Projected Semi-major Axis

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.

Study Astronomy Online at Swinburne University
All material is © Swinburne University of Technology except where indicated.