The earth hasn't really got an exact centre because it's constantly being pulled around by the sun and moon, etc., not to speak of all the wobbly stuff going on in its runny bits inside, and is changing shape a bit all the time. All those oceans keep moving around too and you can't not count them. Anyway, I'm still happy with the height given for Everest by Arthur Mee's Children's Encyclopaedia, the ten volumes of which I devoured as a child and which made me what I am. That height was 29002 feet. That extra "two" on the end made it so easy to remember. I always remember that Penyghent is exactly one hundred feet lower than Ingleborough and that Liathach is 3456 feet. I wasn't bad at rods, poles, perches and chains either. I'm quite good at converting fat into stones too, unfortunately.