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In the second minimum (the 3rd step) there is a smaller decrease in light intensity. For this to happen, wouldn't you need to be looking at the plane of orbit from above rather than directly along the plane of orbit? Because if a person was looking directly along the plane, then it would just be a full eclipse of the star behind in step 3?

But then if a person is not directly looking along the plane but just sees the plane from above, doesn't this mean the definition of eclipsing binaries is wrong? The definition is 'when the plane of orbit of stars is in the line of sight from the Earth'. By 'line of sight' I assumed it meant a person from Earth is able to look directly along the plane. So was my assumption wrong?

  • 2
    $\begingroup$ I think your understanding of the situation is pretty good and the problem may simply be the source of the wording "'when the plane of orbit of stars is in the line of sight from the Earth". Is this an exact quote, or perhaps you are not remembering the exact phrasing? Eclipsing binaries are definitely viewed from a variety of different (but small) angles away from the orbital plane. $\endgroup$
    – uhoh
    Jul 14, 2020 at 10:00
  • $\begingroup$ @uhoh yes, it's the wording issue. If it is from a variety of angles, that would solve the problem. It's just because that was the exact wording, so I'm assuming the variety of angles isn't applicable here? $\endgroup$
    – XXb8
    Jul 15, 2020 at 14:03

1 Answer 1


I assume that the diagram indicates what the observer sees (if they had a big enough telescope!). i.e. The viewpoint is nearly in the orbital plane but not quite. Why then are the eclipses asymmetric, with the secondary eclipse being shallower than the primary? Well probably because the surface brightness of the two stars is different - i.e. they have different surface temperatures. The secondary would be cooler, so when its surface is obscured by the primary, then that results in less flux being eclipsed. Note that this would be the case whether you were looking along a line of sight exactly in the plane of the orbit or at a small angle to it.

There is no definition of an eclipsing binary that says the plane of the orbit must be in the line of sight from Earth. It has to be close to that in order to get eclipses, but it does not have to be exactly true.


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