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).

[image/video deleted]

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

This page is a permanent link to the reply below and its nested replies. See all post replies »

helenS · 36-40, F

I've never heard a word for it. "Focal distance" means something entirely different.

Bel6EQUJ5

@helenS No, it's not the focal distance, which has to do with lenses and mirrors in basic optics.

helenS · 36-40, F

@Bel6EQUJ5 It's the distance between a lens and its focus.
I think the distance of a point on the circumference of an ellipse from any of its foci is also called "focal distance".

Bel6EQUJ5

@helenS No, no, no. Not the distance to the circumference. Look at the image above. See the two foci, marked F? THAT distance, the distance between them.

helenS · 36-40, F

@Bel6EQUJ5 I know what you mean. I didn't say the distance between the foci is called "focal distance".

Bel6EQUJ5

@helenS Okay, sorry 😟

helenS · 36-40, F

@Bel6EQUJ5 It's an interesting finding that there is no generally accepted word for it. I mean ellipses have been the subject of many studies for more than 2000 years!

Bel6EQUJ5

@helenS Exactly! It's weird that something that's been known about for thousands of years now doesn't even have a proper name. 😳