After discovering Jupiter's moons, Galileo tracked their movements for several days. To measure with precision the distance of each satellite from the planet, Galileo designed a device known as a micrometer.
Giovanni Alfonso Borelli described the micrometer as a rule with twenty equal divisions. The device was fitted on the telescope and could slide along the body tube.
Galileo observed Jupiter's system through the telescope with one eye, while his other eye watched the micrometer lit by a lantern. He then set the micrometer distance so as to make the interval between two divisions of the graduated scale coincide with the planet's apparent diameter.
This procedure enabled Galileo to superpose the telescope's field of view on the micrometer. He could thus determine the distance of each satellite from the planet, with the radius of Jupiter as the unit of measurement.