# Visual binary star with circular apparent orbit

Does anyone know a good example of visual (or interferometric) binary star apparent orbit of which looks circular due to projection though in fact is markedly elliptical? I need that for teaching.

• Having an orbit look circular, but knowing it is elliptical implies you know the inclination of the orbit wrt to our line of site. That's a tall order as usually inclination is not known (unless we know we're looking at the system edge on). Feb 24 at 20:38
• I asked about visual or interferometric binaries, not spectroscopic. Feb 24 at 21:27
• Not sure that's possible, the opposite (circular looking elliptical) is far more likely. Only a face-on (inclination=0), eccentricity=0 binary is going to look like a circular orbit; any higher inclination or any eccentricity>0 is going to look like an ellipse isn't ? Maybe a diagram is needed ? Feb 25 at 0:35
• This is a cool question! The two ellipses will have the same eccentricity and share a focus, but when viewed obliquely so as to appear circular, while the will still appear to orbit around a common point it won't be the center of either circle. This reminds me of this situation
– uhoh
Feb 25 at 0:54
• @astrosnapper you can always make an ellipse look like a circle. Imagine slicing a cylinder obliquely to make the ellipse, then looking down the axis of the cylinder
– uhoh
Feb 25 at 0:55