February 18 – Pluto Day

Posted on February 18, 2014

Quick! Let's quickly celebrate Pluto, today, on the anniversary of its 1930 discovery by Clyde Tombaugh, before we get to the silliness of New Mexico's new holiday, on March 13: Pluto is a Planet Day. (That's awfully reminiscent of Pluto Demoted Day, which I wrote about here.)

Pluto was once considered a planet, because we didn't know about all the other objects in the solar system that are about as large as Pluto, and that also orbit around the sun far from the gas giants. Because we have discovered so many of these objects (at least one of which is larger than Pluto, and several of which, like Pluto, are accompanied by moons), the International Astronomical Union decided by vote to relabel Pluto as a [cue dramatic horror-movie music] dwarf planet.

Which is so not a big deal, right? Yet some people still get upset about this so-called demotion...

Other labels for Pluto are Plutoid and Kuiper Belt Object. Basically, Plutoids are dwarf planets that lie beyond (or mostly beyond) the gas giants (as opposed to the only dwarf planet in the asteroid belt, Ceres). Plutoids are the Kuiper Belt Objects that are large enough to be near-spherical—most KBOs are shaped more like potatoes or other knobbly shapes.

Google says about Pluto:
  • Pluto is the largest known object in the Kuiper Belt...
  • ...the tenth largest body observed directly orbiting the Sun (as opposed to orbiting a planet)...
  • ...and the second largest known dwarf planet...

Okay, now back to Pluto's discovery:

Once upon a time there was a wealthy man who loved astronomy. His name was Percival Lowell. He had heard that scientists had carefully observed Neptune's orbit around the Sun and had decided that it was possible that yet another planet, even farther away than Neptune, was also disturbing Uranus's orbit. Scientists speculated that there might be a ninth planet circling the Sun.

Lowell wanted to discover that possible ninth planet, which he called Planet X. He founded the Lowell Observatory in Flagstaff, Arizona, and he started a project in search of Planet X. From 1906 to Lowell's death in 1916, they searched but didn't find a ninth planet.

When Lowell died, he left a lot of money to his wife Constance (of course), but he also left a million dollars to “his” observatory to keep up the search for Planet X.

But Constance didn't care a whit about Planet X. She wanted the million dollars for herself! So she contested the will and dribbled away most of the money in court fees. She finally (ten years later!) lost the case, and the director of the observatory asked a young astronomer named Clyde Tombaugh to resume the search for the ninth planet.

Tombaugh was just 23 years old at the time.

Tombaugh used a machine called a blink comparator. He quickly shifted back and forth between two photos of the night skies that were taken of the same spot in the sky but two weeks apart. What he was looking for was something that had changed position or appearance in those two weeks.

Finally, after more than a year of blink-comparator searching, Tombaugh discovered a teeny light that had, as expected moved. More searching of other photographic plates revealed that the new thing did indeed move as a planet should. The news shot around the world: a ninth planet had been discovered!

By the way, loads of suggestions for the new name poured in; even Constance Lowell made suggestions! Even though she had almost single-handedly wrecked the planet-finding project by taking the Lowell Observatory to court, she asked that the new planet be called Zeus (even though Jupiter is named after that same god—the planets use the Roman, not the Greek, names for the ancient gods and goddesses), and then she suggested Percival, and then she suggested – wait for it! – Constance!


Luckily, the observatory director ignored her suggestions and chose a Roman god who had not yet been assigned a planet: Pluto, god of the underworld, the Roman name for the Greek god Hades.

Also on this date:

Stained-glass artist Louis Comfort Tiffany's birthday

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