Circumcircles

Free Math animations on the web. 

Circumcircle is a circle that passes through every vertex of a triangle.

https://www.mathopenref.com/trianglecircumcircle.html

Play with the vertexes to change the size of the triangle and change the circumcircle. 

Right triangles are a special case:

In the case of a right triangle, the hypotenuse is a diameter of the circumcircle, and its center is exactly at the midpoint of the hypotenuse. This is the same situation as Thales Theorem, where the diameter subtends a right angle to any point on a circle's circumference. (www.mathopenref.com).