Already in the Starry Messenger he had given rough periods of the four moons of Jupiter, but more precise calculations were certainly not easy since it was difficult to identify from an observation which moon was I, which was II, which III, and which IV.
Na sa bituin Messenger niya given magaspang panahon ng apat na Buwan ng Jupiter, ngunit mas tiyak na mga kalkulasyon ay tiyak na hindi madaling dahil ito ay mahirap na makilala mula sa isang observation na buwan ay ko, na kung saan ay II, III na kung saan, at kung saan IV.