On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he only saw three of them, and he believed them to be fixed stars near Jupiter. He continued to look at them from January 8 through March 2. In these observations, he found a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.<ref name="Galilei/Helden, 15-16">Galilei/Helden, 15-16.</ref>