First use of “Secchi disk” in measuring water transparency

        Being a simple and useful tool, the “Secchi disc” or “Secchi disk” is widely used as a simple tool for measuring the transparency in water bodies including fish ponds.  The disk is commonly identified by its circular appearance with its black and white quadrates. The simplicity of the disk goes also  to its use that only requires the lowering of the disc into the water until it disappears then raising the disc until it reappears whereas the average of the two readings (disappearance and reappearance) is known as the transparency (or turbidity) of this particular waters. Based on such measurements along with other analysis, management actions could be taken. The credit of the Secchi disc goes to an astrophysicist, Pietro Angelo Secchi (1818-1978) who was the scientific advisor to the Pope and who was requested by the head of the Papal Navy to measure transparency in the Mediterranean Sea. Subsequently, Pietro Angelo Secchi developed the first 30-cm diameter disk which was lowered from the papal yacht, on April 20, 1865. The disk was plain white. There have been several versions to the first disk used by Secchi reaching to the disc of about 20 cm in diameter which –for easier visibility- is divided into four equal sections (quadrants) painted alternately black and white like the attached calibrated rod, PVC pipe or rope such as the ones used at the present.