Photographer's Survival Manual: A Legal Guide for Artists in the Digital Age (Lark Photography Book): Edward C. Greenberg, Jack Reznicki: 9781600594205: Amazon.com: Books
(HIGHLY recommended book that everyone here could benefit from reading and having as a reference)
See pages 22-25, "What Is Copyrightable?"
In short, it must have "protectable elements" to be copyrightable (and enforceable - the copyright office will take your money and register it because they don't actually review the photos submitted. But that doesn't mean it will hold up in court).
Example from the book: "Two Hands Shaking", shot hundreds of thousands of times and applied to business-type imagery - we've all seen them - not copyrightable or protectable because there are no "protectable elements". It's an oft-used generic image - an "idea" and you can't copyright an idea - only a particular artistic expression of an idea.
To be copyrightable and protectable, the image must have some very specific elements that make it singularly unique from the generic idea. And that doesn't mean one of the hands is wearing a ring or a watch or cuff links; it doesn't mean different skin colors or gender or age or any of that stuff. It has to be truly one-of-a-kind unique, like one of the hands needs to be of a scaly green creature maybe.
So - Not copyrightable:
Photos of the moon are not copyrightable for the same reason. Unique photos that feature the moon in them may be, as long as they themselves are not simply non-unique photos of generic ideas.
So, this very generic, non-unique photo I made of an eclipsed moon
is not copyrightable:
Again, the copyright office will take the money and issue a certificate saying that it is, but certificate or not, a copyright claim for this image will not hold up in court.
On the other hand, this very unique image I made that features the moon
is copyrightable:
Get the book. Seriously. It's worth every penny.