"There are 6.6 billion people in the world, each weighing an average of 12 stones. Thus the total weight of mankind is somewhere in the region of 495 million tonnes."
Indeed. I commend your grasp of mathematics, if not that of physics.
"Now, that sort of weight added to that of the planet would have a massive impact on its orbit."
True, but irrelevant. The fact is that all this weight has been subtracted from other parts of Earth's biosphere, just aas all the weight of our tools, vehicles, houses and other buildings has been subtracted from Earth's other weight.
Earth is a CLOSED SYSTEM. This means, fundie, that if you add mass to any part of it, you must subtract it from another. In addition, as noted by several other posters (if less politely than by myself), all the mass of all these humans and all their stuff is insignificant compared to the mass of the Earth.
Mental exercise, to help your brain grow:
Go get a basketball. Thne draw the continents, cities etc. on the surface.
Congratulations. You have successfully approximated the depth to which our occupation affects the planet Earth.
Glad to add to your education.
