Like the majority of the Slavic languages, Russian has no articles. In Russian, Луна (Luna) is ‘Moon’. It's a proper noun when you talk about it astronomically, like the Apollo missions to the Moon, but when you describe a summer night it becomes луна (luna), a common noun, like a tree, the wind, the sound of the stream – just a part of the landscape. Even if you describe an alien planet which has three moons, in a description of the way the sky there looks like you'll use it as a common noun, взошли три луны 'three moons rose'. The same thing with Солнце (Solntse) ‘Sun’ and Земля (Zeml’a) ‘Earth’. When you talk astronomically, like about researching these Solar system bodies, they are proper nouns and capitalized, but when you speak about what you lie on and what makes your skin brown, they are common nouns, like the rest of the landscape details or elements.
Note: for languages, spelling is secondary, quite a recent invention, if it is only the spelling that allows you to differentiate between common and proper nouns, it looks like a sign that this category is not at all relevant. It is just the modern European tradition of capitalization in general and capitalization of proper nouns in particular that makes us be able to tell if the word is common or proper (or make it be one or the other). In German where every noun is capitalized you will have problems to tell exactly which noun it is, common or proper, not to speak of the languages which know no capitalization whatsoever, like Chinese or Arabic, or Hindi. That difference is pretty semantic, so pretty subjective. The difference could be objective only in a language without articles where common nouns have grammar properties different from those of the proper nouns, but I'm afraid I don't know of such languages (but I know about languages where all the proper nouns are used with articles, e.g. Greek and Portuguese).
Also consider the situation in fairy tales for children where the characters are animals and their names are the same as the names of their species, e.g. Wolf, Fox, Hare, Bear. In Russian, where there are no articles, a child listening to such a fairy tale cannot tell it for sure if those are personal names (proper nouns) or species' names (common nouns).