how about this explanation: think of an arc as a series of very short line segments. Now take one of those line segments and lengthen it (don't change its angle). That line is tangential to the arc. How did I do?
looking at the arc - draw a line from the centre of the circle that describes the arc to the place on the arc where you want the tangential line to begin. now construct a line 90 degrees to the radial line at the point where it hits the arc. that line is tangential to the arc.
strictly speaking thats not a tangent - as any line will be inside or outside of the circle of the arc no matter how short it is and so what youve described is a very close approximation to an arc..... might not seem like a lot until you want to get to mars.... LOL
