The elements of astronomy; a textbook . prob-able,1 though certainly not proved as yet, that the orbital motionof the pair is under the law of gravitation, we know that the 1 As has been often pointed out, the question can be decided by spec-troscopic observations whenever we become able to observe separately thetwo spectra of the components of a binary and so can determine the radialvelocity of each at several different points in the orbit. The difficultiesare great, but probably not insurmountable. 336 ORBITS OF BINARIES. [§464 larger star must be in the focus of the true relative orbitdescr
The elements of astronomy; a textbook . prob-able,1 though certainly not proved as yet, that the orbital motionof the pair is under the law of gravitation, we know that the 1 As has been often pointed out, the question can be decided by spec-troscopic observations whenever we become able to observe separately thetwo spectra of the components of a binary and so can determine the radialvelocity of each at several different points in the orbit. The difficultiesare great, but probably not insurmountable. 336 ORBITS OF BINARIES. [§464 larger star must be in the focus of the true relative orbitdescribed by the smaller ; and, moreover, that the latter mustdescribe around it equal areas in equal times. By the help ofthese principles, we can deduce from the apparent oval the trueorbital ellipse ; but the calculation is troublesome and delicate. The relative orbit is all that can be determined from micrometerobservations of the distance and position angle measured between thetwo stars. tf 58 ( £ Cancri. f-~7-90e ^^1827. 270°>61 Cygni Fig. 114. — Orbits of Binary Stars. Fig. 114 represents the orbits of four of the best determined double-star systems. In but a few cases, where we have sufficient meridian-circle obser-vations, or where the two components of the pair have had theirposition and distance measured from a neighboring star not partak-ing of their motion, we can deduce the absolute motion of each of thetwo stars separately with respect to their common centre of gravity,and thus get data for determining their relative masses (Art. 466).The case of Sirius is in point. About 60 years ago it had been found [§ 464 SIZE OF THE ORBITS. 337 from meridian-circle observations to be moving for no assignablereason in a small oval orbit with a period of about 50 years. In1862, Clark found near it a minute companion, which explainedeverything ; only we have to admit that this faint acolyte, which doesnot give Tshw as much light as Sirius itself, has a mass more
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