But what one sees is the projection of this orbit on the “plane of the sky”. (The “plane of the sky” is the phrase almost universally used by observational astronomers, and there is no substantial obj...But what one sees is the projection of this orbit on the “plane of the sky”. (The “plane of the sky” is the phrase almost universally used by observational astronomers, and there is no substantial objection to it; formally it means the tangent plane to the celestial sphere at the position of the primary component.) The projection of the true orbit on the plane of the sky is the apparent orbit, and both are ellipses.
