The existence of Planet X was announced to the world on March 13, 1930, but this was not the day it was discovered.
The events leading up to Pluto’s discovery are documented in the book, Out of the Darkness, The Planet Pluto by Clyde W. Tombaugh and Patrick Moore.
Tombaugh wrote: ‘In the January (1930) lunation, I re-photographed the entire Gemini region.
On 21 January 1930, I set the l clinch telescope on Delta Geminorum again.
A good night seemed in prospect...but a northeast wind sprang up.
In another ten minutes, it was a howling gale.
With succeeding gusts, the image would swell up so badly that it became invisible.
The plate was spoiled…but I decided to finish out the exposure just to see how bad the images would be after I developed the plate.
After developing the plate the next day…the images were swollen to several times their normal diameter.
Nevertheless this horrible plate did record the image of Pluto, but I did not know it then.
Some astronomical textbooks state that Pluto was discovered on 21 January 1930.
Only on the date that the images are recognized as those of a planet does it constitute a discovery.
On January 23 I photographed the Delta Gem region again. It was a good plate. I was unable to photograph the region again until 29 January.
On the morning of 18 February, I placed the 23 January and 29 January Gem plates on the Blink-Comparator.
By four o’clock that afternoon, Mountain Standard Time, l had covered one-fourth of the pair.
Then after scanning a few fields to the left, I turned the next field into view.
Suddenly I spied a fifteenth magnitude image popping out and disappearing in the rapidly alternating views.
Then l spied another image doing the same thing about 13 millimeters (or .1 25 inches) to the left.
“That’s it”, I exclaimed to myself.
A terrific thrill came over me. I switched the shutter back and forth studying the images.
Oh! I had better look at my watch and note the time. This would be a historic discovery.
Estimating my delay at about 3 minutes, it would place the moment of discovery very close to four o’clock.’
Clyde Tombaugh announced the discovery of Planet X on March 13, 1930.
This date was chosen to coincide with the 149th anniversary of the discovery of Uranus by William Hershel and the 75th birthday of Percival Lowell.
It caused enormous excitement in the astrological community.
Neptune had been discovered 84 years earlier in 1846 and the discovery of another planet was always going to be big news.
Astrologers immediately began to speculate on its astrological influence, but there were so many unknowns.
Where was Planet X located in the zodiac?
On March 13, 1930 it was thought to be at 17 degrees Cancer 34 minutes but it could not be inserted into a birth chart until the astronomers produced and published an ephemerides.
Then its progressed aspects could be investigated to discover its influence on life.
What was its psychological correlate?
And if Uranus had ushered in the machine age and the age of republics and Neptune the age of oil and gas what age would Planet X usher in?
And amidst all the excitement there were those who were aware of the words that Thomas Burgoyne had written 41 years earlier.
On May 24, 1930 Planet X was officially named Pluto.
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