| 22 | The Solar System |
on the vast scale of our solar system, the gap which sunders it from the nearest fixed star is something enormous. Two hundred and seventy-five thousand times our distance from the sun is the space that divides us from the next sun, the star α Centauri. This distance is found by noting the shift in the star's position due to the extreme swing of the Earth in her orbit called the annual parallax. It is a very minute displacement at most, and requires perhaps the most delicate of all astronomical refinement to detect. Incidentally it affords conclusive evidence of itself that the Earth goes round the Sun, not the Sun round the Earth.
Fortunately α Centauri, our nearest stellarα Centauri. neighbour, is a double star, a binary system, and thus of itself affords us information of the region over which it exercises control. Assuming that gravity acts there just as it does here,—any other possible assumption implies that the force depends on the orientation, which does not seem rational,[1]—we can deduce from the motion of the
- ↑ Binaries move in apparent ellipses. Parallel projection keeps an ellipse an ellipse and the centre the centre. From the general polar equation of a conic and the differential equation of the orbit,
, it appears that the only laws of force which do not depend on θ, that is, on the orientation, are , which is negatived by the fact that no star has yet been found in the centre of the apparent ellipse, and , which is thus the only law possible which is rational. It is thus ably put by Moulton (Celestial Mechanics, 1902).
