earth, moon :

earth moon

Soil is lower-case “earth.” And in most uses even the planet itselfremains humbly in lower-case letters: “peace on earth.” But inastronomical contexts, the Earth comes into its own with a proud initialcapital, and in science fiction it drops the introductory article andbecomes “Earth,” just like Mars and Venus. A similar pattern applies toEarth’s satellite: “shine on, harvest moon,” but “from the Earth to theMoon.” Because other planets also have moons, it never loses itsarticle.

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