Capitonyms Are Separate Cases
Some words, in a class called capitonyms, have distinct meanings or senses when they are capitalized as opposed to generic senses. Writers should take care to render these words as appropriate to the context.
Among capitonyms are several words denoting astronomical bodies. For example, one writes that Earth is orbited by the Moon and in turn orbits the Sun. However, when we refer to the surface soil of that planet, or employ an idiom such as “down to earth,” the proper form is earth. Likewise, if we write about the planet’s satellite or the star around which our world orbits, but from the terrestrial perspective, we generally lowercase the names — for example, “The moon is full tonight” and “The sun passed behind the clouds.” (References to moons and suns beyond our solar system are also lowercased.)
Geographical capitonyms include arctic, often capitalized in reference to Earth’s northern regions but generic when referring to cold temperature or mood (Antarctic, by contrast, is generally styled as a proper noun), and alpine, which is capitalized only in reference to the Alps, in Europe.
In politics, such words as democratic, republican, conservative, liberal, socialist, and communist, generic references to concepts of political thought that serve as nouns and adjectives, are capitalized when referring to a political organization or a member of such an organization.
The treatment of god depends on whether one refers to a deity in general or to that of the Abrahamic religions (Christianity, Islam, and Judaism). In philosophical or religious contexts, words for qualities such as truth and beauty are often capitalized to signal the significance of the conceptual connotation.
Another religious term that may or may not be capitalized is mass, referring to a religious ceremony. It is often capitalized when referring to a specific religious ceremony (for example, “High Mass”) but is lowercased in generic references to such events (“He performed several masses”), as is the word when it refers to the unrelated meaning of physical phenomena.