ANU Hidden Gems: the calculator that helped discover Pluto

5 November 2018

How is it that an instrument used in the discovery of Pluto ended up halfway around the world in another country, a quarter of a century later, where it resides at Siding Spring Observatory?

On 18 February 1930, American astronomer Clyde Tombaugh made a discovery that would broaden our understanding of our solar system.

Using a device known as a Zeiss Blink Comparator at the Lowell Observatory in Flagstaff, Arizona, he compared the positions of specks of light recorded on photograph plates taken just weeks earlier.

Of the hundreds of specks, all remained stationary as the plates were sequentially "blinked" - except for one. One miniscule speck of light appeared to jump back and forth as Tombaugh watched. Little did he know, but 23-year-old Tombaugh had discovered the celestial body that would bear the name Pluto.

This set off a flurry of activity from astronomers around the world trying to determine Pluto's orbit, including astronomers Seth Nicholson and Nicholas Mayall at Mount Wilson Observatory in California. They used a Marchant Model H9 calculating machine to measure the orbit of Pluto, which they then published about in January 1931 entitled "Positions, Orbit, and Mass of Pluto". 

In 1976, 24 years after another American astronomer named Tom Cragg was hired at Mount Wilson, he was appointed Chief Night Assistant at the ANU Siding Spring Observatory where he looked after the recently opened 3.9 metre Anglo-Australian-Telescope.

Shortly after arriving in Coonabarabran, Tom was unpacking his possessions when he discovered that the removalists at Mount Wilson had accidentally shipped to Australia the machine that had computed Pluto's orbit. Tom informed Mount Wilson of the mistake and suggested that he ship it back - Mount Wilson said, "Don't bother, it's too out of date to use, you can throw it away".

Fortunately, Tom had an appreciation of the historical significance of the old Marchant H9 Calculating Machine and safely stored it away. It is now on display at Siding Spring Observatory.

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