June 22 – A Moon for Pluto!

Posted on June 22, 2015

What was up with Pluto?

That's what U.S. Naval Observatory astronomer James Christy was wondering as he studied Pluto through a telescope. It looked...bulgy!

Left, Pluto bulgy.
Right, Pluto round
and non-bulgy.
As Christy continued to make observations, he realized that the bulginess was different at different times. It didn't make sense that Pluto itself would be have a bulge that moved around its middle and sometimes disappeared. It made more sense that Pluto had a companion whose reflected sunlight seemed to merge with the sunlight reflecting from Pluto.

Christy made this discovery on this date in 1978. The discovery was announced to the world in early July.

Actually, others had a chance to make the discovery. Christy wanted to check his observation, and he was able to discover the periodic Pluto-bulges in photos taken of the planetoid as far back as 1965. For more than a decade, nobody had noticed the teeny elongation.

Pluto is tiny, but Charon is large.

You've probably heard that Pluto is so teeny that it lost its former label of planet and is now considered a dwarf planet, planetoid, or Plutoid. As a matter of fact, Pluto is the second largest of the Plutoids in our solar system (only Ceres is larger).

But Charon is a bit more than half the size of Pluto. (Compare this to Earth's moon, which is only a tiny fraction of the size of the Earth.) Charon isn't all that large compared to other solar system moons (even Pluto is smaller than Earth's Moon, let alone Charon!) - but the two are much closer in size than are other planet-satellite comparisons.

What's in a name?

Ancient peoples had various names for the five planets that they could see with their naked eyes, but European scientists ended up using the names used by Ancient Roman astronomers. Those names were the names of their gods and goddesses: Venus, Mars, Mercury, Jupiter, and Saturn. When more planets were discovered in the modern era, scientists decided to maintain the custom of naming the planets after Roman deities. That's how we ended up with Uranus, Neptune, and Pluto.

Pluto is the Roman name for the Roman god of the underworld, the god who the Greeks called Hades.

One of the beings associated with Pluto-the-Roman-god is Charon. Charon is the ferryman who transported the dead to the underworld. Coincidentally, the guy who discovered Pluto's moon—and therefore got to name it—had a wife named Charlene. He called her “Char.” Christy loved the nod to his wife while sticking to Roman mythological sources for the name of his discovery.

But the Greeks pronounced “Ch” as a hard “K” – and so the name of the Roman ferryman was pronounced Karon. And Christy's wife's name had “Ch” pronounced like “Sh” – “Shar” for “Sharlene.” This explains why many speakers of other languages, and also many English-speaking astronomers, call Pluto's moon “Karon,” but some English-speaking astronomers, the discoverer himself, and NASA all pronounce the name “Sharon.”

Actually, the two different ways of pronouncing the name has led to the “Sh” version being a shibboleth of sorts. A shibboleth is a version or pronunciation of a word that is used by an in-group, the people “in the know.”

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