Treat the 3D sphere as a 2D circle. The radius for the sphere is the same radius as for the circle. No matter where on the sphere you place a mark, the distance (radius) from the mark to the centre of the sphere will always be the same as the circle.
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A spherometer is an instrument for the precise measurement of the radius of a sphere. The spherometer directly measures a sagittal. Since the spherometer is essentially a type of micrometer, it measures very small things like millimeters.
The formula is 2pie(radius).
To find the circumference of a circle the formula is: 2*pi*radius or diameter*pi
to find the radius you have to measer trow the circle then divid by 2
Circumference=2*pi*radius