1. Mick West

    Mick West Administrator Staff Member


    Source: https://www.youtube.com/watch?v=KLufSkz-et0

    If there were no atmosphere, and somehow the Earth still had oceans, then the curvature of those oceans would be readily apparent, with a pin-sharp horizon cutting off distant objects with mathematical precision. You'd be able to zoom in clearly on ships vanishing over the horizon, and calculate the radius of the earth with great precision.

    But back in the real world, we have an atmosphere. That atmosphere is generally denser as it gets lower down, and this makes the light bend down a bit, which lets you see beyond the mathematically calculated horizon.

    This is normally illustrated with a side view, with the camera on one side and the target object on the other, and a greatly exaggerated curve of the Earth, like this:
    Metabunk 2019-02-06 12-13-03.

    This is where the explanation starts to get lost on people. This abstract side view is something that often causes confusion.

    So I'm seeing if I can get some kind of physical demonstration of what is going on. I've got a fishtank partly filled it with water, layered some sugar on the bottom so it would dissolve and create higher refractive index the lower down you went, and I've got a laser with a beam splitter:
    Metabunk 2019-02-06 13-36-07.

    The idea is that from the side it's like the diagram. There's rays of light, they curve slightly, and there's the slight curve of the earth:
    Metabunk 2019-02-06 13-39-06.

    The curve is slight, but there. More visible with some compression.
    Metabunk 2019-02-06 13-40-17.

    At the other end of the tank, I put a photo of Toronto, and flipped the laser to a single beam and put it behind it.
    Metabunk 2019-02-06 13-43-56.

    I then moved the camera up and down.
    IMG_4788-Toronto-Up-Down-Laser-Spot.

    This compression of the scene near the horizon is very similar to that observed in the Jenna Fredo video (@jenna1789), discussed in this thread: https://www.metabunk.org/views-of-t...rt-niagara-illustrate-earths-curvature.t8149/
    jenna-beach-a. jenna-beach-b.


    Metabunk 2019-02-06 14-36-27. Metabunk 2019-02-06 14-36-01.


    And this setup can be duplicated in the refraction simulator, with a very cool lower layer, taking it from 80 feet to 1 foot above the water:
    https://www.metabunk.org/refraction/?~(profile~(~12.41~0~13.084~6.017~17.352~32.883~17.315~15.205~16.48~287.6~16.52875552~262.976)~useRefraction~true~useStandard~false~useFlat~false~useNarrow~false~useNight~false~showSideView~true~showSideGradient~true~useDebug~false~showEyeLevel~false~vFOV~0.022689280275926284~tilt~0.0008726646259971648~showEveryLines~10~viewerHeight~1~viewerOffset~773~minX~12~maxX~18~minY~-10~maxY~300~RH~50~wavelength~550~computedParams~false~name~'Toronto*20From*20Hamilton*20Beach~src~'Toronto*20From*20Hamilton*20Beach*2032*20miles.png~sourceURL~'https*3a*2f*2fwww.youtube.com*2fwatch*3fv*3dAFJnrMZT7KA~targets~(~(distance~168960~height~1815~name~'Toronto~multiple~0~gap~0)))_
    Metabunk 2019-02-06 14-42-55. Metabunk 2019-02-06 14-41-44.


    So, a good practical results. But I wonder how best to communicate it to people. I think the key concept is how the laser line is the same as a line of sight. If you shine a laser in a particular direction it will hit a spot. If you look in exactly the same direction your were shining the laser, and from the same position, then your line of sight will follow that same (possibly curved) line, and you will be looking directly at the spot.
     
    Last edited: Feb 7, 2019
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  2. Mechanik

    Mechanik Member

    Awesome experiment, @Mick. Beautiful and easily understood analog that can be replicated by anyone with minimal equipment.

    The communication part is going to be tricky. Putting on my tinfoil hat, my first response is “What does sugar water have to do with atmospheric refraction?” Is there some (indisputable) math that can equate atmospheric density to liquid density over distance? Perhaps some added math to your refraction calculator that lets you swap air for water? I’m sure the math exists, but this is well beyond my expertise.

    Just trying to red-blue team the argument. As a recreational sailor who has navigated beyond the horizon using nothing but a compass and a map, I’m baffled that there is a flat earth controversy. If any of these flat earth folks had tried to sail to an island that was not visible from shore, I’m sure many of them would be converted. None of them can imagine the relief one feels when the expected peak first shows over the horizon.
     
  3. deirdre

    deirdre Moderator Staff Member

    if you used some little aquarium heaters and had the tank more full of cold water to start, and filmed how the refraction changed over time as the heaters heated up? would that work? if the heaters were small enough (or could be lowered in temp enough) and placed on the two ends it might sort of demonstrate how the sea is colder in the middle than at the shore. ?? might need a bigger tank though.
     
  4. Mick West

    Mick West Administrator Staff Member

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  5. Mendel

    Mendel Member

  6. Mick West

    Mick West Administrator Staff Member

    Metabunk 2019-02-08 11-21-19.

    I took these seven 2" screws and put them in the tank along one side.
    Metabunk 2019-02-08 11-23-24.

    View from the front of the tank, minimal refraction.
    Metabunk 2019-02-08 11-24-17.

    View from the end, with progressively more refraction the deeper you go.
    Metabunk 2019-02-08 11-29-52.

    The shrinking you see is mostly not from perspective. It's mostly vertical compression (notice how the far screws look much fatter).

    See the compression is much more pronounced lower down. There are equally spaced black bands on the lower parts. You can also see the thread spacing change.
     
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  7. Mick West

    Mick West Administrator Staff Member

    Repeating it with visible light, to show it bends just like the laser.
    Metabunk 2019-02-08 13-17-13.

    Metabunk 2019-02-08 13-17-55.

    The light comes in angled slightly upwards, and hits the ground at screw #3
     
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  8. Astro

    Astro Active Member

    Brilliant, is that sunlight you're letting in there through that slit? I assume this is to put to bed Mr. Riley's excuse that the laser reacts differently than sunlight would.
     
  9. Mick West

    Mick West Administrator Staff Member

    It's a powerful flashlight through a slit. No sun today
     
  10. Mendel

    Mendel Member

    Suggestion: you could draw equidistant parallel lines on the outside of the tank, see how they look from inside
     
  11. Mick West

    Mick West Administrator Staff Member

    You can't see out of the side of the tank from the end, because of total internal reflection
     
  12. Leifer

    Leifer Senior Member

    Along with a version of this and many other "home experiments" prevoiusly described....might I suggest that you post or create a separate area for DIY (do it yourself) home experiments and tests ?
    Maybe a place for interested people to either replicate, prove / disprove, or expand on the ideas ?

    My thrust about this, is to entice people to experiment for themselves, in an effort to "learn from doing".....instead of simply " learn from reading".
    .
     
    Last edited: Feb 11, 2019
  13. JFDee

    JFDee Senior Member

    There is a video response from "Dr John D.". As I understand, his critique mainly revolves around the claim that air above the sea is normally less dense so that the density gradient is reversed.

    Comments are disabled for the video.

     
  14. Rory

    Rory Senior Member

    Looking through, his main objections seem to be that sugar in a fish tank can't represent the actual atmosphere we live in, and his idea that the air 0-1km above the sea is less dense than the air above it, therefore refraction actually bends light up (he also shows pictures of lasers bending up, and claims to be unable to find any of lasers or lights bending down).

    Here are some of the most pertinent slides:

    Screenshot (96). Screenshot (97). Screenshot (98).
     
    Last edited: Feb 15, 2019
  15. Mick West

    Mick West Administrator Staff Member

    There's a good reason for that. I'm adding lasers to the refraction simulator to explain. Still a work in progress, but here's the basic illustration of what is happening.

    Laser over warm water:
    Metabunk 2019-02-15 14-52-28.

    Laser over cold water
    Metabunk 2019-02-15 14-53-13.
     
  16. JFDee

    JFDee Senior Member

    He obviously thinks that the standard atmosphere applies always and everywhere which is of course not true. There are local deviations all the time, like with inversions where the gradient actually reverses direction (warm air above cold air).

    Also, the decreasing temperature with altitude in the standard atmosphere is a consequence of the decreasing density, so it's a misunderstanding to assume that density normally increases with altitude.

    Cold air is necessarily denser than warm air only when at the same altitude.
     
    Last edited: Feb 16, 2019
  17. Mendel

    Mendel Member

    Last edited: Feb 16, 2019
  18. Mick West

    Mick West Administrator Staff Member

    The standard atmosphere isn't just "it gets colder with altitude", it's getting colder at a specific linear rate (0.0065 °K/m) that results in a fairly linear refractive index gradient. He's discussed different conditions over water at night, where warm water creates a much steeper (and non-linear) gradient, resulting in inferior mirages.

    He's got several mistakes there, but his basic point seems to be that the density gradient (density decreasing with altitude) isn't something we see in reality. Clearly, though it is, and it's easily observed in daylight.

    The graph above though is for the standard atmosphere, it's also for a largely irrelevant range, up to 10km. We are really interested in the first 100m or even less. Over water, there can be a significant effect. Here's a simple rendering of the refractive index in white (representative of density) with temperature in red. The white line is like a zoom in on the lower left 15m of the graph above, so looks linear. Red is linear temperature.
    Metabunk 2019-02-16 06-21-02.

    If we then compare that to a typical inferior mirage situation where warm ground or water is heating the air directly above it, causing a more rapid decline in temperature (red), we see that the density DOES increase with altitude, but only while this steep temperature .gradient exists.
    Metabunk 2019-02-16 06-24-25.

    To make this consistent up to 1km would require a constant steep gradient up to 1km, roughly around -3°C (or more) per 100m - something that obviously does not happen.

    Getting back to the topic of the thread here though. It's "how refraction helps you see over the horizon" it is NOT "this fishtank of sugar water is exactly like the atmosphere". The idea here is to help people understand what is going on with the paths of light when they observe things looming over the horizon.
     
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  19. Mick West

    Mick West Administrator Staff Member

    And with the laser rendered.

    Warmer lower:
    Metabunk 2019-02-16 12-34-51.

    Colder lower
    Metabunk 2019-02-16 12-35-44.

    The laser is level at five feet, the viewer is 2 feet above and 3 feet to the side. Even though the laser has more of a curve in the cold situation (where it bends around the curve) it's less apparent to the viewer.
     
  20. Mick West

    Mick West Administrator Staff Member

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  21. Mechanik

    Mechanik Member

    With regards to Dr John and his temperature gradient, I have to wonder whether he has ever heard of the Southern California "June Gloom" or a temperature inversion. June gloom refers to the fog that forms in summer due to the cold ocean air mixing with the hot air off the land. When that cold moist air comes inland in the evenings (as the deserts cool), it forms summer fog in the coastal basin (atmosphere cold at the bottom, then warm, then gradually cooling as you go up). The temperature inversions (remember the famous Los Angeles smog?) have a cap of colder air over the warmer ground air, trapping heat and pollution below (warm, cold, warm or cold and eventually cooling?).

    I guess my point is that, in a geophysical sense, the air density decreases with altitude, but in the local sense i.e. the first 2,000 feet above the surface of the earth, relatively tiny pressure and temperature variations have a profound effect on air density and thus, refraction.

    @Mick West, was just re-watching the original video and noticed that you misspoke at about 10:03. Missed it the first couple of times. Doesn't look like anyone jumped on this but you said "sea level" when it's obvious you meant water level as Toronto (not Chicago) is a couple hundred feet above sea level. ;)
     
  22. DavidB66

    DavidB66 Member

    Not sure if this is the most relevant thread for this, but I wanted to draw attention to a new video by Al K, showing some more examples of extreme refraction effects, mainly from his own observations on the coast of Scotland:

    Source: https://www.youtube.com/watch?v=fR3YctWFj00&t=27s