2 Draw a circle with the Ellipse tool (L). Hold Shift while drawing to constrain it to a perfect circle.
3 Select the circle and click on the fill icon in the Gradient panel. Click on the Gradient thumbnail to apply the black-and-white gradient to the fill of the circle.
Editing Gradients
4 Double-click on the color stop below the gradient ramp. This will open the Swatches panel under the ramp.
5 Choose a color from the Swatch list.
6 Select the other color stop; double-click and choose white.
Offset Path
7 This is how the circle looks with the gradient applied to the fill.
8 Choose Object > Path > Offset Path. Set the Offset amount to -15 and click OK. This creates an exact duplicate of the shape inside (because of the negative number). A positive number will create a shape larger than the original.
9 Select the inside shape and click on the Reverse Gradient button in the Gradient panel to flip the gradient direction.