Why do we say 'The earth' and not 'An earth'?

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As I understand, noun which is singular and start with vowels, we put an article "an" in front of it. ( e.g an eye, an ear )
But why it is not "an earth" and why "the earth"?

This might be relevant from the EL&U site: http://english.stackexchange.com/q/33282/80039 It's not a duplicate - just more information about articles and the Earth/Sun/Moon.

– ColleenV – 2015-04-07T18:51:36.920

The place a female fox uses to raise her young is called 'an earth' – Rick – 2020-08-03T21:02:29.030

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You seem to be mixing up two rules here.

We use an instead of a when the next word starts with a vowel sound. Indeed, 'a ear' would sound clumsy, so we say 'an ear'.

We do not use an instead of the; the English have no problem at all saying 'The ice is thick' or 'The ears of a cat are fluffy' (in fact, I would be hard-pressed to find a sentence with 'an ice' in it, since 'ice' is a mass noun).

The distinction between a / an and the is well explained in the other answers.

2..unless we're talking about "an ice cream sundae" :9 Jokes aside, great post. The difference between "a" and "the" is hard enough for people to learn without adding "a" vs "an" to the mix. – Keiki – 2015-04-07T13:44:42.057

Funnily enough, when I looked up what the 'ice' is called in that case (noun modifier), they used 'ice ....' as an example

– Sanchises – 2015-04-07T13:51:18.333

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It does not have to be the definite article (the), even with an "unique object" such as earth or moon. Websites and other sources that say this are either incorrect or giving generalizations. It depends on the speaker's intent whether to make a definite reference (the earth) or an indefinite reference (an earth). This is true even when we are talking about the planet we live on.

An earth without warfare would be preferable to an earth with warfare, some say.

Lucy hopes that global policies are working toward an earth that is healthy rather than an earth that is sick.

An earth with volcanoes, earthquakes and tsunamis is the only earth we have.

If global policies do not change, then by 2055 we will inhabit an earth incapable of sustaining the human race for more than another 100 years.

As for moon, even when restricting our meaning to our moon, we can still use the indefinite article. See my own question on ELU: How on earth can we say 'a moon'?

9It's worth clarifying that all the times you use 'an' you're talking about multiple possible earths that are imagined. You ask the reader to imagine 'an earth without warfare'. It's equally valid to write 'An earth without warfare is not the earth we live on'. – Pureferret – 2015-04-07T14:20:43.117

1I guess it's worth pointing out that it should be a "unique object", not an. – JiK – 2015-04-07T14:21:37.390

Yes, @Pureferret that is correct, It is up to the writer to decide whether to make an indefinite or definite reference to any noun. I'm just pointing out various possible indefinite references. One sentence I wrote makes both an indefinte and definite reference: An earth with volcanoes, earthquakes and tsunamis is the only earth we have. I could have left out only. – None – 2015-04-07T14:26:44.727

3One thing to note is that "Earth" is capitalized if and only if it is used without an article. – Panzercrisis – 2015-04-07T14:27:44.090

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@Panzercrisis That is debatble, seems to be a matter of choice and/or style, and is refuted by three real world examples that Maulik V shows in this question. Some folks may wish us to refer to Earth or earth or Planet Earth or to the earth or to The Earth or so on, in a certain manner, but this is prescriptive..

– None – 2015-04-07T14:33:12.130

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The question needs more context.

You would use 'the Earth' when 'Earth' is a proper noun, referring to a specific place or thing, the exact planet on which we live. You would use 'an earth' when 'earth' is used more as a pronoun referring to an imaginary or hypothetical Earth-like planet.

1

The things that are unique (celestial bodies: the Sun, the Moon, the Earth, the Milky Way) take the definite article. However, not all planets take the definite article (just Mars and not 'the Mars'; 'Mercury' and not 'the Mercury').

Worth noting that when the noun starts with a 'vowel', the pronunciation of the definite article is 'dhee' and not 'dhu'

So, it's 'dhu' moon in 'the moon', but 'dhee' earth in 'the earth'.

However, some references say that the article with 'earth' is optional.

Addtiional note: When used as an intensifier, the word does not take any article: Example: what on earth, who on earth

The use of the article with the Sun, the Moon, and the Earth is similar to the use of an article with the Thing or the Punisher (comic book characters) but not with Superman; names which are common nouns which are also descriptive of the thing named are often preceded by articles but capitalized. The Sun is a sun; the Moon is a moon. The Earth is the most notable chunk of earth. Although mercury is a common noun, it is a metal which, while named for the same God Mercury as the planet, has nothing to do with the planet. – supercat – 2015-04-07T15:32:12.320

2I would suggest that the article be omitted from "Earth" when the term is used in relation with the other planets (to achieve parallelism with them), and included when it is used as a reference to the one planet anywhere in the known universe which supports habitable life (emphasizing its uniqueness). – supercat – 2015-04-07T15:35:12.143

@supercat this is a very useful comment +1 :) – Maulik V – 2015-04-08T05:11:25.583

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In addition to the science/science fiction use of multiple Earthlike planets, we can also use "an earth" when we're speaking of soil, as for instance "Thanks to years of applying compost, my garden has an earth that's fertile and easy to work."

2I don't think this is correct. In this case "earth" is a mass-noun, so it would be "My garden has earth that's fertile and easy to work". – Mooing Duck – 2015-04-07T21:39:13.560

@Mooing Duck: I think it can properly be used either way, and I have seen it so used. There would be a subtle difference in meaning, as to whether you're speaking of the earth (= soil) in general, or contrasting yours with some other(s). – jamesqf – 2015-04-07T22:41:46.947

1Mass noun an earth here analogous to a shaving cream, meaning a type or kind of earth (clay, rocky, arable) or shaving cream (gooey, aerated...) @Mooing Duck – None – 2015-04-08T00:27:49.067