The depth at which a white disc lowered into the sea vanishes from sight, one of the oldest measures of water clarity. It is named for Angelo Secchi, the Jesuit astronomer who in April 1865, at the invitation of its commander Alessandro Cialdi, ran systematic trials with white discs from the papal steam corvette Immacolata Concezione off Civitavecchia, testing how sun, cloud, the disc’s colour and the side of the ship the observer stood on changed the reading.
Why it matters for sightings under water
A Secchi depth is a calibrated answer to the question every report of Unidentified Submerged Objects raises: how deep can a pale object be and still be seen from above. Preisendorfer’s NOAA technical memorandum of 1986 sets out the physics: a target’s contrast falls away exponentially with depth at a rate set by the water’s beam and diffuse attenuation together, so a measured Secchi depth fixes that rate for the water it was measured in. Nils Jerlov’s classification of natural waters by their light, five oceanic types from I to III and nine coastal types, gives the same rate for water nobody measured on the day.
For Submerged Egg 2002, CalCOFI survey casts made five weeks later in the waters off Southern California recorded Secchi depths of 18 to 24 m; the site’s Submerged Egg Buoyancy Bench runs a white body through that water and through Jerlov’s types to see what shows from the air. How a body stays at such a depth at all is the subject of Neutral Buoyancy.
Connections
Unidentified Submerged Objects · Submerged Egg 2002 · Neutral Buoyancy