Expert: Courtney Seligman - 1/27/2011

Question
Dear expert,

Does our technology enable one to measure in a decent way the difference of weight (that's provoked by centrifugal force contribution to gravity) between Earth's poles and equator?

                                        Sincerely

Answer
The difference is about a third of a percent of the weight of the object, so you just need something that will measure weight to that accuracy. I have a postal scale that weighs objects up to 50 pounds, supposedly to within 0.2 ounce accuracy. For a 50 pound (= 800 ounce) object, 1/3% of its weight is almost 3 ounces; so my scale should be able to measure a difference in weight almost 25 times smaller than the one caused by the Earth's rotation.

I don't know the technical specifications of high-accuracy scales, but am sure that the best ones could beat the accuracy of my $30 scale by several decimal places. So yes, I think that it would be easy to find equipment to measure the difference to any reasonable accuracy.

I might note, however, that this technique would only apply to objects of constant mass, not to things such as people which might gain or lose weight during the time it took to transport them from the Pole to the Equator, or vice versa. So if you weighed 150 pounds at the Equator, then got on a plane and flew to the Pole, the half-pound difference in weight that might be expected could easily be offset by eating, drinking, or even breathing (water vapor being expelled in the process, and for that matter even through the pores, as a result of the oxidation required to keep you alive).

Finally, although it isn't as obvious as putting something on a scale, the rate at which a pendulum swings back and forth depends on the force of gravity; so a pendulum swings a little slower at the Equator than at the Poles, and if it can be kept going long enough, the difference in its rate of swing can be used to measure the difference in the force of gravity to substantial accuracy. (Supposedly, even moving a pendulum to the top of a tall building can cause it to gradually but noticeably lose time, due to the slower rate of swing caused by the lower gravity at higher altitudes.)