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It's hard to understand meanings of words by just reading their definitions in dictionary

In the Oxford dictionary

space: the area outside the earth’s atmosphere where all the other planets and stars are

sky noun /skaɪ/ /skaɪ/ [countable, uncountable] (plural skies) the space above the earth that you can see when you look up, where clouds and the sun, moon and stars appear

Some people say "the sky" means the space within earth atmosphere where we ave oxygen and "space" the space outside the earth atmosphere.

I feel that understanding is not correct because it does not match the definitions.

This is what I understand based on the definitions but I am not sure.

Say a spacecraft is flying up. Now if we can still see it, then we can say "the spacecraft is in the sky". Now it keeps flying until we ca not see it, then we can say "the spacecraft is in space".

Another example, we can see a star with the naked eye, we can say "the star is in the sky" but there are stars that we can not see with the naked eye, we can say "the star is in space"

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    We say we can see the International Space Station in the sky, but if the astronauts have to suit up and go "outside" to carry out maintenance, they go spacewalking. Commented Nov 17, 2023 at 17:28
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    I don't know anyone who insists that "in the sky" means within Earth's atmosphere. All the celestial objects we can see -- Sun, Moon, stars -- are routinely considered to be "in the sky", and none of these is within the atmosphere. Commented Nov 18, 2023 at 16:15
  • When I first read this, I thought you were joking. Are you? Surely, in Vietnamese you distinguish the sky from space? Try a good bilingual dictionary.
    – Lambie
    Commented Nov 20, 2023 at 20:08

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Say a spacecraft is flying up. Now if we can still see it, then we can say "the spacecraft is in the sky". Now it keeps flying until we ca not see it, then we can say "the spacecraft is in space".

You can say a spacecraft is in the sky when you are referring to an observer potentially seeing it with the sky as the background. It does not matter if you can actually see it with the naked eye. A person could show you the position of a black hole in the sky by pointing at it.

"Sky" refers to the apparent two dimensional surface visible or potentially visible from the earth or other heavenly body. Something is said to fly in the sky as it appears to move across this surface with respect to a potential observer. On a cloudy day or night, you can still say in New York that the North Star is in the sky, even when it is not visible. In the Southern Hemisphere, you would not normally say that the North Star is in the sky, because it is never visible to a typical observer from there, since the Earth is in the way.

The "sky" is not a real object or physical place and is not suited to describing objective things divorced from a potential observer. A plane can be said to fly across the sky, because that is how it appears to an observer; however, we would be much less likely to say that a plane flies across the sky at a certain speed, since that is not dependent on an observer.

"Space" refers to a real place. Strictly, it refers to the near vacuum between heavenly bodies, but can loosely apply to anywhere in the universe that contrasts with the Earth and its atmosphere.

If a spacecraft has left the atmosphere, it is fine to say it is in space, even if still visible in the sky, as space stations and satellites occasionally are. If a spacecraft has not left the atmosphere, you cannot say it is in space, regardless of whether or not it is visible.

Another example, we can see a star with the naked eye, we can say "the star is in the sky" but there are stars that we can not see with the naked eye, we can say "the star is in space"

A star is in the sky if it is potentially locatable in the sky above the typical observer, whether or not it is visible. The stars are still in the sky during the daytime. They are just not visible in the glare of the Sun.

It is odd to say the simple sentence "the star is in space," because the primary definition of space in this usage excludes heavenly bodies like stars. It would be fine to say: "Stars are actually floating in space far away from the earth and are not floating in the air above us." Here there is a contrast between the Earth and space that allows the looser usage.

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  • “If a spacecraft has left the atmosphere, it is fine to say it is in space, even if still visible in the sky, as space stations and satellites occasionally are.” — And contrary to the strict definition which excludes heavenly bodies, I would say it’s perfectly fine to say that the Apollo Lunar Module was ‘in space’ even while it was standing on the surface of the Moon with the astronauts going for a walk. (I also don’t find “the star is in space” odd as such, just redundant: all heavenly bodies, including Earth, are by definition in space, so if anything, it’s an odd statement to make.) Commented Nov 19, 2023 at 16:36
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You have to be careful about interpreting dictionary definitions. They're trying to condense the idea behind the meaning of a word into a sentence or two. Trying to take a definition that you found in some dictionary and applying it absolutely literally and then supposing that this interpretation is gospel truth can get you into trouble. Dictionary writers have to struggle to catch subtleties and nuances of meaning. Especially if you compare definitions of related words.

When English speakers say "space", we mean the region beyond the atmosphere, where stars and other planets are found.

When English speakers say "sky", we basically mean the area within Earth's atmosphere. But we might also mean what you see when you look up, as in, "I saw the stars in the sky." You can see stars in the "sky" even though they are not within the atmosphere.

If a fluent English speaker said, "The spacecraft is in the sky", we would probably understand him to mean that it is within the Earth's atmosphere. But he might mean that it is visible if you look up. For example, a spacecraft in orbit could be "in the sky" in the sense that you can see it when you look up. (Probably as a tiny point of light, so you'd have to know exactly where to look, and you'd need a telescope to see any detail. But that's another matter.)

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    Yes. I might be wary about a statement such as 'the International Space Station is in the sky' if it was over the other side of the earth. Commented Nov 17, 2023 at 18:18
  • Agreed, and things can vary a lot with context. "The sky" has a dictionary definition - but if something is "in the sky", it carries the additional implication of being within sight. Commented Nov 19, 2023 at 19:47
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space, often outer space, from the point of view of the average earthling, refers to the expanse of emptiness outside the atmosphere and beyond the gravitation pull of planet Earth, whereas the sky is what we see above us, the clouds, the blue dome, and at night the twinkling stars, moon, and seasonally visible planets.

The spaceship has left the Earth's atmosphere and is on its journey into outer space.

The planet Mars will be visible just above the horizon in the night sky.

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  • Above the atmosphere, but not necessarily so far as to be largely beyond Earth’s gravitational field. After all, Newton would say that the International Space Station, which is in space, orbits the Earth precisely because of the attraction that it and the Earth exert on one another. Commented Nov 17, 2023 at 18:31
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    @PaulTanenbaum I'd agree. That's where I'd use space for space stations and satellites that are above the atmosphere but not beyond earth's pull and remain in orbit, and outer space to being both above and beyond.
    – TimR
    Commented Nov 17, 2023 at 23:20
  • "beyond the gravitation pull of planet Earth" – At the height of the International Space Station, the gravitational pull of planet Earth is still 90% of what it is on the ground. In fact, according to your definition, the Moon is not in space, since Earth's gravitational pull is what keeps the Moon orbiting around Earth. Commented Nov 18, 2023 at 11:51
  • @JörgWMittag See the comment immediately above yours. NASA calls this area within the pull of Earth "near space", FWIW.
    – TimR
    Commented Nov 18, 2023 at 11:57
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Before people understood how the solar system works, we talked about 'the sky' (or whatever the word was in our own language) as the thing you see when you look upwards. At one time it was thought to be like a big upturned bowl with the stars sprinkled in it.

'[Outer] space' is, as you say, the region outside the earth's atmosphere, to which spacecraft are sent. When a spacecraft orbiting the earth can sometimes be seen, we could say that we see it 'in the sky'.

We talk about 'stars in the sky' because we see them there at night, but most of us know that they are really in outer space. In everyday speech, we tend to refer to visible objects as 'in the sky' in accordance with traditional usage.

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  • I mean, even now that we know how the solar system works, the sky is still "what you see above the horizon". There is no boundary between "sky" and "space", a sky doesn't even require an atmosphere - if you were standing on the moon, you could see the earth in the moon's black sky. A star may be both in the sky and in space, it's not an either-or as the last sentence suggests. Commented Nov 17, 2023 at 17:56
  • @NuclearHoagie - I didn't mean to imply that it was an either-or situation. Commented Nov 17, 2023 at 18:42
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Normally, “in the sky” means above the ground, either in the atmosphere or in outer space. “In space,” means outside the atmosphere (of Earth or any other planet). Originally, space was a synonym for vacuum. Sometimes, though, “in space” just means away from Earth. Someone might for example say that a rover on Mars is “in space,” even though I’d consider that incorrect. People don’t use “in the sky” that way. “In space” can also be used as a disparaging shorthand for giving an unoriginal story a science-fiction setting, like many children’s TV shows of the late 20th century (“A girl band in space,” “Giant fighting robots in space,” “Murder mystery in space”), even though almost all of these were set on Earth-like planets.

An example would be Superman. The radio show from the 1940s would get kids to imagine him flying through the air by saying, “Look! Up in the sky! It’s a bird! It’s a plane! No, it’s Superman!” But when Up in the Sky was used as the title of a comic book in 2019, the story (written by Tom King) was about Superman going on an adventure in outer space.

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Lots and none at all, because the whole depends on who is speaking.

From the other end, the problem seems to be that though, yes, stars seen with the naked eye are 'in the sky' and those we can't see are 'in space' that's about optics, not physics or cosmology.

Those views aren't mutually exclusive; all can be true at the same time.

Who calls 'the sky' the space in our atmosphere where we have oxygen and 'space' the space outside, must trip over those meanings of 'space'.

That spacecraft seen are in 'the sky' and unseen, in 'space' might well work philosophically…

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