ôô

# Can We Feel the Earth Moving?

As the Earth orbits the Sun and spins on its own axis, can we actually feel it? Everyday Einstein describes how fast we are moving and explains what it means astronomically to start a new year.

By
Sabrina Stierwalt, PhD
Episode #129

Well, we don't end up feeling anything because we are moving along with the Earth, and all of the motions are nearly constant. Similarly, when you take a smooth plane or train ride, you don't feel the motion unless the plane or train slows down, speeds up, or hits a bump in the road. So as long as the motion is constant, we don't feel it.

There is one way that we could, in theory, at least indirectly measure the Earth's spin. Earlier we said that the Earth spinning is an example of nearly constant motion. The reason we said "nearly" is that the Earth's spin carries us around in a circle, not in a straight line. It's a very big circle, and it takes 24 hours to go around, but qualitatively it is the same thing that happens on a spinning amusement park ride, where it feels like you're being flung outward as the ride spins around. The spinning Earth is flinging us away from its surface a tiny bit, so that we weigh a little less than we would otherwise, simply because we are not being held down to the surface as tightly.

However, this is a very small effect. At the equator, your weight is reduced (compared to a non-spinning Earth) by about 0.3%; the effect weakens as you go north or south. Once you reach the north or south pole, it disappears completely because the Earth is not spinning there. So if you can feel differences in your weight of 0.3% (about half a pound for a 150-pound person), and if you travel from the equator to the north or south pole, then technically speaking, you could feel the effects of the Earth spinning.

On the other hand, people's weight naturally fluctuates more than 0.3% in any given day, so it's unlikely that you would be able to disentangle this change from other effects (like whether or not you had just eaten lunch).

## Newton’s First Law

However, if the Earth did begin to slow down or speed up, we would be able to feel the change.

If our planet were to suddenly begin spinning faster, we would be thrown backwards, just like when you are in a car, and you feel pushed back into your seat as the car accelerates or increases speed. This phenomenon, the idea that we only feel changes in motion, is summed up by Newton’s First Law.

The law states that an object in motion will stay in motion and an object at rest will stay at rest unless acted on by an external force. In the case of the car, your body and the car are moving together until the car accelerates. At first your body tries to maintain its original motion and thus you feel like you are being pressed backward into your seat.

## The Leap Year

So before we end for today, let’s get back to that quarter of a day I mentioned earlier. The Earth completes one trip around the Sun every 365 ¼ days. That means nearly every 4 years, we have what’s called a leap year – a year with an extra day to add to the calendar which we tack on as February 29th.

Julius Caesar first introduced leap years into the calendar over 2,000 years ago with the rule that every year divisible by 4 would be a leap year. Under these original guidelines, however, too many years were being added. So an adjustment was made to not include years divisible by 100 unless they are also divisible by 400. That means our next leap year will be 2016.

That ends our show for today. Remember to become a fan of Everyday Einstein on Facebook or follow me on Twitter where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. Thanks for listening!

Earth spinning image courtesy of Shutterstock..

## About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.