The relativity of imperfections

Related Content

Don’t believe what people say about the shape of our planet, because it’s not true. The Earth isn’t round! At least not if “round” means having the shape of a perfect sphere.
   The word sphere is derived from the Greek word sphaira, meaning a three-dimensional body in which every point on the surface is an equal distance from the centre. This description is a far cry from the shape of Earth. In fact, the difference between Earth’s radius at the equator and the one at either pole is about 21 kilometres. It looks more like a ball being stepped on.
   Well, then, does any item exist that is perfectly spherical? Perhaps not – but some do get pretty close. Take a steel ball made for a ball bearing, for example. If you turned it in your fingers and looked at it against the light, you would not notice any imperfections. Still, in the photos taken during space missions the Earth looks pretty round, too.
   Let’s make a comparison. Imagine the 10mm steel ball you are holding between your fingers, enlarged to the size of the Earth. If you measure the radius of this ball in different places, you will find that it doesn’t vary more than about five metres anywhere. And on a ceramic ball of similar size, the variation would be about half a meter. Compare that to Earth’s 21-kilometre difference. Clearly, these balls more closely approach the perfection of a sphere. They are “rounder.”
   Now remove your fingers from the steel ball and polish it thoroughly – because if you held an Earth-sized steel ball with fingers of proportionate size, you would leave fingerprints about 50 metres high.

Keep me updated

Want to learn more about what is driving change in the engineering world? EVOLUTION helps you to stay up to date with emerging trends as well as the latest technology. Sign up for EVOLUTION updates to receive new content directly to your inbox.

Sign up