Robert McNees
Clyde Tombaugh's discovery of Pluto was announced #OTD in 1930. Here it is dashing across his photographic plates, alongside an image taken just after closest approach by NASA's New Horizons probe in July 2015.
Images: Lowell Observatory/National Geographic (July 1939); NASA/JHUAPL/SwRI
Robert McNees
Here’s a gif version of the blink comparator used by Tombaugh to spot Pluto.
Images: Lowell Observator
Gif: @tomruen
Robert McNees
The announcement was made on that date for two reasons.
First, to honor Percival Lowell, who was born #OTD in 1855. Lowell first hypothesized the existence of Pluto to explain irregularities in the orbit of Uranus.
And second, to commemorate the discovery of Uranus, smooth as an egg and monstrously featureless to the human eye, by William Herschel #OTD in 1781.
Uranus Image: NASA/JPL
Robert McNees
@alec Great question. I don't know for sure, but I would guess it is an asteroid or minor planet. I suppose one could use modern planetarium software to plot the sky on those dates (January 23 and 29, 1930) to see what else was in that patch of sky!
Alec Perkins
@mcnees what is the second arrow?
Robert McNees
Tombaugh made his discovery at Lowell Observatory.
It was Lowell who first hypothesized the existence of Pluto to explain irregularities in the orbit of Uranus that couldn't be explained by Neptunian perturbations. He searched for the planet from 1905 to 1914, but never found it.
Here's where the story gets a little weird!
Robert McNees
Lowell observatory was funded by Percival's wealthy family, but *not* to hunt for Pluto.
The observatory was set up in 1894 so Lowell could prove his theories about the existence of canals on Mars built by an alien civilization.
Image: Wellcome Trust
Robert McNees
The caption in that last image reads:
"Discoverer of Most of the Six-Hundred-And-Ninety Canals of Mars: Professor Percival Lowell
Professor Lowell, the famous American astronomer... firmly believes that organic life exists on Mars and has advanced many striking proofs in support of his assertion."
Robert McNees
Anyway, Lowell passed away in 1916, so he didn't live to see Tombaugh's discovery of the planet he predicted at the observatory he founded. He didn't live to see Martians, either.
Here's Tombaugh at the blink comparator, and his 13-inch telescope.
Images: Lowell Observatory
Robert McNees
(Pluto's official discovery was in 1930, but it was actually spotted eleven years earlier in 1919. Milton Humason captured it on four photographic plates while searching for trans-Neptunian planets, but didn't recognize it for what it was!
https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1931ApJ....73....1N&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf )
Robert McNees
Now maybe you've heard most that stuff before. But here is something you may not know.
Many of the calculations that Lowell used in his hunt for a trans-Neptunian planet were made by a woman named Elizabeth Langdon Williams. She was an MIT grad who worked for Lowell at the observatory.
Image: Arizona Memory Project
Robert McNees
Elizabeth Langdon Williams's calculations led to predictions for the location of the mysterious "Planet X." Here is a scan of some of her work.
I don't know whether or not any of her predictions were accurate, or investigated by others. Unfortunately, Constance Lowell (Percival's widow) fired her in 1920. Elizabeth Langdon Williams had married another astronomer, and Constance felt that it would be inappropriate to employ a married woman.
Image: Arizona Memory Project
Robert McNees
We finally reached Pluto in 2015 when the New Horizons probe sailed past, carrying some of Clyde Tombaugh's ashes.
The little space robot sent back images of a world unlike anything Lowell might have imagined.
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/ Alex Parker (@ahp)
Robert McNees
As it flew off, New Horizons caught this image of the sun illuminating Pluto's thin atmosphere. The blue tint is due to scattering of sunlight by particles called "tholins.”
https://solarsystem.nasa.gov/resources/637/blue-rays-new-horizons-high-res-farewell-to-pluto/
Image: NASA / Johns Hopkins University Applied Physics Laboratory / SwRI / Alex Parker (@ahp)
Jason Bowen
@mcnees @ahp I didn't know that New Horizons was carrying some of Clyde's ashes. That almost brings a tear to my eye.
Robert McNees
@jbowen Yeah, an ounce of his remains is on board. Here's a little thread about their whereabouts.
simonbp
@mcnees Using the Millionaire mechanical calculator, now on display because Lowell never throws anything out (great for history, bad for finding storage space)
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