Blog Redux: Secchi Disk Celebrates 150 Years of Clarity

Pietro Angelo Secchi
Pietro Angelo Secchi Image from: United States Navy

Tomorrow will mark the one hundred and fifty-second birthday of the Secchi disk. We dusted off this post from two years ago to tell you all about our favorite scientific instrument. Enjoy!
(Originally published April 20, 2015) – A time-honored instrument of limnology turns 150 today. The Secchi disk, the black and white plate at the bottom of many a limnologist’s rope, was invented by Pietro Angelo Secchi, an Italian Jesuit priest in 1865. In fact, the story goes, he first deployed the instrument into the waters of the Mediterranean, dropping the disk in over the side of the papal yacht. But, before Secchi made his indelible mark on our field of study, he made waves in another field.
Secchi was a prolific scientist, publishing more than 700 papers in his lifetime (1818-1878).  But the bulk of those papers were in his true discipline – astrophysics. Secchi is credited with developing the first spectral classification system for stars. He also discovered a comet (named after him), drew one of the earliest maps of Mars and served as a professor of astronomy and director of the observatory at the Roman College in the Vatican.
150 years later, scientists still use the Secchi disk for water clarity measurements. Photo: Adam Hinterthuer
150 years later, scientists still use the Secchi disk for water clarity measurements. Photo: Adam Hinterthuer

Luckily for limnology (if not for Secchi), the Jesuits were expelled from Rome in 1848, an exile that eventually led Secchi to Georgetown University in Washington D.C., where he befriended Commander Matthew Fontaine Maury, who would become superintendent of the United States Naval Observatory. This friendship led Secchi to devote at least a little bit of his attention to aquatic, rather than astronomic, matters. This served him well when, as one of the scientific advisers to the Pope, Secchi was asked to quantify water clarity in the Mediterranean Sea. The rest, as they say, is history and, on April 20, 1865, Secchi stood on the papal yacht and plopped the first disk into the (presumably clear) Mediterranean waters.
Mediterranean Sea Photo: Inga Vitola, Flickr
Mediterranean Sea Photo: Inga Vitola, Flickr

While water clarity measurements undoubtedly pre-dated Secchi by many, many years, he was the first to standardize the procedure and, crucial to anyone’s scientific legacy, publish his results. hence, we now use a disk named after him to this day.
New technology exists that can provide more accurate water clarity readings but, since people have been using Secchi disks for 150 years, continuing the practice allows us to track water clarity trends over long periods of time. Plus, since anyone can use a Secchi disks, Pietro Angelo’s namesake device is now in the hands of citizen scientists across the globe, allowing scientists to ask big picture questions across immense amounts of space and time.
Here’s to 150 more years of one of our favorite limnological tools – and a big thank you to the Jesuit priest who started it all. Happy 150th anniversary, Secchi!
“The Secchi Dip-In” –  North American Lake Management Society
“The Secchi Disk, Unplugged” – UNC-CH “UNder the C” Graduate Student blog
“Pietro Angeo Secchi” – Encyclopedia