What is the role of the Earth's rotation in the Michelson-Morley experiment?

[abhilash.s]

I understood that the basic idea of the Michelson-Morley experiment was to measure the velocity of light, as seen from a frame fixed relative to the earth, in two perpendicular directions. Assuming the existence of the ether medium (only for the purpose of understanding the rejection of the ether medium), while intending to measure the ether wind velocity, should not we also take into account the spinning motion of the Earth with respect to the ether in addition to its orbital velocity around the sun and the sun's velocity with respect to the ether?

 

[HallsofIvy]

Not if the experiment is done quickly enough- like within a few minutes. The rate at which the Earth is spinning on its axis is very small compared to the speed of light. As far as the "orbital velocity around the sum" and the "sun's velocity with respect to the ether", those are being taken into account when you measure "the Earth's velocity with respect to the ether".

 

[ghwellsjr]

I understood that the basic idea of the Michelson-Morley experiment was to measure the velocity of light, as seen from a frame fixed relative to the earth, in two perpendicular directions. Assuming the existence of the ether medium (only for the purpose of understanding the rejection of the ether medium), while intending to measure the ether wind velocity, should not we also take into account the spinning motion of the Earth with respect to the ether in addition to its orbital velocity around the sun and the sun's velocity with respect to the ether?

You probably already know this but the experiment was not measuring the speed of light in two perpendicular directions but rather merely comparing them for all orientations in a plane parallel to the surface of the earth. It had no ability, for example, to determine if the speed of light was constant from one moment to the next.

 

[Meir Achuz]

The speed of the Earth around the sun is much larger than the speed due to the rotation of the earth, which can be neglected.

 

[ghwellsjr]

The speed of the Earth around the sun is much larger than the speed due to the rotation of the earth, which can be neglected.

Nevertheless, the experiment was sensitive enough to detect an expected ether wind caused by the rotation of the Earth sometime over a twelve hour period. They didn't have to wait for several months. There was no guarantee that the sun was at rest in the ether.