Any observable phenomena due to this cause depend on the square of the ratio of any velocity entering into the observation to the velocity of light.

Now light takes about fifty minutes to get round the earth's orbit; and the earth takes rather more than 17,531 half-hours to do the same.

Hence all the effects due to this motion are of the order of the ratio of one to the square of 10,000.

Accordingly an earth-man and a sun-man have only neglected effects whose quantitative magnitudes all contain the factor 1/10^8.

Evidently such effects can only be noted by means of the most refined observations.