Not accurately rotating elements around center of circle

[sgswink]

I found an odd issue with rotating elements around the centerpoint of a circle. To recreate the issue:

1. Create a circle
2. Create a triangle with points aligned to the circumference of the circle
3. Rotate the triangle around the centerpoint of the circle 

Theoretically, the points of the triangle should remain aligned with the circumference of the circle. However, Illustrator is shifting the tringle off the circumference. The issue is only visible at high magnification, but  causes issues when trying to create precise compound shapes. 

I was able to recreate this issue across multiple machines, running Illustrator 29.7.1 and 29.8.1. No snap-to preferences are in use. 

Is this user error I am overlooking, or is there an issue with how Illustrator is computing the rotation? 

[Ton Frederiks · Accepted solution]

See this discussion:

https://community.adobe.com/t5/illustrator-discussions/is-there-a-way-to-draw-a-perfect-circle-in-il...

You may want to create a circle using a script that uses more than 4 anchor points for added precision.

Like the Circle scipt here: https://shanfan.github.io/Illustrator-Scripts-Archive/

Or the Ovalize script here: https://github.com/shspage/illustrator-scripts/blob/master/etc/ovalize.jsx

[sgswink]

Fascinating, somehow haven't run into this issue before. 

[Ton Frederiks]

It is hardly visible, so you only notice when zoomed in very far.

[sgswink]

Only noticed it by the hard-to-align elements when trying to build complex compound shapes, and by the odd remnent pieces when combining/subtracting sub-shapes. Makes for  "meh, good enough" set up and fussy clean up. But yes, not visible unless you zoom in to a crazy amount. 

[Doug A Roberts]

It used to be more easily noticeable, but the way bounding boxes are drawn for ellipses was changed slightly to lie to you:

This 1000 px circle is rotated 22.5 degrees with the rotate tool. It still says it's 1000 px, but the circle is drawn past the edge of the bounding box.

If we expand the live shape and reset the bounding box, we can see that the bounding box is actually now 1000.2617 px:

[sgswink]

@Doug A Roberts -- I had previously assumed Illustrator acted like AutoCad and created primitive geometric shapes with its primitive shape tools (circle) and converted to bezier arcs once the primitive shapes were altered or otherwise "released."  Looks like it's all bezier all the time. The perfect circle we get when we first make one with the tool is a mirage, only true because the anchor points for the bezier curves are on the horizontal/vertical planes. Thought I was going crazy.

Still kinda maddening, but at least there's a why behind those thousandths of a pixel, extra overlapping and misaligned items.