Canadian astronomer and engineer who discovered many new binary stars, including Plaskett's Twins, previously thought to be a single, massive star. He also carried out research into stellar radial velocities.
Plaskett was born near Woodstock, Ontario, and trained as a mechanic before studying at Toronto. From 1903 he was in charge of astrophysical work at the new Dominion Observatory in Ottawa. He designed a 1.8-m/72-in reflector telescope for the Dominion Astrophysical Observatory in Victoria, and was its director 1917-35. He then supervised the construction of a 205-cm/82-in mirror for the MacDonald Observatory at the University of Texas.
Plaskett's work on the radial velocities of galactic stars enabled him to confirm the contemporary discovery of the rotation of the Galaxy and to indicate the most probable location of its gravitational centre. This led to a study of the motion and distribution of galactic interstellar matter, particularly calcium.