, he says, mean to assert that these real mo- tions can be actually reduced to the low quantities above mentioned; but, whatever may be the sum of real motions required to account for the phenomena of proper motions, the foregoing arguments can. not be affected by the result; for, as it is known that proper motions exist, and no solar motion can resolve them entirely into parallactic motions, we ought to give the preference to that direction of the motion of the sun that will take away more real motion than any other.