# Why is euclidean geometry also called parabolic geometry?

Given that the three fundemental geometries are euclidean geometry (zero curvature), riemannian geometry (positive curvature) and lobachevskian geometry (negative curvature), I am curious as to what the link is between these curvatures and the conic sections of an ellipse, an hyperbole and a circle.

I have seen euclidean geometry be called parabolic geometry, riemannian geometry be called elliptical geometry and lobachevskian geometry be called hyperbolic geometry but I have not found any further explanation. How can the 2D shapes be mathematically linked to the 3D surfaces? Why is a parabola associated with flat space while a hyperbole is associated with a saddle shaped 3D space and an ellipse is associated with spherical space?

I have tried to look at curvature for a connection, however, I have only found explanations of curvature and Gaussian curvature seperately but not anything connecting the curvature of 3D space with 2D space.

I also read that gaussian curvature = 1 - eccentricity. This would theoretically make sense as for example a parabola has an eccentricity of 1 giving a curvature of zero which would relate it to flat space. However, I can only imagine this being true for some special cases as I have struggled to find a published explenation. Is there any proof for this formula?

• The currently unanswered question Parabolic, Hyperbolic, Elliptic appears related. I myself once asked about the term “elliptic” in particular. If I understood the last point in the answer to that correctly, elliptic has a greek origin related to deficit while hyperbolic would indicate surplus. Both could relate to curvature or interior angle, but the relationship to the conic sections of these names would be rather loose.
– MvG
Commented Sep 22, 2018 at 15:18
• I think you are partially mistaken. Riemannian geometry covers all three cases (positive, zero and negative curvature), and I have never heard anybody applying this term only to positive curvature. Commented Jul 27, 2020 at 21:48

From Greenberg's Euclidean and Non-Euclidean Geometries:

[I]n the early nineteenth century two alternative geometries were proposed. In hyperbolic geometry (from the Greek hyperballein, "to exceed") the distance between the rays increases. In elliptic geometry (from the Greek elleipein, "to fall short") the distance decreases and the rays eventually meet.

So the terms are not directly based on the names of conic curves, but come from the same roots. The Greek parabole had connotations of juxtaposition, comparison of one thing with another, likeness, similitude. So it was the "just right" that separated hyperballein and elleipein.

Some more background at Perisho, The Etymology Of Mathematical Terms

Update: I was leafing through Klein's Elementary mathematics from an advanced standpoint : geometry and noticed his claim to naming these geometries.

Interested parties can follow the URL for context, and the article he refers to is here (in German). (See also Campo and Papadopoulos, On Klein’s So-called Non-Euclidean geometry, page 18).

His reference to asymptotes of conics may sound strange because we are taught that only the hyperbola has asymptotes. But if asymptotes are defined as tangents to ideal points (points on the line at infinity) on the conic, then the ellipse has these but they are imaginary.

AFAIK these names come from far-fetched analogies, and other meanings of these three Greek words, rather than any specific similarities to the similarly named curves.

The name "elliptic geometry" is especially confusing to people who hear it for the first time, because it sounds like it would describe ellipsoids, while it actually deals with perfect spheres. At least it makes some sense if you consider circles/spheres to be special cases of ellipses/ellipsoids (but why not name based on the special case then?).

The name "hyperbolic geometry" feels quite appropriate though if you think of the Minkowski hyperboloid model, and also as you would use hyperbolic functions (sinh, cosh) for most distance-related computations in this geometry.

One quite far-fetched relation between Euclidean geometry and a shape of parabola is that horospheres in Minkowski hyperboloid model are represented as paraboloids, and the geometry on a horosphere is Euclidean.

• The terms elliptic, hyperbolic, and parabolic are used in many mathematical settings to describe the situations where some quantity is positive, negative, or zero. In the setting of geometry, the quantity is curvature. According to Wikipedia (en.wikipedia.org/wiki/Non-Euclidean_geometry#Terminology), Klein applied the terms to geometry. Commented Jul 21, 2020 at 17:06