 Originally , earth was written in lowercase , and from early Middle English , its definite sense as " the globe " was expressed as the earth . By early Modern English , many nouns were capitalized , and the earth became ( and often remained ) the Earth , particularly when referenced along with other heavenly bodies . More recently , the name is sometimes simply given as Earth , by analogy with the names of the other planets . House styles now vary : Oxford spelling recognizes the lowercase form as the most common , with the capitalized form an acceptable variant . Another convention capitalizes " Earth " when appearing as a name ( e.g. " Earth 's atmosphere " ) but writes it in lowercase when preceded by the ( e.g. " the atmosphere of the earth " ) . It almost always appears in lowercase in colloquial expressions such as " what on earth are you doing ? " 
