It is approximately 4000'C at the centre of the Earth. To put this in context:
The centre of the Sun is approximately 15 million'C
The surface of the Sun is 5500'C
Iron melts at 1535'C (when at atmospheric pressure)
Water boils at 100'C (when at atmospheric pressure)
Human skin is comfortable with temperatures up to about 60'C
The highest temperature recorded on the Earth's surface is 58'C (Libya 1922)
It is not possible to directly measure the temperature at the centre of the Earth and four thousand degrees is nothing more than our most well-established piece of guesswork to date. Most modern calculations rely on the fact that we believe the inner core to be made up of iron and nickel that is just about at melting point. It is under a lot of pressure, which prevents it from melting, even at such high temperatures. There is also a lot of evidence regarding how the outer core of the Earth convects and that helps to establish the temperature. However, recently British scientists have suggested that the temperature of the Earth's core may in fact be as high as the surface of the Sun, so the question is still open.
Sally Riordan, M.A., Management Consultant, London
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'