The simplest and most convincing proof that the Earth's is "round" is to examine a photo of our planet taken from space or to orbit it in the space shuttle.
There can be no doubt from such evidence that the Earth is round, or more correctly, a "sphere." The word "round" refers to a circular shape, like the shape of a quarter when held flat in your hand. The proper term to describe the Earth's shape is spherical. A sphere appears round when viewed from any direction in space. It's okay to use the term "round" to refer to the shape of the Earth, so long as you're aware of this distinction.
Transparency master [figure 2]
Now if 7 degrees subtends 5,000 stadia, then 360 degrees -- the circumference of the Earth -- corresponds to just over 250,000 stadia. If 1 stadium was 0.16 km as we believe, then Eratosthenes obtained a value of 40,000 km for the circumference of the Earth which is remarkably close to today's accepted value. This underscores how powerful simple geometry can be to learning about our world.
Have you ever seen a lunar eclipse? A lunar eclipse occurs when the Earth comes between the Sun and the Moon causing the Moon to enter the Earth's shadow. [show figure of lunar eclipse here] While not as spectacular as a Solar eclipse (when the Moon comes between the Sun and the Earth), the Moon turns a wonderful copper or blood-red colour. The important point for us, however, is the shape of the Earth's shadow as it sweeps over the Moon's surface; it is circular, exactly as one would expect if the Earth were spherical.
A skeptic -- someone who needs more proof before accepting such an important idea -- might protest that a sphere is not the only shape which casts a circular or round shadow. And she would be right. For instance, a quarter is certainly not spherical, but if held in the right way, casts a circular shadow. How then can we convince her? The Earth's shadow on the Moon as it enters or leaves a lunar eclipse is circular regardless from where it is viewed; whether from Canada or Australia. Only a sphere casts a circular shadow when viewed from any direction. A quarter when tilted or turned on its side has a distinctly non-spherical shadow. Try it some time.
The other well known proof for the roundness of the Earth is often associated with Christopher Columbus who first sailed from Europe to North America in 1492. At that time, many people in Europe, including the well educated, believed that the Earth was flat. Common sense told them this. If one sailed too far in one direction, one would fall over the "edge." Columbus thought that the Earth was round (spherical) and gave the following proof: when a large ship (with a tall mast) sails away from a port, it appears to get smaller and smaller before it completely disappears from sight at the horizon. The curious thing, however, is that while the entire ship is visible as it leaves port, only the mast remains visible just before it vanishes. This is a natural consequence of sailing on a spherical Earth as figure 3 shows.
Transparency master [figure 3]
When a large ship is far enough away, the body first disappears under the horizon followed by the mast. Columbus was right!
There are other interesting proofs of the Earth's roundness which don't require a trip into space, though they do require some careful planning! Suppose you had several friends who lived in various cities across the country; say Halifax, Montreal, Winnipeg, Regina, Calgary and Victoria. You phone each of them up and ask them to phone you tomorrow the instant they see the Sun rise on the eastern horizon. (Let's assume you have enough money to pay for the phone calls, that your family doesn't mind getting calls in the middle of the night, and that it is clear right across Canada the morning of the experiment.) What would happen? Your first phone call would come from your friend in Halifax, followed later by your friends in Montreal, Winnipeg, Regina, Calgary and finally Victoria in this order (and a few hours after your first). Each friend can confirm that he or she is observing the same Sun since our Sun's surface is normally covered in spots with a definite pattern which would be the same for all observers. The simplest explanation for why the times of sunrise differ across the country in this manner is because the Earth is round. (Of course, we all know the Sun doesn't actually rise; rather it is the Earth turning on its axis which gives the illusion of the Sun rising and setting.)
You can easily demonstrate this by performing the following experiment. You'll need three things; some string, a globe, and a flat world map. Suppose you'd like to fly from Vancouver, British Columbia to London, England. On the flat world map, locate both cities. Now take the string, fix one end on Vancouver, and draw the string tight so that it also passes over London. Record any major cities or bodies of water over which the string passes on the map. Repeat the same process with the globe which represents the spherical Earth. Place one end of the string at Vancouver, and find the route over the Earth which uses the smallest length of string and which terminates at London. Record any major cities or bodies of water over which the string passes.
There is a radical difference between these two paths, isn't there? Apart from the beginning and ending of the path, not much else is the same. The path you've determined on the globe is very close to the actual flight path a modern jet follows; it does get very close to the North Pole! The world is round (or at least spherical) after all.
There are many interesting consequences of living on a spherical Earth. One with religious significance can be illustrated with an experiment similar to the one you've just performed. In the religion of Islam, Muslims are encouraged to pray while facing towards Mecca, Islam's most holy city, at certain times during the day. The question for Muslims who live in Canada is, which direction do they turn to face Mecca? Repeat the experiment above, this time using the cities of Toronto and Mecca (in Saudi Arabia). Since Mecca is east of Toronto and much closer to the equator, some people believe that one should face south-east. But the globe experiment will quickly convince you that the proper direction to face is actually north-east!
Photos of Mars and other nearby planets from the Hubble Space Telescope also show these bodies to be spherical, just like pictures of the Earth taken from the Moon or space. (In fact, one can watch them rotate on their axis.) Do you recall seeing a picture of an asteroid, a chunk of rock leftover from the formation of the Solar system and which can be the size of a large city (the largest is the size of the distance between Halifax and Ottawa). Asteroids are far from spherical in shape, looking like pieces of driftwood. Have you ever wondered why all the planets and our Sun are spherical, while smaller bodies are not?
When it comes to energy, nature doesn't like to be wasteful. It much prefers to have bodies in the lowest possible energy state. This is true for atoms as well as planets. Planets are spherical for two reasons; first, a sphere is the shape which has the lowest energy for a given size so far as gravity is concerned, and second, planets are massive enough that their gravity can force material to change its shape over a long period of time. In the case of a rock or an asteroid, gravity is not strong enough to force it to become spherical from its original shape.