How to prove that the Earth is round

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 April 2009

Here is an interesting question: What do they mean when they say that the world is a circle? The answer is simple: “They” never said that! Even those who believe that it is flat say that it extends to infinity in all directions. “They” said that the earth is a sphere… approximately.

But what is the difference between the two? Well, one is a two dimensional figure while the other is a three dimensional object. Strictly speaking, it is impossible to make a circular object. We can only draw it…but some might argue that even that is impossible.

Still, most people take it for granted that the geography teacher was right when she said the earth is round, but they cannot prove it to themselves.

Two observations are readily quoted as evidence that the earth is round: first is that wherever you are, the horizon appears circular all the way around you. This, however, is not a very good explanation.

On an infinitely large flat earth model, the horizon would also appear circular. The reason being that our eyes can only see a limited distance in normal lighting. Incidentally, we are only able to see far away stars because they are very bright.

The second popular observation to prove that the earth is round is that when one stands at the sea shore and watches ships as they sail outwards, they appear to be slowly dropping below the horizon. If you live at the coast, however, you will probably say that you have never observed this phenomenon.

Two reasons might explain why. First: due to misty nature of the air above the sea, the visibility is limited to only about three kilometers on most days. That is, you cannot see farther than this. Therefore, the ships seem to suddenly disappear from your field of view.

The second reason has to do with the reducing apparent size of the ship as it sails into the horizon.

Now, assuming that the earth is round with a diameter of 12,800km, it is easy to calculate the distance to the horizon as observed by a person of average height (about 1.75m)… you simply form a right-angled triangle with one corner at the feet, the second one at the eyes and the third one just touching the surface of the earth at the horizon; then apply Pythagoras’ (Pytha-who?) theorem and voila! Any secondary school pupil can do that.

It turns out that the horizon is about 5km away from the sea shore; but it increases as you climb higher elevations, say, when you stand on a cliff.

At 5km away, a 20m-tall ship appear the same height as a 5mm pebble held at an arm’s length. It is therefore difficult to discern the “sinking” under the horizon.

To observe this effect therefore, one must then use a set of binoculars to magnify the ship, or alternatively sail in the ship and observe the tall buildings on land.