Who decided, and when , that a right angle was 90 degrees ?

Probably the Greeks or Egyptians, I dunno, but whatever, the dude that decided on 90 degrees couldn’t have given the matter much thought.

Like, 90 is divisible by, 2, 3, 5, 9 , 10 , 15, 30 and 45. What use is that ?

Now imagine if the dude had decreed, “we’re going to have 120 degrees in a right angle …” …WOW …

Straight off, the right angle is divisible by 2,3,4,5,6,8, 10, 12,15,20, 30, 40 and 60. Only problem is that it ain’t divisible by 9 …well DUH …

How cool is that ?

Is there any valid reason why the international standards body couldn’t still adopt this concept ?

Nobody decided that a right angle was 90 degrees. The concept of 90 degrees was named “right angle.” It is what it is. I suppose there could be a calculus in which a circle is 720 degrees. Far out.

Are you asking why the word “right” is used to describe it, or why it’s divided up into 90 degrees? I think it’s the latter. That was thanks to the Babylonians, who divided up a circle into 360 degrees. It was the ability to divide up 360 lots of different ways that they liked. The 90 degree thing was just a side effect.

Just divide your circles into 480 degrees and your right angles will be 120 degrees.

I think the question here is “why is the circle divided into 360 degrees, rather than some other number of divisions?”

(not “why is a right angle square?”)

ETA: dammit - took too long composing that.

The Babylonains divided the circle into 360 degrees. Apart for the divisibility reasons stated, I suspect that the year having 365 days is not a complete coincidence.

Because all the really influential mathematical texts in western civilization have used 360 degrees.

Then, there are all those crazies who decided that a full circle should consist of 6.28318531 (approximately) angular units.

I mean, would it fly if we had 6.28318531 inches to the foot? Or 6.28318531 ounces to a pound?

It is a right angle, not “The Correct Angle”. Mathematicians use words differently.

The question is nonsense, NONSENSE ! A right angle isn’t 90°. It’s π/2 radians.

Left angles don’t come out right if you use anything other than 90 degrees for the right angle.

No, but it flies with circles. If your circle was 12 inches around, would it fly that the diameter was 3.81972… inches? Of course, if your diameter is one inch, it doesn’t help that there are an integer number of degrees in a circle when your circumference is 3.141592… inches.

Irrational numbers are a fact of life with circles. Might as well make that plainly obvious with 2*pi radians, and simplify all the math while you’re at it.

You mean τ/4 radians.

100 grads in a right angle. You know it’s right, because it’s metric.

A lot of geometry would get screwed up if there were no 90 degree angles.

Equilateral, Isosceles and Scalene triangles. Acute, Obtuse and Right triangles. We studied those for weeks. There’s always 180 degrees in any triangle.

Right, but unAmerican.

Radians, it’s the right thing to do, and the right way to do it.

/Wilford Brimley

provided you are using Euclidean geometry.

Angus Right the famous mathematician.

Observing just how many degrees of separation there are among the responses so far …

If we limit it to three degrees, we can just sing about it. Increase it to six and we can make a movie or play. Or just ask Kevin Bacon.