title: What is the distance in terms of millimetre between each degree on a circle?
body: <p>I'm totally a beginner in Trigonometry so my question may seem so trivial for many of you.
  If my understanding is correct, based on the trigonometry a degree is defined by dividing the circumference of a circle into 360 equals parts so that each of those parts is called a degree. Now imagine that you open a circle and roll it on the table to form a simple straight segment (as if you actually drew a segment on a piece of paper using a ruler). You would then have a straight segment divided by 360 equal parts. What would be the distance between each degree ( = each division) in terms of millimetre on that segment? The reason that I ask this question is that I was looking to a protractor as you can see in the picture below:</p>
  <p><a href="https://i.stack.imgur.com/lAX23.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lAX23.png" alt="enter image description here" /></a></p>
  <p>The bottom of this protractor is an ordinary ruler and above of that we can see the measures of the degrees from 0 to 180. When I compare <strong>visually</strong> the measures on the ruler on the bottom with the degrees measures on the top of the protractor, it seems that they are the same and each degree has a distance of 1 millimetre from the next or previous degree. Is this true? Sorry if the question seems somewhat trivial for many of you but I'm completely a beginner in the field and I just try to understand how these units were actually defined.</p>