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The Question

(Submitted February 18, 1998)

My question for you is this: Throughout the better part of the 20th century people have always believed that the earth remained stationary.

I would like to know who the first person was to prove that the earth does in fact spin around its axis, and by what means did he/she use to come to this conclusion. Was it a tangible experiment?

The Answer

Thank you for contacting our Ask an Astrophysicist service. It has been generally accepted that the earth is not stationary for hundreds of years.

I suppose the first tangible proof that the earth rotates was provided by Jean-Bernard-Leon Foucault in the nineteenth century. He found that if a long pendulum with a heavy bob was set swinging, the plane in which it was swinging would appear to rotate.

The plane of a Foucault pendulum only rotates with a period equal to the Earth's sidereal period (23hrs 56min) at the poles. At lower latitudes, the period is (23h 56m)/sin(latitude), and at the equator, the plane doesn't rotate at all.

There is no reason why the pendulum's plane should rotate. It just appears to rotate because the ground under it is rotating. If you do a web search on "Foucault pendulum" or "Foucault's Pendulum" you should find lots of links.

Variations in gravity due to centrifugal force have been measurable for some time. There is also the subtle Sagnac effect (now the principle of operation of optical gyroscopes) that causes a phase difference in light moving in opposite directions along the components of a path parallel to the direction of rotation. Specs for the Canterbury Ring Laser based on this principle are described in

Some commentators attempt to find some contradiction of Einstein's predictions here; but in fact it is a general relativistic effect due to the rotation frame of reference, which is precisely why it is evidence in situ that the Earth rotates.

We won't discuss what astronauts have seen, because that might be a matter of perspective... but the equatorial bulge of the Earth, and the measured Doppler shifts from opposite sides of spiral galaxies and various disks viewed edge on certainly provide strong evidence that other things in the universe rotate. We could in principle measure such a Doppler shift of the Earth from space, using lasers... at least from the Moon.

Damian Audley, Mark Kowitt, Eric Christian, John Cannizzo and Kevin Boyce
for Ask an Astrophysicist

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