Astronomy Tonight for - 06-20-2025

On June 20th in the realm of astronomy, one of the most exciting events occurred back in 1944 with the discovery of Pluto's moon, Charon. This discovery was made by James Christy at the United States Naval Observatory in Flagstaff, Arizona. Christy was examining photographic plates of Pluto when he noticed a peculiar bulge on its image. At first, he thought it might be a defect in the plate or an anomaly in Pluto's shape. However, upon closer inspection and comparison with other images, he realized this bulge was actually a separate object orbiting Pluto! The discovery of Charon was a big deal in the astronomical community. It was the first time a moon of Pluto had been observed, and it dramatically changed our understanding of the Pluto system. Charon is unusually large compared to Pluto - about half its diameter and one-eighth its mass - leading some astronomers to refer to Pluto and Charon as a "double planet" system. Christy named the moon after his wife, Charlene, nicknamed "Char." Coincidentally, in Greek mythology, Charon was the ferryman of the dead, which fits nicely with Pluto being named after the god of the underworld. The discovery of Charon led to a more accurate measurement of Pluto's mass, which turned out to be much less than previously thought. This revelation played a part in the later decision to reclassify Pluto as a dwarf planet in 2006. So, on this day, we can raise a toast to James Christy and his sharp eyes, which spotted a tiny moon nearly 4 billion miles away from Earth. Who knows what other celestial secrets are waiting to be uncovered on future June 20ths? Perhaps by 2025, we'll have discovered a moon orbiting a planet in another solar system on this very date! The cosmos never ceases to amaze us.