In 1978, the U.S. Naval Observatory was making measurements to refine Pluto's orbit. Astronomer James Christy noticed that a number of the Pluto images appeared elongated. These Pluto images show a normal, roughly circular image on the right, and an elongated one on the left. Upon further study, Christy concluded that the only explanation was a previously unknown moon, now called Charon, with an orbital period of just over six days.