Charon was discovered on June 22, 1978, by James Christy

Pluto’s largest moon, Charon, was discovered on June 22, 1978, by U.S. Naval Observatory astronomer James Christy, who noticed a small bulge on photographs of Pluto taken with the 61-inch telescope at the observatory’s Flagstaff Station in Arizona.

Christy was not looking for a moon. He was trying to refine Pluto’s orbit when the bulge kept appearing, then vanishing. Working with colleague Robert Harrington, he checked archived images back to 1965 and found the bulge recurring on a regular cycle that matched Pluto’s rotation. After ruling out other causes, they concluded that Pluto had a companion orbiting about 12,200 miles away, and the discovery was announced on July 7, 1978.

6.39 hours or 6.39 days?

NASA’s own anniversary history of the discovery says the bulge matched a Pluto rotation of “6.39 hours”. That is a slip. The Smithsonian’s National Air and Space Museum says the blob reappeared every 6.39 days, and NASA’s Pluto fact sheet gives Charon’s orbit as about 153 hours, the same as Pluto’s day. The two bodies are locked face to face, so Charon hangs over one spot in Pluto’s sky.

Proof that the moon was real came later. From 1985, Pluto and Charon began passing in front of each other as seen from Earth, and Harrington watched the eclipses happen as predicted. In 1994 Hubble’s Faint Object Camera took the first image showing Charon as a separate object.

The bigger payoff was a scale. A moon’s orbit reveals the mass of the body it circles, and Charon showed that Pluto has less than 0.25% of Earth’s mass. The orbital irregularities behind Lowell’s search would have needed a body of at least roughly Earth’s mass. At about 753 miles across, Charon is half Pluto’s width, the largest moon relative to its planet in the Solar System. Christy named it after the mythical ferryman of the dead, and for his wife Charlene, known as “Char”.