What is the centre of the world?

The scientific answer is that the centre of the earth contains superheated iron and nickel, which gives the earth its magnetic field (and, in certain ways, has kept us alive by deflecting solar radiation). The northern lights is one effect of this.

If you mean the centre more politically, there are various answers. One would be Greenwich in London (because it's the "zero" timezone). For a similarly human perspective, you could count the horn of Africa (because that's where modern human life started). For a non-human perspective, you could look at the world's land surface. If we treat the land area as being from the Aleutian Islands on the left, to Eastern Russia on the right (it always seems strange that the US and Russia are only 50 miles apart), the centre geographically becomes Western Europe or North Africa.