the shape of the curve depends on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight.

It is impossible to determine individually the semi-major axis a and the inclination of the orbit plane i. However, the product of the semi-major axis and the sine of the inclination (i.e. ) may be determined directly in linear units (e.g. kilometres). If either a or i can be determined by other means, as in the