Page:Philosophical Transactions - Volume 095.djvu/272

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Dr. Herschel on the Direction of the

position of other double stars point out, and this cannot be probable.


Direction of the solar Motion.

From what has been said, I believe the expedience of admitting a solar motion will not be called in question; our next endeavour therefore must be to investigate its direction.

To return to the before mentioned intersections of the arches, in which the proper motions of the stars are performed, I shall begin by proving that when the proper motions of two stars are given, an apex may be found, to which, if the sun be supposed to move with a certain velocity, the two given motions may then be resolved into apparent changes, arising from, sidereal parallax, the stars remaining perfectly at rest.

Let the stars be Arcturus and Sirius, and their annual proper motions as given in the Astronomer Royal's Tables.

When the annual proper motion of Arcturus, which is -1",26 in right ascension, and +1",72 in north polar distance, is reduced by a composition of motions to a single one, it will be in a direction which makes an angle of 55° 29' 42" south-preceding with the parallel of Arcturus, and of a velodty so as to describe annually 2",08718 of a great circle.

The annual proper motion of Sirius, -0",42 in right ascension, and +1",04 in north polar distance, by the same method of composition, becomes a motion of 1",11528, in a direction which makes an angle of 68° 49' 41" south-preceding with the parallel of Sirius.

By calculation, the arches in which these two stars move, when continued, will meet in what I have, called their parallactic center, whose right ascension is 75° 39' 50", and south polar