What is the simplest way to prove the Earth is round?

Question

Assume you've come in contact with a tribe of people cut off from the rest of the world, or you've gone back in time several thousand years, or (more likely) you've got a numbskull cousin.

How would you prove that the Earth is, in fact, round?

Answer 1

The shadow of the Earth on the Moon during an eclipse and the way masts of ships are visible when they are out of sight are the classical reasons.

Answer 2

Simplest, you say? There are two that strike me as being simple to demonstrate. Luckily someone on the internet has already spent some time to help us here to make these easy to illustrate:

• Shadows differ from place to place:

Eratosthenes carried out this experiment to determine the circumference of the Earth, already assuming its spherical shape; incidentally, the proof of such being consequential of the procedure.

However, a demonstration can be achieved by a simple, local experiment (as opposed to having a party venture to a distant enough point):

Take a piece of card (A3, or so), attach two obelisks to the card by their bases and, with a light source, produce shadows - now, slowly bend the card so that it becomes convex (that is, the side with obelisks attached bulging out) and watch the effect.

• You can see farther from higher:

There are numerous other ways of demonstrating that the Earth is round, or curved, at least, from analysing the center of gravity to simply observing the other round objects that are visible in space; but I believe these illustrations to be the simplest to comprehend.

_{Images sourced from SmarterThanThat}

Answer 3

You can build a simple pendulum and observe how it rotates as the day progresses. You can then put a pendulum on a stick or something that you can rotate yourself in order to demonstrate that when you rotate the stick, the pendulum will continue to swing in the same direction. This shows that the direction of movement of the pendulum will change relative to its base only if its base is rotated.

Pendulums can also be used to measure your latitude (its direction will change at different rates for different latitudes), and to measure the local value of g (the amount of time it takes to go through one cycle, or its period of oscillation, will vary with gravity).

Answer 4

Sitting for a while by the seashore ought to make it clear the Earth isn't flat, even if you don't happen to see a ship go over the horizon. The edge of the discworld Earth would have to be just a few miles away, and there's no way that the entire, circular world would fit inside the circle that the ocean horizon seems to make.

Humans have not just known the Earth was spherical but actually have been measuring its radius for thousands of years. http://en.m.wikipedia.org/wiki/History_of_geodesy

Answer 5

Besides the going back in time option, you could just show your "numbskull cousin" a picture of the Earth taken from the moon like the one below.

http://nssdc.gsfc.nasa.gov/imgcat/hires/a17_h_148_22727.gif

Answer 6

If the person in question is from a temperate latitude, take them to the tropics to feel the heat of the noon sun, preferably trapped out on a sailboat without water. Point to the very high sun and make your point when they are the most miserable. Next, take them to a very high latitude. As they freeze and become exhausted at 3:00 AM while out walking the tundra, point out the low, non-setting (or non-rising) sun, and re-iterate your point in their heightened state of misery. Through suffering and a sense of pride, the object of your demonstration will now likely feel that they have "been there" and "seen it" with "their own eyes". If convinced, that person will gladly proselytize the "truth" of aforementioned roundness of said planet, and will confront the heretics who do not believe.

I think that there are no simple answers to provide "proof" of anything. "Proof" is relative, much in the way "truth" is relative. If simple means "without using science or technology" then you are without hope, as the receiver of the "proof" must accept the truth of the methodology.

Photos from space are photoshopped.

Ships at sea look below the horizon because Osirus/Neptune/Odin/Jesus/Bhaal does not wish man to see to infinity (which also proves that the heavenly bodies are not very far away).

Sticks in the dirt and shadows prove nothing unless you accept that other bodies are permanent, in orbital motion, and far away (at which point the person will already believe that the planet is round).

Don't try to prove anything. You can't. Instead, "Demonstrate and educate", because all you can do is convince, not prove.

Answer 7

I think the simplest way is to have two sticks of same size put both of them perpendicular to the surface of the earth in the mid day sunshine and the gape between them is to be few miles and exact time mesure the angle of elevation or mesure the size of the shadow so both will be differ! By several exams in a sysmetic order we can find that the earth is round.

Answer 8

There's a video on youtube of a island a few miles away such that when you see the island from an elevation, you can see further to its base than you can when you see the island from the shoreline ( a demonstration of answer 1 above). I think this is the simplest way given that now we have zoom ability, anyone can do this kind of experiment on a clear day from any shoreline viewing something a few miles away.

http://www.youtube.com/watch?v=bco_p4V7-QU&feature=related

Answer 9

The occurrence of noon (i.e. meridian passage of true Sun) isn't simultaneous for two observers situated along an east-west line. Hmmm...okay perhaps even simpler. Sunrise and sunset aren't simultaneous for those two observers.