Original question (my answer is below): Little doubts the accuracy of the usage of “the” instructed by English syntax manuals. I think to use “the” to mean something unique should be incorrect.  For example: When I say that there is a pocket in front of me. I might describe it as“The packet is mine”, not " Packet is mine. ” Similarly, A sun means a planet, while a moon means a smaller planet.But each galaxy has its own planets and smaller planets as well. As long as there is "the" before sun or moon,we can then make it narrowed down to the solar system to which we belong. Another case is that if there are two individuals with the same name as Bill Gates. I might say that The Bill Gates I met yesterday.  So, the explanation, I consider, that unique creature or thing require to be modified by "the"shouldn't be appropriate.