Plakidis made systematic observations of variable stars, novae, planets (Mars, Jupiter, Saturn, and Mercury), minor planets, stellar parallaxes, orbits of comets, and double stars, also contributing to the accurate geographic coordinates of the Athens Observatory.

Plakidis independently discovered V1500 Cygni several hours after Minoru Honda claimed the find in Japan.

He attended the University of Athens and became an assistant astronomer at the National Observatory under Demetrios Eginitis eventually traveling to Europe to study in Italy, France, Germany, and England.

While in England he collaborated with Arthur Eddington on a paper entitled the Irregularities of the Period of Long-Period Variable Stars and by 1931 Plakidis was awarded a Ph.D.[4] He returned to Greece and became an astronomy professor at the University of Athens.

Because of his academic achievements in the field of astronomy, Plakidis received a scholarship and spent two years at different European observatories.

He also participated in the establishment of the Penteli Observatory containing a 63 cm Newall telescope which was donated by Cambridge University in 1955 because of Plakidis's lifelong research relationship with Sir Arthur Eddington.

[10] Plakidis continued his research on variable stars throughout his life writing papers for different local and international publications.

That same year he was honored by a publication of his works entitled In honorem S. Placidis :special volume dedicated to Professor S. Plakidis on his 80th birthday.