Why the reference for kilogram has to be round in the Avogadro Project?
The question popped into my mind after reading this post and the corresponding Wikipedia article. From what I've understood (please correct me if I'm wrong), they wanted to redefine the Kilogram. So they made the roundest object on earth whose mass is exactly $1$kg. But why they went through all the trouble for making a $1$kg perfect sphere?
I mean, couldn't it be an easier-to-make shape? Or why it's shape should be perfect? From a layman's point of view, when you want to make something a perfect $1$kg and a perfect sphere simultaneously, these two perfectnesses may compensate each other. You may smoothen some surfaces to make it more round but instead you might lose some material, or vice versa.
Here are the reasons that I could think of, which did not sound totally convincing:
1. The object is intended to be a reference for kilogram, which is the base for all weight measurements. This kind of object is surely a special one, so it should be special in some other ways as well to make it unique. Being the roundest object serves this purpose.
2. To illustrate some technological advancements. 439
I mean, if they are showing their ability to make such a perfect silicon sphere, it indicates that they can handle making some ultra-sensitive devices as well, such as ultra-accurate gyros and other stuff. This would justify the expenses in the long (or even short) term.
3. The molecular structure of silicon in its monocrystalline form is spatially symmetric. So the sphere shape is more compliant with this crystal form than other shapes. (Sorry for possibly inaccurate terms, I'm not so familiar with the details).
These reasons are not so related to physics except somehow the last one. Here are my questions:
* Are there any other physics-related or technical reasons for the spherical shape of the kilogram reference?
* Wouldn't it be better to put more focus on the accuracy of the weight and be less worried about its perfect shape?
P.S. Some side-notes:
I tried my best to add relevant tags, but since I am not so familiar with this site and this is my first question, some tags might be missing or irrelevant. Please feel free to edit and improve the question.
I am also having trouble understanding why the kilogram should be redefined at first place and what was wrong with the good old platinum-irridium alloy. Since this was not so relevant, I didn't mention it directly. But a really perfect answer may shed some light on this matter as well. 441