Bridges (1995) described how a modern computer spreadsheet could be used to deduce orbital parameters for the planets from binocular observations of their positions amongst the stars. The present paper describes how a spreadsheet may be used to fit a circular orbit model to observations of Jupiter's moons made with a small telescope. Kepler's third law, and by implication the inverse square law of gravity, may be observed in action in this "mini solar system", and the mass of Jupiter may be deduced as a fraction of the mass of the Sun.