Now that we know the size of the earth, and a little history, let's get back to the subject at hand ­ Geodesy, the study of the size and shape of the earth.

From much studying over the years and using some very high tech tools (such as GPS and VLBI), many of which we will look at in other modules in this series), we've discovered that the shape of the earth is very complex. It's certainly not the nice round or 'spherical' planet we once thought it was. In fact, it actually 'bulges' outward all along the equator.

Measuring 'The Bulge'

Here's an example of what we mean. Let's pretend that we have a special tape measure. This special tape measure can be stretched ­ directly - between any 2 points on earth. It can go right through the centre of the earth so we can measure the shortest distance between any 2 points. If you have a globe, now would be a good time to get it out and have a look.

Now, let's measure the straight-line distance between 2 points on the equator. The equator is an imaginary line that runs around the earth half way between the North and South poles. Let's measure the distance between two points on this imaginary line that are on the "opposite sides of the earth" from each other. Example ­ Point 1 is in Borneo (check out Indonesia, above and to the left of Australia) and Point 2 is near the Columbia/Brazilian border in South America.

If we stretched our special tape measure between these 2 points the distance would be 12,756,274 metres or about 12,756.3 km.

Now, let's also measure the distance between the North pole and the South pole  - again, we stretch our special tape measure right through the centre of the earth. If the earth were round, the distances would be the same right? Well, let's check the tape. This time, our measurement is 12,713,505 metres, or about 12,713.5 km. Almost the same, but not quite. Turns out the straight-line, pole to pole distance is shorter than the distance between the two points along the equator.

So, the Earth's not as round as we think is it?