The micrometer and Jupiter's satellites
Opera Laboratori Fiorentini

Galileo's "micrometer" seems to have consisted of a ruler divided into 20 equal parts, which slid along the telescope tube. During observations, the micrometer was fixed at a distance from the eye at which the interval between two divisions coincided with the diameter of the planet observed through the telescope. When one eye was applied to the telescope while looking at the micrometer with the other, the two fields of view overlapped. In this way Galileo could measure the distance of each satellite from Jupiter expressed in units equal to the planet's radius.