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.
[39] 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?"