<p>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.</p>

CC BY-SA 4.0
2021-02-24T20:38:48.713Z
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).
zephyr
CC BY-SA 4.0
2021-02-24T21:27:15.863Z
I asked about visual or interferometric binaries, not spectroscopic.
Leos Ondra
CC BY-SA 4.0
2021-02-25T00:35:47.727Z
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 ?
astrosnapper
CC BY-SA 4.0
2021-02-25T00:54:21.023Z
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](https://space.stackexchange.com/q/26578/12102)
uhoh
CC BY-SA 4.0
2021-02-25T00:55:13.543Z
@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
CC BY-SA 4.0
2021-02-25T19:56:15.653Z
@astrosnapper Elipse is nothing but a circle compressed in one axis (x, for example). And similar transformation will change the ellipse back to circle. Of course, the focus of the ellipse will not be in center of the circle, but that's another problem.
Leos Ondra
<orbit><binary-star><orbital-elements>
Visual binary star with circular apparent orbit
Leos Ondra
# Visual binary star with circular apparent orbit

(Asked by: username_0 on 2021-02-24)

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.

**Tags:** orbit, binary-star, orbital-elements

> **Comments:**
> **username_1**: 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).  
> **username_0**: I asked about visual or interferometric binaries, not spectroscopic.  
> **username_2**: 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 ?  
> **username_3**: 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](https://space.stackexchange.com/q/26578/12102)  
> **username_3**: @username_2 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  
> **username_0**: @username_2 Elipse is nothing but a circle compressed in one axis (x, for example). And similar transformation will change the ellipse back to circle. Of course, the focus of the ellipse will not be in center of the circle, but that's another problem.  

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## 0 answers

