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.

  • $\begingroup$ 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). $\endgroup$
    – zephyr
    Feb 24, 2021 at 20:38
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    $\begingroup$ I asked about visual or interferometric binaries, not spectroscopic. $\endgroup$
    – Leos Ondra
    Feb 24, 2021 at 21:27
  • $\begingroup$ 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 ? $\endgroup$ Feb 25, 2021 at 0:35
  • $\begingroup$ 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 $\endgroup$
    – uhoh
    Feb 25, 2021 at 0:54
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    $\begingroup$ @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 $\endgroup$
    – uhoh
    Feb 25, 2021 at 0:55


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