I got a book about the philosophy of mathematics, Stephan Körner’s **The Philosophy of Mathematics: An Introductory Essay**. It’s a subject I’m interested in, despite my lack of training. Made it to the second page before I got to something that I had to stop and ponder. I thought to share that point and my reflections with you, because if I had to think I may as well get an essay out of it. He lists some pure-mathematical facts and some applied-mathematical counterparts, among them:

- (2) any (Euclidean) triangle which is equiangular is also equilateral
- (5) if the angles of a triangular piece of paper are equal then its sides are also equal

So where I stopped was: what is the (Euclidean) doing in that first proposition there? Or, its counterpart, about being pieces of paper?

I’m not versed in non-Euclidean geometry. My training brought me to applied-physics applications right away. I never needed a full course in non-Euclidean geometries and have never picked up much on my own. It’s an oversight I’m embarrassed by and I sometimes think to take a proper class. So this bit about equiangular-triangles not necessarily being equilateral was new to me.

Euclidean geometry everyone knows; it’s the way space works on table tops and in normal rooms. Non-Euclidean geometries are harder to understand. It was surprisingly late that mathematicians understood they were legitimate. There are two classes of non-Euclidean geometries. One is “spherical geometries”, the way geometry works … on the surface of a sphere or a squished-out ball. This is familiar enough to people who navigate or measure large journeys on the surface of the Earth. Well. The other non-Euclidean geometry is “hyperbolic geometry”. This is how shapes work on the surface of a saddle shape. It’s familiar enough to … some mathematicians who work in non-Euclidean geometries and people who ride horses. Maybe also the horses.

But! Could someone as amateur as I am in this field think of an equiangular but not equilateral triangle? Hyperbolic geometries seemed sure to produce one. But it’s hard to think about shapes on saddles so I figured to use that only if I absolutely had to. How about on a spherical geometry? And there I got to one of the classic non-Euclidean triangles. Imagine the surface of the Earth. Imagine a point at the North Pole. (Or the South Pole, if you’d rather.) Imagine a point on the equator at longitude 0 degrees. And imagine another point on the equator at longitude 90 degrees, east or west as you like. Draw the lines connecting those three points. That’s a triangle with three right angles on its interior, which is exactly the sort of thing you can’t have in Euclidean geometry.

(Which gives me another question that proves how ignorant I am of the history of mathematics. This is an easy-to-understand example. You don’t even need to be an Age of Exploration navigator to understand it. You only need a globe. Or a ball. So why did it take so long for mathematicians to accept the existence of non-Euclidean geometries? My guess is that maybe they understood this surface stuff as a weird feature of solid geometries, rather than an internally consistent geometry. But I defer to anyone who actually knows something about the history of non-Euclidean geometries to say.)

And that’s fine, but it’s also an equilateral triangle. I can imagine smaller equiangular triangles. Ones with interior angles nearer to 60 degrees each. They have to be smaller, but that’s all right. They all seem to be equilateral, though. The closer to 60 degree angles the smaller the triangle is and the more everything looks like it’s on a flat surface. Like a piece of paper.

So. Hyperbolic geometry, and the surface of a saddle, after all? Oh dear I hope not. Maybe I could look at something else.

So while I, and many people, talk about spherical geometry, it doesn’t have to be literally the geometry of the surface of a sphere. It can be other nice, smooth shapes. Ellipsoids, for example, spheres that have got stretched out in one direction or other. For example, what if we took that globe and stretched it out some? Leave the equatorial diameter at (say) twelve inches. But expand it so that the distance from North Pole to South Pole is closer to 480 miles. This may seem extreme. But one of the secrets of mathematicians is to consider cartoonishly extreme exaggerations. They’re often useful in getting your intuition to show that something must be so.

Ah, now. Consider that North Pole-Equator-Equator triangle I had before. The North-Pole-to-equator-point distance is right about 240 miles. The equator-point-to-other-equator-point distance is more like nine and a half inches. Definitely not equilateral. But it’s equiangular; all the interior angles are 90 degrees still.

My doubts refuted! And I didn’t have to consider the saddle shape. Very good. “Euclidean” is doing some useful work in that proposition. And the specification that the triangles are pieces of paper does the same work.

And yes, I know that all the real mathematicians out there are giggling at me. This has to be pretty near the first thing one learns in non-Euclidean geometry. It’s so easy to run across, and it so defies ordinary-world intuition. I ought to take a class.