At the same time as the 15-inch telescope was installed, the DO acquired a large coelostat, a telescope specifically designed for observation of the Sun, at the CEF. From around 1930 to 1936, solar astronomer Ralph DeLury undertook a series of photos of the Sun. These were measured to determine the rate at which sunspots traverse the solar disc. It was found that the rate of rotation at the solar equator was greater than at the poles by several days.

In analyzing the images, DeLury had to correct for foreshortening, an effect where the spots coming or disappearing around the Sun’s edge appear to move more slowly. This is a geometrical effect that can be mathematically calculated. To speed the process (remember this is long before electronic computers) DeLury had the Observatory’s shops construct an analogue computer. This consisted of a globe made of polished steel with lines of latitude and longitude engraved on it. Using the photographic negatives, he then simply projected the solar images onto the globe and read off the angles of latitude and longitude for each of the sunspots he was studying.

Canadian Heritage Information Network
Canadian Heritage Information Network, Canada Museum of Science and Technology, Musée de la civilisation, Stewart Museum, Canadian Medical Hall of Fame, Museum of Health Care at Kingston, University Health Network Artifact Collection, University of Toronto Museum of Scientific Instruments, University of Toronto Museum Studies Program, Suzanne Board, Dr. Randall C. Brooks, Sylvie Toupin, Ana-Laura Baz, Jean-François Gauvin, Betsy Little, Paola Poletto, Dr. James Low, David Kasserra, Kathryn Rumbold, David Pantalony, Dr. Thierry Ruddel, Kim Svendsen

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