Galileo's observations of Neptune

Abstract

The planet Neptune was discovered in 1846. As its period of revolution is almost 165 years, Neptune has not yet completed one revolution since its discovery. Largely as a result of this, its orbit is not known with a precision comparable to that of the inner planets. A pre-discovery measurement of Neptune's position by Lalande in 1795 differs from the predicted position by 7 arc s. There is some debate about whether this discrepancy is real or an error of measurement¹. Clearly, it would be worthwhile to find other pre-discovery observations of this planet. One possible way of finding such observations is to search for close approaches of Neptune to other objects which were frequently observed. Neptune was actually occulted by Jupiter in January 1613 and September 1702 (ref. 2). By 1702 the telescope was in widespread use, and examination of manuscripts of that period should reveal cases where Neptune was seen near Jupiter and mistaken for a star. The abundance of possible material, however, makes a search for such observations lengthy. We have found that Galileo observed the planet Neptune on 28 December 1612 and 28 January 1613. The latter observation may be of astrometric value, and differs by 1 arc min from the predicted position of Neptune. Galileo also detected the motion of Neptune.