This question refers to Earth's moon only.

This is really two questions:

  1. Our Earth has only one moon. So why and even how can we say 'a' moon?
    1b. Restated: What other moon than 'the' moon is in the class of Earth's moons so that I can say 'a' moon? See C) below.

Here is my second question:

Consider the following:
A) There's a fly in my soup.

  1. Does the use of the indefinite article restrict a fly to one of a finite number of real flies which have a realistic probability of being in my soup? I know that a generic or prototypical fly cannot be in my soup.

For instance I am in a diner in San Francisco. If I say, There's a fly in my soup, I think I am referring only to one of the 5,000 flies that are in San Francisco. I can't be referring to flies in Los Angeles or flies in Chicago, can I? I can't be referring to fictional flies or flies from outer space either. I can't really be referring to a fly that I saw yesterday, can I? If there are, say, 1 million flies in the world I can only be referring to some subset of that 1 million, not the whole 1 million, because 500,000 are in the Orient and could not possibly be in my soup (a fly from the Orient could be).

In any case, is what is being said the following:

There's a fly in my soup and I don't care to say which one it is, and I don't care which one it is--but I am referring to one of 5,000 flies in San Francisco.


B) There's a full moon in the sky tonight. (Or: There's a full moon out.) The moon is full once every lunar month. I don't know which lunar month it is. So am I saying:

There's a full moon out tonight and I don't care to say which one it is, and I don't care which one it is--but I am referring to 1 of the 12 or 13 possible full moons?


C) There's a moon in the sky tonight (Or: There's a moon out tonight.)

So am I saying: There's a moon out tonight and I don't care to say which one it is, and I don't care which one it is--but I am referring to 1 of the 1 that there is circling Earth?

The moons of other planets are not under consideration, because only a geekazoid would know when the moons of other planets are 'visible' from Earth on any given night.

What other moon than 'the' moon is in the class of Earth's moons so that I can say 'a' moon?

  • 2
    You might start by reading Birner & Ward 1994. – snailboat Oct 14 '14 at 19:09
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    A fly is not restricted to any subset of flies. It can be any of all potential or real flies in the entire universe. A moon doe snot refer to the physical object but to its current appearance. It is no different from "a weak sun" or "a clear sky". Nobody actually says this with the intention to express a lesser activity in the fusion-processes inside the physical sun, or to indicate there are multiple skies over our head. – oerkelens Oct 14 '14 at 19:14
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    I see nothing interesting in the fly example. For the moon examples - my intuition says that the indefinite article may apply to members of sets which are real or imaginary, actual or hypothetical, literal or figurative. Moons could refer to possible moons belonging to a number of hypothetical universes in which Earth's one moon always appeared in one specific form, for instance. I could also say, "My friend's an amateur detective - a real Dick Tracy!" figuratively to mean that my friend has qualities in common with the imaginary character of the same name. – Patrick87 Oct 14 '14 at 19:47
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    The word "moon" can refer to the natural satellite of any planet. Io, Ganymede, Callisto, and Europa are the four largest moons of Jupiter. You may ping astronomy.stackexchange.com as well. – barrycarter Oct 14 '14 at 19:51
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    Many moon ago, on Earth, there was only one moon. Still, we called it by many moon-names: blue moon, harvest moon, honey moon, Cosmo's moon,... Many, many moon-names. Many moons. Many moon ago, and still. – Drew Oct 14 '14 at 20:38

You're asking two separate questions. The first one is easy. We say There is a full moon, because there are many full moons, across the course of time. The fact that it could only be one particular full moon on a given day is not relevant.

There is a moon out tonight is a little tougher. Most people would usually say the moon is out tonight. I would argue that in case that the word "a" is used, the moon is actually being viewed in relationship to all the other moons on other nights, as with the full moon. However, the reason for the ambiguity is that all the moons on all the nights are really one object, and we know and accept that fact. This contrasts with the full moons, which are arguably discontinuous and separable, since we're actually referring here to a perceptual phenomenon, not an object.

You might compare There's a moon over Miami tonight. Again we're referring in this case to our perceptions. Of course it's the same celestial object over Miami and over New York. But the experience of perceiving the moon over Miami can be distinguished from the experience of perceiving the moon over New York. It's the same case with your moon spoiling the meteor shower. It's the experience of the moon shining that spoils the show, not the celestial object itself.

  • Thanks for your answer. I get a full moon. It is when it is just a moon that I still do find tougher. I'll think on your answer. – pazzo Oct 14 '14 at 20:47
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    Any moon you like. The moon is only visible on about half of clear nights; the rest of the time it occurs during daylight. And every night it looks different. So "a moon" can mean any sliver of moon, which explains why "the moon" might not be right when you can only see 1/10 of it. Don't forget, the phrase is much older than the astronomy. – John Lawler Oct 14 '14 at 21:01
  • I edited to add some more examples of the distinction. – Chris Sunami Oct 14 '14 at 21:09
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    @CarSmack That looks like locative there rather than existential there. Pay attention to the pronunciation: in existential there, the vowel is reduced to schwa. – snailboat Oct 16 '14 at 0:36
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    Funny how we can say “There's a moon out tonight”, but we'd never dream of saying “There's a sun out today”. I suppose the sun always looks pretty much the same when we can see it, unlike the moon. – Janus Bahs Jacquet Jun 27 '15 at 12:27

There are 28 "moons" in our sky, which is why we can say, There is a full moon tonight.

We have:

a new moon, a crescent moon, a few various waxing crescent moons, a first quarter moon, a few waxing gibbous moons, a full moon, etc back to a new moon.

Of course they are phases of the moon. But they aren't unique. They happen over and over and over again. How many full moons have you seen in your lifetime? Therefore, one of them is called a full moon. Just as you have seen a squirrel or a cat.

You can, if you insist, call it the first full moon of the month when you happen to have two full moons in a one month period, but that only happens once in a blue moon.

We usually only have one Harvest Moon a year. Still, because it's been seen hundreds of times (though not by us), it's still a Harvest Moon. If you're referring to a Harvest Moon in 2014, though, it is the 2014 Harvest moon.

In short, it's my belief that your reasoning is faulty.

Edited to add: Finally, most people don't say, there's a moon out tonight. They usually say, The moon is out tonight or Look at the moon! It's so big! or The moon is lovely/bright/full/so cool tonight.

However, on googling, There's a moon out tonight, it does get a lot of hits; mostly as lines of a song where the line is meant to harmonize with the other a's:

There's a moon out tonight whoa-oh-oh ooh
Let's go strollin'.
There's a girl in my heart whoa-oh-oh ooh
Whose heart I've stolen...
There's a glow in my heart whoa-oh-oh ooh I never felt before There's a girl at my side whoa-oh-oh ooh That I adore... - The Capris, 1960

Or a poem, or the title of a work. It was even the title of a piece in the New York Times on blood moons.

I wonder if the use of There's a moon out tonight reflects the popularity of the song, or if it has a dialectical/regional origin that is found in the song?

I still maintain that most people either say, the moon is out tonight or, if they do say, there's a moon out tonight, it reflects a phase-usage, not a lack of awareness that we only have one moon.

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    Okay, so that applies to a full moon, according to your first sentence. And a moon? Referring only to Earth's moon? – pazzo Oct 14 '14 at 20:30
  • Please use it in a sentence that you would find objectionable. – anongoodnurse Oct 14 '14 at 20:32
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    I don't find the sentence There's a moon out tonight objectionable. I am just wanting to know the rationale (or whatever) that allows us to use 'a moon' when Earthlings have only one moon circling our planet. Moons of other planets do not count, because 99.99% of us never refer to them in such a sentence. – pazzo Oct 14 '14 at 20:38
  • There's a moon out tonight; I hope it doesn't spoil the meteor shower. – pazzo Oct 14 '14 at 20:45
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    I might well say 'There is a moon out tonight'. – WS2 Oct 14 '14 at 20:56

Because the moon is well covered, let us consider the fly. The indefinite articles "a" or "an" should be used when referring to a thing in general and not a specific instance of that thing:

"There is a fly in my soup."

The definite article "the" should be used when referring to a specific instance of a thing:

"The fly in my soup is doing the back stroke."

English Page

  • Well, if I am notifying the waiter that a fly is in my soup, I am initially not going to say the fly. And when I notify the waiter that there's a fly in my soup, my question is: out of what subset of flies am I talking about when I say a fly. – pazzo Oct 14 '14 at 21:59
  • ............any subset............ – Gary's Student Oct 14 '14 at 22:54
  • Well, can it refer to the subset of flies in Mozambique if I am eating my soup in a Dallas restaurant? – pazzo Oct 15 '14 at 16:07
  • @CarSmack I admit it would require a rather long migration. – Gary's Student Oct 15 '14 at 16:09
  • Yeah, and after migration such flies would no longer be in Mozambique but from Mozambique. – pazzo Oct 15 '14 at 16:23

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