A few assumptions that have gone into the creation of this tool, for the curious.
- The earth's radius at the equator is taken to be: 6,378.137km, this being a number we found on the internet.
- The earth's radius at your latitude is calculated on the basis that the earth is a perfect sphere, and does not take into account the fact that the diameter between the poles is some 20km greater than that at the equator. I'm not a genius.
- The radius at your latitude will represent an average figure, however for people living in (for instance) Cusco (at an altitude of some 3,500m) the calculated speeds and distances will be understated if this is not added into the equation. (It should however be noted that altitude has no bearing on the difference in distance travelled between head and feet).
- It is assumed that while seated, your height is two-thirds that when standing.
- When asleep, head and feet will be level, and so have the same speed. It is assumed that you sleep at ground-level.
In summary - I hope to have accounted for sufficient variables in order to provide an impression of diligent accuracy, and to cause you to ignore minor details like the fact we've assumed you were born at the height you are now, and that you have never moved - other than to stand up and sit down.