All I did was point out a similarity between two illusions, and that similarity remains. This neither adds nor detracts from the simple fact that a shadow, no matter how dark it is, cannot appear on the ion tail of a comet. This would be similar to casting shadow on a lit light bulb. The tail is ionized by the Sun and energized to produce its own light, therefore the proposed shadow will not happen.

Now you can come up with examples of shadows on lit bulbs, but I will simply counter with the fact that the blocked light source would have to be enormous -- and the Sun most certainly fits this requirement. ...But the resolution of the film used and the jpeg format would be lost on this.

Technically, the particles deep within the core of the tail cone will be in shadow of the bow shock, which will keep them from the ionization process and safe from being energized to light. In this aspect, it certainly is a shadow, but not of the nucleus itself.

As a further FYI, the comet nucleus itself is not as large as the supposed shadow would indicate it to be anyway,
Dan