# Can we prove the Earth rotation with a disk mounted in its center on a frictionless axis?

Can somebody prove the rotation of Earth if it places an uniform disk with a hole in its the center on an axis and orient it paralelly to Sun ecliptical disk?Just assure himself that the disk on the axis is in equilibrium and the friction between the disk and axis at the center hole of the disk is negligable. The disk should preserve its absolute orientation with time while the Earth would change its orientation due rotation and this should be visible after several minutes or hours by looking a labeled part of the disk regarding the floor?

• If the disk is not spinning how do you expect its axis to remain fixed? What you propose will not work. – joseph h Nov 5 '20 at 21:56
• @Drjh The axis is horizontal and the disk is vertical... simply like a car wheel on its axis but all elevated.. not touching the ground. If the wheel is uniform(balanced) it can stay with same angular velocity for a long time also because we use a contact to the axis with negligable friction. Now put the wheel to ang. velocity zero... it will stay like so if there is no force on it to change this absence of rotation. Now,as the Earth rotates and the wheel not the wheel should change its orientation respect to the ground if placed East-West(rotation of Earth) ...... – Krešimir Bradvica Nov 6 '20 at 2:06
• You write in your comment: "Now put the wheel to angular velocity zero". You are proposing this as a setup capable of demonstrating the Earth rotation. Which means that using information from outside this setup is disallowed. Therefore presumably with 'puttng the wheel to angular velocity zero' you mean zero angular velocity with respect to the Earth. Can you confirm that? Zero angular velocity with respect to the Earth. – Cleonis Nov 6 '20 at 5:04
• @Cleonis Zero velocity regarding the ground at the start of the experiment.... after that the disk starts apparently rotating.... – Krešimir Bradvica Nov 6 '20 at 5:11

What you describe is reminiscent of a 1913 experiment by Arthur Compton. This setup is referred to as a Compton ring

A circular tube is filled with water (with suspended particles in the water to allow tracking of motion of the water).

The initial position of the tube is perpendicular to the local level surface. The water in the tube is allowed to come to complete rest. This rest state is a state of co-rotating with the Earth rotation. Then the tube is flipped 180 degrees. After that flip the water is seen to have been set in motion, the magnitude of the velocity can be observed with a microscope.

This setup will show the strongest effect at the Equator, and a smaller effect on higher latitudes.

So, contrary to assertions in comments and answer to this question using gyroscopic effect is not the only way to demonstrate the Earth's rotation. However, if a disk is used that is initially co-rotating with the Earth then the setup does need to execute a flip in order to obtain any data

• What You described is about Coriolis effect but this what I asked is similar but not same. – Krešimir Bradvica Nov 6 '20 at 2:07

If the disk is spinning then it will act as a gyroscope and will try to keep its axis oriented in a constant direction in space. This is called a Foucault gyroscope and it can be used to demonstrate the rotation of the earth, although great care must be taken to minimise friction and allow the axis of the gyroscope to rotate freely. See this Wikipedia article for more details.

• Sorry for concusing question... but the disk is not doing any rotation, just standing still vertically on a horizontal axis and so while the Earth is rotating a label on the disk let say at 12 o'clock should be after same time at for example 13 o'clock..... – Krešimir Bradvica Nov 5 '20 at 21:30
• Imagine it like a car wheel on the axis...same position... – Krešimir Bradvica Nov 5 '20 at 21:33
• @KrešimirBradvica It won’t work unless the disk is spinning. A disk that is stationary with respect to the earth actually shares the earth’s rotation in space and so will continue to rotate with the earth due to conservation of angular momentum. – gandalf61 Nov 5 '20 at 21:50
• In that case it would be acting like the Moon which on every revolution does one rotation... should the disk not make any rotation? Now I am confused... – Krešimir Bradvica Nov 5 '20 at 21:56
• @KrešimirBradvica The answer from Cleonis describing the Compton ring suggests a method without spinning the disk. – gandalf61 Nov 6 '20 at 2:28