# By convention, when observing an exoplanet, is the observer's line of sight (on Earth) perpendicular to the plane of sky?

This is just a quick question. In this picture of the orbit of an exoplanet, is the observer's line of sight perpendicular to the plane of reference, which is in the +z direction on the diagram? In the case when the inclination equals to $$90^{\circ}$$, it is ideal for us to measure the radial velocity. Is that right?

• That is not a picture of the orbit of an exoplanet. That is a diagram showing the meaning of various orbital elements. – James K Sep 7 at 23:00
• @James K But when studying an exoplanet orbiting its host star, the diagram is similar to this. There's not too much difference right? – consideration Sep 7 at 23:04