drawGrid20x20On: aCanvas

    | radius_squared |
    active
        ifFalse: [^self].

    radius_squared := self radius squared.
    aCanvas
        strokeWidth: 1 color: Color veryLightGray
        do: [ :canvas |
            -120 to: 120 by: 20 do: [:eachX |
                | edge |
                edge := (radius_squared - eachX squared) sqrt.
                canvas moveTo: eachX @ edge negated;
                lineTo:eachX @ edge].
            -120 to: 120 by: 20 do: [:eachY |
                | edge |
                edge := (radius_squared - eachY squared) sqrt.
                canvas
                    moveTo: edge negated @ eachY;
                    lineTo: edge @ eachY]].