There are an infinite number of points on the circumference of a circle and an infinite number of points on a semi-circle so the answer to your question is "An infinite number of pairs of points."
Measure the distance between the two end points!
I guess 3 . Take these 3 points 2 at a time . These two points would be the diameter of the circle .
They are straight lines within a circle that meets the cicumference at 2 points and the circle's diameter is its largest chord
Measure the perimeter of the wheel using a tape measure. Then use the formula to find out the diameter D = perimeter/3.14 ========================================== You could certainly do it that way. But it would be much more direct to just measure all the way across the wheel, between the two points on the edge of the wheel that are furthest apart. That would be the diameter.
the equator
No. The circles are circles. The poles are points. If the circles were points, then they might be called the Arctic and Antarctic Poles, but they wouldn't be called the Arctic and Antarctic Circles since they would be only points.
Calipers are for measuring diameter of rounded objects. For instance you measure a ball, then carefully not changing the measurement pull the caliper off the ball and measure the gap between it's points with a ruler.
Like most celestial bodies, Jupiter, the Solar System's largest planet, is an ellipsoid rather than a perfect sphere. This means that its diameter is different depending on where you measure it. The distance between Jupiter's farthest points and thus its longest diameter is at its equator. That is about 142,984 kilometers. That is the diameter that is normally used by astronomers. The Jupiter's shortest diameter is the distance between the planet's poles, which are its closest points. It is approximately 133,708 kilometers.
the measure of a straight-line distance between two points is called length.
Small circles
The circles themselves represent elevation, the disatnce between the circles(elevation points) show distances between elevations, therefore showing you rate of elevation increase/decrease, also.
You connect a known voltage across the two points in the circuit, and measure the current that flows between the points. The ratio (voltage) / (current) is the resistance between those points.