What was it like to watch the skies in the first few decades of the twentieth century? I had my first revelation about this when I saw a photograph of Frederick Slocum seated at the 20-inch refracting telescope in Van Vleck’s main dome.
Take a closer look at Slocum’s outfit. The big coat, the fur hat and gloves, the boots: all these point to the the physical realities of doing astronomy in the days before charge-coupled devices and digital photography allowed observers to sit in a warm room nearby, controlling the telescope with a computer and snapping digital images with the stroke of a key. As anyone who has visited an observatory for a viewing night knows, the dome is not really an interior space. Temperature gradients and fluctuations can cause lenses to expand and contract, or lead to condensation of water vapor, both of which make for distortions and inaccuracies. As a result, when you’re in the dome looking through the eyepiece, everything needs to be the temperature of the outside air. And since the best observing conditions in New England usually occur in the winter, when the air is drier, that temperature might be pretty cold indeed.
While this photograph was almost certainly posed, given the lighting conditions that would have prevailed during actual nighttime observing, it reveals some of the daily practices that shaped astronomy in the early 20th century. Slocum’s choice to wear his observing gear indicates that, in addition to telescopes and clocks, warm clothing was a crucial part of the astronomer’s toolkit.