Blue Collar Scientist

On this day in 1930, amateur astronomer Clyde Tombaugh discovered the ninth planet of the solar system the Kuiper Belt Object the Trans-Neptunian Object the Scattered Disk Object the dwarf planet Pluto. Tombaugh was employed by Lowell Observatory at the time.

Tombaugh discovered the planet¹ in photographs on this day 78 years ago by comparing photographs taken at different times using an instrument called a blink comparator. The planet appeared to hop against the background stars in this instrument. The photos that Tombaugh evaluated had been taken almost a month previously.

Prints of Pluto’s discovery plates. The starfield is dramatically cropped; the plates covered a large area of sky and consequently there were many more stars to evaluate in the original. Click to enlarge.

After reporting the discovery to Lowell Observatory director Vesto Slipher, additional confirming photographs were taken and measured. The discovery was eventually announced on March 13. After conducting a search of archives, it was learned that Pluto had been photographed at least five times prior to its discovery, with the earliest observation, in 1915, being found in 1976.

A faithful-color image of Pluto’s surface constructed from photometric data from partial eclipses of Pluto by its moon Charon. Image courtesy Eliot Young (SwRI) et al., NASA. Click to enlarge.

Pluto’s largest moon, Charon, was discovered in 1978 - right at the beginning of the time that I began following astronomy news as a kid. Pluto has two other moons, Nix and Hydra, discovered in 2005.

Today, the New Horizons mission is headed for a Pluto flyby in 2015.

So lift a glass for Pluto Day!

1. I’m confused. “Dwarf” is an adjective in this usage, right? So a “dwarf planet” is still a planet, right? Except IAU says it isn’t a planet. Nothing like being unambiguous in your terminology…. [↩]