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Approximation in understanding the speed of light

  1. Apr 24, 2015 #1
    Hello All,

    Light travels at 1,86,000 m/sec. i.e.2,99,338 km. The distance from Earth to Moon is 3,84,400 km. Is there any constellation or any physical object (to have an idea) which is not near to moon but falls within the range of 2,99,000 km?

    Just for curiosity.

  2. jcsd
  3. Apr 24, 2015 #2

    Simon Bridge

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    When you are trying to get a feel for things, it is useful to be careful about the things you want to understand.
    1,86,000 is three numbers separated by commas. Same with 3,84,400 and 2,99,338.
    I think you mean 186,400 299,338 and 384,400... i.e. digits get grouped into threes to aid reading.

    m/s is "meter pers second" ... I think you mean "miles per second" which is MPS or mi/s.
    km is 1000m ... i.e. it is a distance, not a speed. A speed cannot be equal to a distance.

    There is no constellation 1 light second away because constellations are made out of stars many light-years away. There is no "distance to a constellation" anyway, since the constellations are projections of the star positions onto the celestial sphere ... the stars in the same constellation may be at widely different distances.

    Something that falls "within the range of" 299,000km would be anything less far away than that.

    What you want to know, it seems, is if there is anything of note about 1 light second from the Earth.
    The Moon is about one and a quarter light seconds out. I don't know of anything you'd find meaningful roughly 1ls out, but a graphic can help with the feel.
    ... I think you'll find the moon is close enough.
  4. Apr 24, 2015 #3
    Sorry for misunderstanding the metric

    299792458 metres / s =1,86,282 miles/sec.

    earth to moon 384,400 km=238855 miles

    So, near about close to moon, except the difference amount.
  5. Apr 24, 2015 #4


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    Only passing asteroids and small amounts of gas/dust.
  6. Apr 24, 2015 #5


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    And the occasional spacecraft.
  7. Apr 24, 2015 #6


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    If you are trying get a feel for the distance light can travel in one second, "seven times around the world" may be useful.
  8. Apr 24, 2015 #7

    Simon Bridge

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    Yah - everything is close to everything else "except for the difference amount" =)
    I had a look to see if there were any co-orbiting thingies of note but they are all farther out.
    ... it seems to be a region you just pass through on your way to someplace else. Doesn;t seem to be any spacecraft with an orbit that regularly passes through that range either. Lunar probes orbit closer to the Moon.

    The L1 point is closer at 1.08ls. (322000km) ... but does that help you if you have no intuitive feel for the Lagrange points?

    It is usually best to get a feel for distances etc in terms of things you are personally familiar with.
    "Space is big, really big. You may think it's a long way down the street to the chemist, but that's peanuts to space, listen..."
    -- The Hitchhikers Guide to the Galaxy
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