The title of a letter in the October Harper’s Magazine from Howard Passell (of the Earth Systems Analysis Department, Sandia National Laboratories , Albuquerque NM):
Harper’s Magazine is one of the most progressive periodicals being published, yet it lingers in the dark ages when it comes to referring to the planet on which we live. In “Emptying the World’s Aquarium” [Letter from the Sea of Cortez, August], Erik Vance writes that “there is no better place on earth to look at the future of global fishing” than the Gulf of California. This is a story about what’s in the water, not in the soil, so the word “earth” is obviously incorrect. Referring to Earth as “earth” is a vestige of the Judeo-Christian legacy. You can’t have dominion over our planet or pillage it quite so easily if linguistically you put it on the same level as all the sacred words we capitalize. Please change your style. This is an egregious philosophical error in an otherwise excellent story on the decline of our Earth.
Yes, it’s a rant about capitalization. And it presumes that earth (so spelled) can have only one meaning (‘land’, as opposed to sea and sky) — this despite the fact that every dictionary and style sheet I’ve looked at treats the word as ambiguous between this ‘land’ sense and reference to our planet.
[Added 9/18: The Earth in tlhe title is not some bit of clever commentary on the part of the editors about Passell's position -- emphatic on earth is always lowercased, as I point out below -- but the consequence of capitalizing all the significant words in titles.]
Here’s NOAD2, distinguishing the two senses (and offering the capitalized Earth as an alternative for the first):
earth (also Earth) the planet on which we live; the world: the diversity of life on earth .
the surface of the world as distinct from the sky or the sea: it plummeted back to earth at 60 mph.
and listing a number of other uses of the word, including:
like nothing on earth informal very strange: they looked like nothing on earth.
on earth used for emphasis: who on earth would venture out in weather like this?
(Emphatic on earth is one of a set of “extenders” of interrogative wh words: also in hell, the hell, the fuck, etc. As far as I can tell, earth is never capitalized in this use.)
OED3 (Nov. 2010) says of the case issue:
The world on which mankind lives, considered as a sphere, orb, or planet.
Usu. with the (or our). In later use freq. with capital initial, esp. when compared or listed with other planets of the solar system.
The names of the other planets are all orginally proper names (of gods) and are consequently always capitalized. But our planet’s name originated from the ‘land, surface of the world’ sense (by semantic extension) and so tends to resist capitalization except in contexts where other planets are mentioned. (I have no idea where Passell got his “Judeo-Christian legacy” idea.)
The Associated Press Stylebook attempts to negotiate a middle course, while following its general inclination to reduce capitalization:
earth Generally lowercase; capitalize when used as the proper name of the planet. She is down-to-earth. How does the pattern apply to Mars, Jupiter, Earth, the sun and the moon? The astronauts returned to Earth. He hopes to move heaven and earth. (link)
Roughly, then, if the world can be used in the context, then (the) earth (lowercased) can be too. If you generally favor lowercasing, then the advice above would counsel lowercasing in the Harper’s quote (given “there is no better place in the world to look …”). And it would counsel lowercasing in
The balloon deflated and fell to earth.
(even if the balloon fell into water); compare this with the astronauts returning to Earth.
In actual educated usage, there is clearly variation on this point. In part, this variation reflects a gray area in the classification of nouns as common or proper. The noun refers to a unique entity, but that doesn’t necessarily make it a proper name.