I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here
I would have thought that a sphere would have been the hardest shape to machine accurately.
Answer
If you know the diameter of the sphere, you know everything you need to know about the dimensions. It all comes down to one single value.
Any other shape requires multiple dimensions and thus multiple values. Further, measuring a cube or another shape for accuracy is harder than measuring a sphere.
Making very accurate spheres is not as difficult as you might think - it's no different than making optical glass or mirrors using grinding techniques, and, in fact, they are measured much the same way with lasers for very high accuracy.
This video goes into a little more detail as to why they are doing this, how they achieved it, and how the sphere is made.
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