[gus_massa]

Sorry for nitpicking, but a cow to represent Mars should be 10m away, and the length of a typical cow is about 2m (1m~=1yd=3ft). So I expects a 4% correction, because the main part of gravity goes like 1/d^2 and the first correction like 1/d^4 (with some constants that mix theoretical numbers with the details of the shape of the cow).

So if the cow is 5 cows away, you can get something like a get a 4% correction, that perhaps is measurable if you can measure the pull form the cow.

The cow representing Regulus is 4000 cows away, so the correction is less than 1/1000000 and it's safe to take it as a spherical cow.

[Side question: Can LIGO "hear" a nearby cow? It has filters to try to see the chirp of the merge of two black holes, and another filter to only consider the signals that appear in both detectors, so you probably need to train two synchronized dancing cows to fool LIGO. But, do they have enough precision?]

[ISL]

You are absolutely correct. I'd somehow parsed the Mars number as in miles, rather than meters. Some part of my brain was surprised, but didn't pull the fire alarm.

On the LIGO question: Two GPS-synchronized cows, standing near the end-masses, that mooed in the appropriate chirp (correcting amplitude for polarization) would make people nervous enough to do a few calculations and, out of an abundance of caution, put up a fence. I expect the gravitational effect from any mooing (it's a tiny quadrupolar deformation of the cow) to be extremely small.

In practice, even the end-station buildings are large enough to keep cow-moos from happening close-enough to bother the instrument gravitationally. Furthermore, there are no cows at Hanford, and I doubt that there are any near Livingston.

[gus_massa]

Just to be 100% sure, I don't expect LIGO to detect the sound waves of the moowing cow, or the deformation of the cows while moowing. But can it detect the movement of a cow nearby?