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Thread: Earth is round?

  1. #61
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    I think you meant this:


  2. #62
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    On 2001-10-26 11:28, Bob S. wrote:
    The gist of most replies seems to be: no, you cannot just stand in your backyard and point to the sky or sun or clouds and say "See, there's proof the Earth is round." All suggestions for determining proof seem to involve either travel or some contact with people far away.

    People of ancient times believed the Earth was flat because locally it looks flat, and most people never traveled far from whatever valley or plain or town in whiched they lived. And for those that did travel, transportation was so slow, one would never really experience any form of jet lag. Understanding the spherical nature of the Earth had to wait for the invention of accurate timepieces and an age of enlightenment when people would wonder about such things as shadow length (and be able to travel far to make such measurements and compare).
    What's your definition of ancient people? Educated people since the third century B.C. have known the Earth is round. Aristotle had these reasons to believe the Earth is round:

    1.) Lunar eclipses.
    2.) A ship's mast disappears last.
    3.) The sun rises and sets at different times as you move east-west.
    4.) Different stars as you move north-south.

    Granted reasons three and four require travel or foreign contact, but reasons one and two Ari could do himself. About one hundred years after, in the second century B.C., Eratosthenes calculated the size of the Earth within about 16% error.

    We did not have to wait for "accurate timepieces". (For accurate calculation of longitude, we did.) And we definitely did not have to wait for the age of enlightment.

    You seem to making a mistake I've seen many other people make - my apologies if I'm infering something that's not there. Just because a culture is less technologically sophisticated does not make them stupid. These cultures are just smart and curious as we are and have a sophisticated and intimate knowledge of the world in which they live.

  3. #63
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    I have been told in this thread a couple of times that you can't just point up into the sky and say "Look! The Earth is round." However, several people have pointed out that if you have a clear day and live in a fortunate location, you can just look out your window. So it is possible. And it is possible without precise measurements or fancy equipment. Apparently nobody can think of any other ways to demonstrate this.

    Ben

    Before everybody jumps all over me again, I will clarify: no one can come up with anything else which is does not require fancy equipment, precise measurements, travel (in person or by telephone), or several years of waiting for lunar eclipses.


  4. #64
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    On 2001-10-26 17:53, Ben Benoy wrote:
    I have been told in this thread a couple of times that you can't just point up into the sky and say "Look! The Earth is round." However, several people have pointed out that if you have a clear day and live in a fortunate location, you can just look out your window. So it is possible. And it is possible without precise measurements or fancy equipment. Apparently nobody can think of any other ways to demonstrate this.

    Ben

    Before everybody jumps all over me again, I will clarify: no one can come up with anything else which is does not require fancy equipment, precise measurements, travel (in person or by telephone), or several years of waiting for lunar eclipses.

    Perhaps a better way is to try to prove the Earth is round by contradiction. What would be a required property for a flat Earth? And can this be "easily" disproved?

    For instance for a flat Earth the sun must rise at the same time everywhere. However this can't be disproved without a telephone or travel.

    <font size=-1>[ This Message was edited by: Wiley on 2001-10-26 18:14 ]</font>

  5. #65
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    On 2001-10-26 14:09, Wiley wrote:

    Aristotle had these reasons to believe the Earth is round:

    3.) The sun rises and sets at different times as you move east-west.
    I'm puzzled as to how Aristotle could know that the sun sets at different times as you move east and west.

    Local noon was just the time the sun was in the south, and it still is. If it weren't for rapid communication, travel and world-wide business interests, who in Athens would care what time it is in Rome?

  6. #66
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    I know this doesn't exactly prove that the Earth is spherical but what about the stars?
    I have never seen Polaris by looking into the sky, because I live in Australia. So how do I know that it exists? Of course I believe it does exist, so where is it? Obviously it must be on the other side of the planet, so at the very least the planet has two sides, (although technically a sphere only has one side) and without actually bothering to do any calculations or anything, I'm sure that at different latitudes different stars can be seen, so at least this infers the Earth has opposing sides (without being technical), and is curved from South to North, and I'm sure observing celestial motion will help to prove the spherical nature of the earth.

  7. #67
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    Before everybody jumps all over me again, I will clarify: no one can come up with anything else which is does not require fancy equipment, precise measurements, travel (in person or by telephone), or several years of waiting for lunar eclipses.

    I don't agree Ben. You earlier state that my suggestion of taking the documented flight path(s) of trans-Pacific or Atlantic flights and drawing them on a globe and showing they are indeed on the "great circle" between the origin and destination as not conforming to your original question, but I see nothing in your first post that would disqualify this method. Methinks you're unwilling to accept anyone's answer at this point! [img]/phpBB/images/smiles/icon_wink.gif[/img]

  8. #68
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    Well, as far as I can see it, there is no way to directly prove the earth is round just from standing in one place.

    Determining roundness requires measuring angles; from the relative positions of heavenly bodies to different points on the ground, or from one point on the surface to others. Both of these conditions require either travel or time to calculate. There just isn't any other way to get these measurements.

    The only way mentioned that doesn't require direct measurement of angles is from observing lunar eclipses, and that still requires multiple observations over time and/or from different locations.

    So there are many ways to prove sphericity, but they do require some effort. You can't just walk out of your house and point to it.

  9. #69
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    I think Wiley was on the right track. To explain this to a child start from what they are willing to accept and use it as evidence that a flat Earth does not fit with that.

    Here is my approach. First find out whether the child believes that Australia (or some place on the other side of the world) exists. Then find out if the child agrees that TV can show something happening now at a place far away. Now show the child a live TV broadcast from the remote location where it is daytime while it is night in the child's location. In order for this to happen, the sun must be on the other side of an Earth of some shape other than flat.

    This is the time zone demonstration except that the child is viewing the difference in time directly through the TV camera. Although this might not be completely conclusive to the child, it should get them thinking.

    It just occurred to me that letting a child monitor a webcam showing something like the Eiffel Tower would let them have this experience as well.

    <font size=-1>[ This Message was edited by: Kevin J. Ashley on 2001-11-02 17:28 ]</font>

  10. #70
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    Earth is an oblate spheroid.

    ljbrs [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

  11. #71
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    On 2001-11-25 14:40, ljbrs wrote:
    Earth is an oblate spheroid.

    ljbrs :wink: :wink: :wink:
    Earth is approximately an oblate spheroid.

    I'll see your three winks, and raise you one.

  12. #72
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    On 2001-11-26 09:14, GrapesOfWrath wrote:
    On 2001-11-25 14:40, ljbrs wrote:
    Earth is an oblate spheroid.

    ljbrs [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]
    Earth is approximately an oblate spheroid.

    I'll see your three winks, and raise you one.
    Touché!!! You win hands down!

    However, on second thought, that was a *.com* site, and I always prefer the *scientific* kind from websites graced with *.edu* or *.gov*, with the exception of *Bad Astronomy* (because it has a bona fide astronomer at the helm).

    Now, seriously, where did I put my horoscope?

    ljbrs [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

  13. #73
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    I still think there is a way to do it by observing the phases of the moon. I'm not sure my previous post on this topic was entirely correct, but I'd be curious for the BA's take on how phases of the moon appear.

    Here is my thinking from some limited and non-scientific observations. If you watch the phases of the moon, you note how it goes from a tiny crescent to a "half moon", all the way to full. Watch from wherever you are and note that the line between the points of the crescent or the tops of the gibbous part do not run perfectly vertical. It is fairly easy to conclude that the moon is spherical just from that, and it is also pretty evident that the phase of the moon depends upon the moon to sun geometry. However, I think it can be shown that the appearance of the phases also depends upon the relationship of both the moon and sun to the Earth.

    That is a direction perhaps someone might want to pursue. I would, but I'm not meticulous enough to mess with it right now.

  14. #74
    I used to work at a 12-story building in Mountain View CA, from which I could see San Francisco about 30 mi away, with water in between. This guy in an adjacent cubicle had a telescope for examining the view more closely.

    The bottom few floors of the Transamerica building appeared to be missing, although we never counted the number of floors to see how many were below the horizon.

    The Richmond bridge, about 50 miles away was completely invisible, although its towers were probably some 400 feet above the water.

    The earth was definitely round.

  15. #75
    Sorry if this thought has been posted before:

    Couldn't Eritosthenes's results also be explained in terms of a flat earth and a sun that was much closer?

    I don't mean to suggest that this is true, but couldn't it have been an alternate explanation with the facts available to the ancient greeks?

    Or, if an eclipse had already convinced them that the earth was round, why not a much larger round earth and a much closer sun?

    Like this:
    <pre><code>
    O sun
    |
    |
    |
    |
    |
    |
    |
    |
    |
    |_________
    no yes
    shadow shadow
    </code></pre>



    <font size=-1>[ This Message was edited by: Phil S on 2001-11-29 11:01 ]</font>

  16. #76
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    Couldn't Eritosthenes's results also be explained in terms of a flat earth and a sun that was much closer?
    I don't mean to suggest that this is true, but couldn't it have been an alternate explanation with the facts available to the ancient greeks?
    Or, if an eclipse had already convinced them that the earth was round, why not a much larger round earth and a much closer sun?
    See my second "class project" posted on Oct. 23, page two of this discussion. I mention the math and geometry for such a proof.

  17. #77
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    The Globe

    What did the ancients know?


    Akkadians an old race of Mesopotamia knew lots of stuff, people of the Indus valley, the famous Greek Aristarchus and Aristotle, the Chinese astronomers in history, the Celts and builder of Stonehenge 2 700 BC ? , the Babylonians and the Romans, Ionians and Inca.

    They used many models and measurements to predict that the Earth was a sphere, geometry, observation, math, eclipse, trigonometry....
    Galileo agreed with the Copernicus ideas that Earth is not the center of the planetary system. The Galileo discovery that Venus has phases just like Earth's Moon confirmed that venus orbits the Sun and supported the heliocentric mode
    The giant Library in Alexandria was one of the most famous centers of learning in the ancient worlds. The Museum was a center for scientific, geographical and mathematical research. One of its geographers, Eratosthenes knowing that the Earth was a sphere, said that the Sun was at least 20 times farther away than the Moon ( wrong answer but good work!! )
    Chinese astronomers record a solar eclipse 2136 BC, Huainanzi relates a legend in which the sage of the king Yu race determined the size of the earth by ordering officials to walk to the ends of the earth, east-west and north-south, and gives both distances as 233,500 li and 75 steps ( a "Li" is an old measuremnet equals about half of a kilometre or one third of a mile ) it is a set of measurements that could, in principle, have been made by people on foot.
    Other mathematical works use gnomon shadow measurements and the properties of right triangles to calculate this information. They assumed that the heavens were a round hemisphere (or canopy) over an earth that was both square and flat. Then other values could be taken. They also used gnomon shadow measurements to measure the “height of heaven,” the distance of the apex or the hemisphere from the earth’s surface. Measurements were taken at noon from a series of gnomons 1000 li apart due north-south.
    An 8-foot gnomon cast a shadow of 1 foot 6 inches the shadow decreased an inch for each thousand li due south, and increased an inch for each thousand li north. Thus each thousand li of terrestrial distance corresponded to an inch of shadow length. The passage concludes that the distance from the northernmost gnomon to the subsolar point where the gnomon has no shadow was 60,000 li. It appears to use the properties of similar triangles to calculate the vertical distance from that point to the sun as 80,000 li. The similar triangles are ABC (whose height is the gnomon BC) and ADE, where DE is the height of the sun. The same techniques are used to calculate both terrestrial and celestial distances where no alternative direct measurement is available, or as one text puts it: “Heaven cannot be climbed like a staircase” .
    The Greeks knew lots of stuff. Their knowledge that the Earth and Moon are spherical was widespread in the Greek world by the fifth century B.C. Aristotle argued that the circular shadow projected by the Earth when it eclipsed the Moon was clear evidence of the Earth's spherical nature. This argument had been known for a long time. Hipparchus was one of the greatest men of time. He lived around 120 BC. He developed the first star catalog, listing some 850 stars, and he calculated the lenght of a year within 6 minutes. He knew of the concept of the the circular motions. He also calculated precession, or wobble, of the Earth's axis by comparing his own observations with those made in Alexandria 150 years earlier and in ancient Babylonia.
    Early celtic peoples and tribes in parts of England, France, Scotlands were also skilled astronomers created calendars from changes they saw in the Moon. Some ancient people around 5,000 years ago set up large stones to mark the movement of the Sun and other stars such as Stone henge. These people alos understood the earth was a sphere? One possibility is that the shadow of the earth cast on the Moon during an eclipse is always round. However, the idea that we are seeing the Earth's shadow cast on the Moon is, if you think about it, a very bold one
    Back to another great man was from the Mediterranean world,his name Eratosthenes , he had the idea that the world was a sphere, and from the earlier maths of Aristarchus that the Sun was at least 20 times farther away than the Moon (the correct value is nearer 400), reasoned that rays of sunlight ought to be almost parallel when they reach the Earth, enabling him to measure the Earth's circumference. Eratosthenes chose observing stations at Alexandria ( famous library of the wonders of the world ) and Syene to the south, on the Nile River. For the time of the experiment he chose local noon on the day of the summer solstice, which comes at the same moment at both sites because they are very nearly on the same meridian of longitude. He probably selected that day, since the Sun was as far north as it would be during the year, meaning that it would pass very near the zenith at local noon at Syene.
    At noon he had an observer in Syene observed that the Sun was directly overhead, while another observer in Alexandria found the Sun to be 7o south of the zenith. Measurers had paced off the distance between the two cities as about 4900 stadia (1 stadium is an old measurement and = 0.16 kilometer). Because a straight line cuts two parallel lines at equal angles, the angle at the center of the Earth is equal to the zenith angle, 7o. Working a simple proportion, Eratosthenes found that the Earth's circumference was as follows: C/4900 stadia = 360o /7o, or C = 252,000 stadia, or about 40,320 kilometers (km). In principle, Eratosthenes' experiment was correct. Although his measuring technique was very inaccurate by modern standards, his results were surprisingly close to today's true mean value of 40,030 km.
    Zhang Sui from the Chinese Tang era (700 ) also understood that the earth was a globe and was the first man to describe proper stellar motion, or the apparent motion of stars across the plane of the sky relative to more distant stars, but Edmond Halley is credited with this discovery in 1718. What happen the west, did you peoples forget about Ancient romes and old Chinese charts?
    The Polish astromoner Nicolaus Copernicus also knew the earth was round and used mathematical proofs to revive the earth centred heliocentric models of the Greeks and other people.without the aid of a telescope, concluded from his observations that it made more sense to have the sun as the centre of the solar system. He kept his ideas of a heliocentric solar system secrets to himself until he published them at the age of seventy in "De Revolutionibus Orbium Coelestium"
    Another inventor from China, Zhang Heng thought Pi was the squares roots of 10 = 3.16 he was a Chinese astronomer and mathematician. He constructed a celestial globeknowing that the world was round, "The sky is like a hen's egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying alone at the centre. The sky is large and the Earth small."
    Galileo came from Italy, he done lots of research into physics. He studied at pisa. Galileo Galilei built his first telescope without ever having seen one assembled. This telescope had two lenses and magnified objects to three times their size. Galileo Galilei soon after in 1610 finds Callisto, Europa, Ganymede, and Io. The West was kind of in a dark ages witch hunt. His books were banned, he was forbidden to teach the Copernican view, and he was confined to his house until death.

  18. #78
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    Quote Originally Posted by Ben Benoy
    Before everybody jumps all over me again, I will clarify: no one can come up with anything else which is does not require fancy equipment, precise measurements, travel (in person or by telephone), or several years of waiting for lunar eclipses.
    How about some garden hose for non-fancy equipment?

    Stretch out a several hundred feet of garden hose and make it level by either line of sight or tight stringline. Put a "T" in the middle then turn each end up to the same height as the top of the "T". Cap the "T". Fill the hose with water, remove cap and watch the water come out. You will see the water level drop on the ends about an 1/8" or so depending on length.

    When sewer lasers were first introduced to utility contractors they learned, the hard way, that the curvature of the earth could not be ignored.

    [They later learned that the glue used for the pvc pipe also had to be taken into consideration due to refraction. Blowers were used to solve this problem.]
    We know time flies, we just can't see its wings.

  19. #79
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    Quote Originally Posted by George
    Stretch out a several hundred feet of garden hose and make it level by either line of sight or tight stringline. Put a "T" in the middle then turn each end up to the same height as the top of the "T". Cap the "T". Fill the hose with water, remove cap and watch the water come out. You will see the water level drop on the ends about an 1/8" or so depending on length.
    I was a little dubious, so I checked this out. Using 600 (several hundred) feet of hose with the outlet in the middle, and using a value for the Earth's radius of 3959 miles, the difference between the sphere and a straight line of sight would be sqrt( (3959*5250)^2+300^2)-3959*5280. That's only a fortieth of an inch--which could be offset by local density anomalies.

  20. #80
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    Quote Originally Posted by milli360
    Quote Originally Posted by George
    Stretch out a several hundred feet of garden hose and make it level by either line of sight or tight stringline. Put a "T" in the middle then turn each end up to the same height as the top of the "T". Cap the "T". Fill the hose with water, remove cap and watch the water come out. You will see the water level drop on the ends about an 1/8" or so depending on length.
    I was a little dubious, so I checked this out. Using 600 (several hundred) feet of hose with the outlet in the middle, and using a value for the Earth's radius of 3959 miles, the difference between the sphere and a straight line of sight would be sqrt( (3959*5250)^2+300^2)-3959*5280. That's only a fortieth of an inch--which could be offset by local density anomalies.
    Your math looks, essentially, fine to me. But, you have to remember, a Texas backyard is a tad bigger. A good hose will run about 1,300 feet.

    ops: Seriously, the laser mfg. required us to get the contractors to check grade after 2 or 3 hundred feet "due to the curvature of the earth". :roll: ....and to think I never checked it out ... :-?
    We know time flies, we just can't see its wings.

  21. #81
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    I know it's been suggested already but.....
    Next time you're in Cape Town South Africa, take a trip to the top of table mouintain and wait for sunset. Look out over the ocean and scan left to right, you will see the curve of the earth. Breathtaking, Especially when I wasn't expecting it! At first I didn't believe it and looked for some other explaination, but the curve of the earth was the only answer.
    The glow of the milky-way is amazing from there as well.

    One thought occurs which hasn't been mentioned yet, (and it's a local effect) is by taking a local Ordnance Survey map (Thats the UK name) and by using the scale, very carefully measure the distance between two points that are a reasonable distance apart and, driveable to.
    Then, actually go and drive between the points, measure it for real and see what the difference in distance is. Granted you may need a pretty accurate measure on both accounts but, in essence the map should show a slightly longer distance because it dipicts a 3D world on a 2D surface.

    To explain: If you take a 3D object say the intact peel of half of an orange and flatten it...well... it doesn't make a nice 'square' flat surface. To make it 'complete' with no holes (ie a 3" square of peel) you would need to stretch the peel in a few directions therefore distorting the surface area. This is similar to all maps. They show a 'stretched' surface which isn't absolutley accurate to the real world.

    After a few short road trips discovering that all map measurments are slightly longer than the 'actual' measurements, should bring you to the conclusion that either the map makers measuring equipment is rubbish, your measuring equipment is rubbish or that the surface you are driving on is spherical! (As if you are driving on top of a very large sphere.) Obviously you could argue that it's the natural lay of the land on a flat earth but, you would discover the same effect anywhere in the world.

    (The smaller the scale of the map the more inacurate -scale wise- it is. I think!)

    I'm not an expert on maps so maybe someone else can confirm or deny this idea.
    The Watcher

  22. #82
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    I would think you would need to do this on the salt flats where ground variations do not become the predominant factor.

    The distance would need to be significant. I get a little more than 6 feet extra distance due to curvature in a 10 mile line-of-sight stretch of perfect grade road (1 meter in about 30 km).

    If you decide to do this, roll out a hose while you go and fill it with water, and.... :wink: (You should get a water column in the middle about 1.5 meters high in a 30 km hose).

    [edit: hope I'm right this time, Milli360]
    We know time flies, we just can't see its wings.

  23. #83
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    Quote Originally Posted by Ben Benoy
    To repeat, not a flat Earther, and not actually trying to demonstrate that the Earth is round/flat to anybody. The original post says that a method is sought to demonstrate the curvature to a young person, using local effects. That is, no traveling great distances, not using satellites, etc. [snip]
    You might call it cheating, but TV/telephone is a simple way to do it in your living room. Watching or talking to someone live on the opposite side world sort of makes it obvious.

    What is traveling great distances? When I visit my brother three states to the West I can easily notice the change in when the Sun comes up.

    I might also bring up the Foucault Pendulum which shows that the Earth is rotating.

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    [edit: holy dead thread, Batman!]

  25. #85
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    Quote Originally Posted by The Watcher
    After a few short road trips discovering that all map measurments are slightly longer than the 'actual' measurements, should bring you to the conclusion that either the map makers measuring equipment is rubbish, your measuring equipment is rubbish or that the surface you are driving on is spherical! (As if you are driving on top of a very large sphere.)
    If your method of measurement is driving, I'm gonna go with option B.

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