|MadSci Network: Science History|
In fact, the Earth is only called that in English. If you go to the Nine planets web page you will find under the section "planetary linguistics" a host of different names for our planet.
The word `Earth' itself has Teutonic origin (check the Oxford English Dictionary, your public library will have a copy), and the words are very similar in German (Erde) and Swedish (Jorden), other languages sharing common origins with English. It had much the same meaning long ago, referring to the thing on which we live. We have a very different understanding now of what the ground/world is than people did back then, but we still call it the same thing.
Try the links in the MadSci Library for more information on Science History.