The distance between the two foci of an ellipse is called... what?

I know all about the axes, both minor and major, about eccentricity, the semi-latus rectum (no, that's got nothing to do with Uranus), and all else there is to know; except this. What's it actually called? Not how to calculate it, nor the symbol for it, but the name (if it even has one).

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If it doesn't have a name, I propose that we Christen it the 'Bi-Focal Displacement'. :)

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helenS · 36-40, F

It's interesting that there is no simple formula to calculate the arc length L of an ellipse.
When I was still a girl I thought L should be equal to
L = π * (a+b),
where a and b are the half-axes; because I thought that, for a circle,
L = 2πr = π * (r+r)
so for an ellipse it should be π * (a+b).
I was hugely disappointed when I learned that my naïve approach was totally wrong... 😕

Bel6EQUJ5

@helenS Finding out that we're wrong is part of the learning process. 😊

helenS · 36-40, F

@Bel6EQUJ5 I made a new maths friend! Nice 🌷

Bel6EQUJ5

@helenS Yay! ☺