在单位半径的球体中心对球表面上的投影面积所张的角。

The Angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius.

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如果你愿意，也可以认为，车轮的半径是一个单位长度。

If you want, let's say that we take the unit of length to be the radius of our wheel.

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也就是说，单位长度等于半径。

So, take the length unit to be equal to the radius.

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