Page:The Solar System - Six Lectures - Lowell.djvu/40



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 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 Centauri, our nearest stellar  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, —we can deduce from the motion of the