View from Mars Hill: The day Lowell Observatory's Clyde Tombaugh discovered Pluto

By February 1930, 24-year-old Clyde Tombaugh had been working at Lowell Observatory for several months. To earn his monthly salary of $125 plus living quarters on the second story of the Observatory’s administration building (known today as the Slipher Building), Tombaugh shoveled coal and wood into the building’s furnace, cleared snow off telescope domes, and presented the daily afternoon tours to visitors. He also ran the observatory’s search for a theoretical ninth planet, an effort that would lead to worldwide fame for the man and his observatory.

On clear, moonless nights, Tombaugh made photographic plates of selected areas of the night sky using a 13-inch astrograph, a type of specialized telescope used for taking pictures—essentially a large camera. This required long, tedious hours in which overcoming the boredom of guiding the astrograph played as big a role in successfully capturing images as mastering the technical aspects of the operation. Tombaugh wrote to his parents, “My consolation used to be: ‘The longest night has an end’ but now it is: ‘The longest exposure has an end,’ when one is guiding in a cold, dark dome, and one’s mind craves for something to occupy its attention, because guiding becomes rather a habit performed subconsciously.”

The plates recorded an area of the sky about the size of a fist held at arm’s length, and with a one-hour exposure, captured an average of about 300,000 star images. During the day, Tombaugh examined pairs of these plates that recorded the same portion of the sky but taken a few days apart. His job was to use a machine called a blink comparator to look at every star-like image from one plate to the other and see if it changed position. Most did not, meaning they were stars, but those that did represented a possible planet that changed position relative to the background stars.

If guiding the telescope was tedious, staring at hundreds of thousands of dots through the eyepiece of the blink comparator for hours on end was nothing short of mind-numbing. Yet Tombaugh worked hard to overcome these mental challenges, as well as ongoing technical difficulties that required attention. The only thing he really couldn’t control, as astronomers before and since have also lamented, was the weather. But his days as a farmer prepared him for this and allowed him to draw parallels between astronomy and farming. He wrote to his mother, “Both farming and star-searching depend on the weather, but the difference is that: 1. In farming you get the work alright, but may lose your pay. 2. In star-searching, you get your pay, but may lose your work.”

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