When you finished creating you gradient, select the paint bucket tool and click on your black circle around the spot indicated on fig8
fig8
Here we go we have the first layer of our chrome sphere, press F8 and convert your sphere as a graphic symbol and name it "sphere"
4. above the layer "sphere" create another layer and name it "sphere_shadow" (fig9)
Again draw a 150x150 black circle (fig3) without stroke (fig4)
fig9
5. In the color mixer panel select the Radial fill style and create another gradient as shown on fig10
fig10
the RGB color values and the alpha from left to right are :
R:255 G:255 B:255 A:0%
R:0 G:0 B:0 A:100% When you finished creating you gradient, select the paint bucket tool and click on your black circle around the spot indicated onfig11
fig11
press F8 and convert your sphere as a graphic symbol and name it "sphere_shadow". Select the sphere_shadow graphic and in the properties panel set the _alpha value of the graphic to 80% (fig11a)
and click on your black circle around the spot indicated on 
