I have been reading through a couple of books by, Sir Thomas L. Heath on Greek Mathematics. And had an idea that it might be fun to see if I could get some working geometric shapes going to represent several of the theorems.

One theorem of interest is by Thales an inscribed right angle inside of a circle.

So how do we make a circle in javascript?

This is a very basic script(no functions), we build a container which is our canvas then point that to an id on the page.
I have broken out the x&y coordinates and a radius so we can use them in calculations at a later date. If we wanted a simple arc, then we can adjust the start and finish of the path.
<p> <canvas id="myCanvas"
 width="300"
 height="300"
 solid #d3d3d3;">
 Your browser does not support the canvas tag.
 </canvas></p>

 var c=document.getElementById("myCanvas");
 var ctx=c.getContext("2d");
 ctx.beginPath();
 //circle
 var x = 100;
 var y = 100;
 var r = 50;
 var start = 0;
 var finish = 2\*Math.PI;
 ctx.arc(x,y,r,start, finish);
 ctx.stroke();

To keep this post small and to the point I am not going to add anything else, but the next phase will be to bisect the circle. This will be a horizontal line, and using the x&y with the radius can you see how this might be done?