It is known that early (i.e. 17th-l8th century) visual observations of Jupiter's Galilean satellites are approximately as accurate as modern visually observed eclipses (Lieske 1982). In the early days the clocks were the primary source of error while in present days problems related to the Earth's and Jupiter's atmospheres have become the primary limiting factor. The early observations were generally made in local apparent time (i.e. related to the real hour angle of the sun). After the development by Huygens in 1656 of the pendulum clock, these early observations, when reduced to a modern UT system, have been shown to be quite accurate.