spot-cavities

Updated: May 15, 2025

The absolute depth of spot-cavities at least of their sloping sides was determined by Father Secchi through measurement of the "parallax of profundity" that is, of apparent displacements attendant on the sun's rotation, due to depression below the sun's surface.

Agnes M. (Agnes Mary) Clerke - A Popular History of Astronomy During the Nineteenth Century: Fourth Edition