As early as 1573, Thomas Digges had suggested that this theory should necessitate a parallactic shifting of the stars, and, consequently, if such stellar parallaxes existed, then the Copernican theory would receive additional confirmation.
The moon was the earliest " measurer " both of time and space; but its services can scarcely have been rendered available until stellar " milestones " were established at suitable points along its path.
In 1882 Pritchard commenced a systematic study of stellar photometry.
In 1876 he successfully applied photography to the study of the ultra-violet region of stellar spectra.
Now it is necessary to bear in mind that all observed motions are relative; and, especially in dealing with stellar motions, it is arbitrary what shall be considered at rest, and used as a standard to which to refer their movements.