Moving an object's center node

I am having a hard time selecting a center node and moving it from it's default position.

I am making an arc shaped label with degree marks.  I draw the the first mark using a rectangle.  the rectangle has a default center node.  I will use the Transform > Rotate to make a more degree marks.  To do that I want to move the center node of the rectangle to the center of the arc.  I am having a hard time grabbing the center node to move it to where I want.   I've done it before and I assume there is some sequence of selection clicks on the node that allows it to be moved.

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