Our world is a large sphere, spinning once a day, and moving around the sun once a year. The moon is a smaller sphere which orbits the earth once a month.
The study of the shape of the Earth is called geodesy. The shape was measured originally by taking the length of a degree of latitude in different places.
The Earth's shape was no mystery to the ancient Greeks, who used observations of the horizon, and the planet's shadow on the Moon as proof that it was a sphere.
Some time before 200 BC, the mathematician Eratosthenes, who lived in Alexandria, had estimated the circumference of the Earth with an error of about 4%.
While most educated people and sailors all knew that the world was a globe, many of the less educated people clung to the old notion that the world was flat.
In 1492, Christopher Columbus, like every other educated person of his day, was well aware that the earth was a sphere, and he also knew Eratosthenes' estimate.
Because Eratosthenes gave his estimates in terms of a unit that had no standard, using the stadion, later scholars had troubling understanding his estimate.
Christopher Columbus used a low estimate of the size of the stadion, and an overestimate of the land distance to China, and thought China was closer to Europe.
In 1521, Ferdinand Magellan's crew completed the first circumnavigation of the Earth that he had commenced, establishing for all to see that it was a sphere.
The circumference of the world is close to 40,000 kilometres, so that the distance from the North or South Pole to the equator is close to 10,000 kilometres.
The Earth is not a perfect sphere, being slightly flattened at the Poles. This can be shown by measuring the length of one degree in low and high latitudes.
Isaac Newton showed theoretically what we now know from measurements as a fact, that the planet is an oblate spheroid, a flattened sphere rather like a pumpkin.
The extent of the flattening of the Earth is small, so the difference between the polar radius and the equatorial radius is about 22 kilometres, or about 0.33%.
The rotation of the Foucault pendulum proves that the Earth rotates once in 24 hours. The rate of rotation is a function of the latitude of its location.
A Foucault pendulum at the equator does not appear rotate at all when it is set going, while at the poles, it rotates once in twenty four hours, like the Earth.
The period in hours of a Foucault pendulum's apparent rotation, when it is located at latitude lambda, is given by twenty four divided by sine lambda.
The Earth has a magnetic field, but the magnetic poles are not in the same place as the geographical poles which lie on the planet's axis of rotation.
We do not understand exactly how the Earth's magnetic field is generated, but it is assumed to be something to do with the iron core spinning around.
Karl Gauss worked out where the geomagnetic poles are, placing the southern pole over the ocean and triggering a series of expeditions to locate the poles.
In other words, it is possible for Antarctic navigators to sail to the south magnetic pole or even to the south of the south magnetic pole.
At different places around the world, it is possible to measure the magnetic deviation, the difference between magnetic and geographical north or south.
From time to time, about every 200,000 years, the Earth's magnetic field reverses, exchanging north and south in a period of perhaps a few thousand years.
On average, there are four or five reversals every million years, but there has not been a reversal of the magnetic field in the past 800,000 years.
The strength of the Earth's magnetic field has dropped by about 15% in the past 150 years, and some scientists think we may be heading for a reversal.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsworldstruc.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.