Michelson-Morley Exp.

This is probably a 9th grade level science question but I cannot quite
grasp a full understanding so I thought I'd ask if someone could
provide some detail.

In the Michelson-Morley Experiment they measured the speed of light at
right angles, blah, blah, blah.

What I don't get is where the variability comes from. They expected
that there was going to be differences between the two beams based on
the earths rotation and its movement around the sun.

So, my question is--- Does the speed of the earths rotation vary
enough to expect a difference in the light beams? AFAIK, the
variablity of the earth rotation is approx one millionth of a second
daily.....tiny. Or is there something I'm missing?

Anyone?

Thanks

Joe B wrote:
This is probably a 9th grade level science question but I cannot quite
grasp a full understanding so I thought I'd ask if someone could
provide some detail.

In the Michelson-Morley Experiment they measured the speed of light at
right angles, blah, blah, blah.

Not quite. They looked for time-of-travel difference for light
taking two different round-trip routes.


What I don't get is where the variability comes from. They expected
that there was going to be differences between the two beams based on
the earths rotation and its movement around the sun.

For an Earth immersed in a light-carrying medium (the so called
luminiferous aether) the velocity of the Earth in its orbit and
the velocity due to its daily rotation should have produced
noticeable interference fringes, as the relative round trip
times for the beams would depend upon the beam's orientation
with respect to the aether 'flow'.

A simple analogy would be comparing the return times of identical
boats taking round trips in a river. The boats set off together,
one heading for a spot directly across the river (and hence cutting
across the flow of the water) while the other heads downstream and
back (or upstream and back). Only if there is no current will the
round trips take equal time.


So, my question is--- Does the speed of the earths rotation vary
enough to expect a difference in the light beams? AFAIK, the
variablity of the earth rotation is approx one millionth of a second
daily.....tiny. Or is there something I'm missing?

Variation of the Earth's orbital speed and rotation rate are not
germane to the experiment. It's the speed of the observer with
respect to the hypothetical aether background that's being sought.


Anyone?

Thanks