The projected semi-major axis, x, of a binary system is the projection of the semi-major axis onto the line of sight:

x = a sin(i)

where i is the inclination angle of the binary orbit (with respect to the plane of the sky). This implies that orbits which are viewed face-on have x = 0, while for edge-on systems x equals the actual semi-major axis of the binary system.

This parameter is mostly important because of its direct relation to the Roemer delay, which enables its measurement through pulsar timing.