Astronomer and engineer who created a new design in reflecting telescopes
Following high school education, Plaskett trained and worked as a mechanic. In 1889, he was hired as foreman at the department of physics at the University of Toronto. He took undergraduate courses, graduated, and became a lecturer at the university. His interest in astrophysics led him to the new Dominion Observatory in Ottawa where he helped design and construct instruments as well as conduct research on stellar radial velocities. Plaskett vastly improved the observatory's 38-cm/15-inch telescope by building a spectroscope for it. He lobbied the Canadian parliament for a 1.8-m/72-inch telescope, and was finally appointed to supervise the building of this instrument for the Dominion Astrophysical Observatory in Victoria. Completed in 1918, this was the world's largest telescope of the day. Still in place, although with many updating modifications, it is named the Plaskett Telescope.
Plaskett remained at the Dominion Astrophysical Observatory as Director until 1935. He also served as president of the Royal Astronomical Society of Canada. After he retired, he oversaw the construction of the 2.05-m/82-inch telescope for the MacDonald Observatory at the University of Texas.
Plaskett discovered many new binary stars, including Plaskett's star, a massive binary star previously thought to be a single star. His work on the radial velocities of galactic stars allowed him to confirm the rotation of the Galaxy and to locate the most probable location of its gravitational centre. Together with J. A. Pearce, he published the first detailed analysis of the structure of the Milky Way Galaxy (1935), demonstrating that the sun is two-thirds out from the centre of our galaxy and rotates once in 220 million years. In 1984, Minor Planet No 2905 was named Plaskett in honour of J. S. Plaskett and his son H. H. Plaskett, also an astronomer. The Plaskett Medal of the Royal Astronomical Society of Canada and the Canadian Astronomical Society is also named for this pioneering astronomer.
Sources: The Canadian Encyclopedia, 2000 ed.; The Hutchinson Dictionary of Scientific Biography, Roy Porter and Marilyn Ogilvie, eds. 3rd edition, 2000, The Bruce Medalists website; Image: Dominion Astrophysical Observatory website.