HISTORY OF PHYSICAL ASTRONOMY.

79

planet; it was then exceedingly small, and was discernible only withgreat difficulty. On the 13th he finally saw four stars. Three of them

were on the west side of the planet, and the remaining one on the eastside. They were all arranged in a line parallel to the ecliptic, with theexception of the central star of the three western ones, which declined alittle towards the north. They appeared of the same magnitude, and,though small, were very brilliant, shining with a much greater lustre thanfixed stars of the same magnitude *.

The future observations of Galileo established beyond all doubt thatJupiter was attended by four satellites. He continued to examine themuntil the latter end of March, noting their configurations, and recordingthe stars which appeared in the same field of view with them.

Soon after Galileo’s famous discovery, he perceived the utility ofthe satellites for finding the longitude, and he continued for manyyears to make observations on them, with the view of constructing atheory of their motions. Much has been said about his tables of thesatellites, which were to have been published by his friend and pupil Ri-mieri, but which, by some unaccountable accident, disappeared at the death ofthat person, and could nowhere he found, until they were finally discovereda few years ago in a private.library at Rome . We know that Galileo him-self was very sanguine of their practical utility, but his opinion of theirmerits does not seem to he borne out by the actual examination of themconsequent on their rediscovery. Indeed, when we reflect on the manypainful efforts which it cost his successors to arrive at even a tolerableknowledge of the elements of the satellites, we might very reasonablyconclude, a priori, that his tables can only be regarded in the present dayas an object of scientific curiosity. An interesting fragment of his earlyresearches on the satellites is to be found in one of his letters to Welser,the person through whom he carried on the controversy with Schener theJesuit , relative to the discovery of the solar spots. At the end of a letterdated December 1st, 1612, he gives a sketch in rough drawings of theconfigurations of the satellites from 1st March till 7tli May of the following year.

Simon Mayer, the German astronomer, who contended for the inde-pendent discovery of the satellites, resolved to strengthen his claims bythe construction of tables of their motions. The crude labours of tliis im-pudent pretender were, however, no sooner given to the world than theyfell into deserved oblivion. Hodierna, a Sicilian astronomer, is the nextperson wh@ is mentioned as having devoted his attention to this subject.In 1656 he published his observations on the satellites, accompaniedwith remarks on the theory of their motions. He is the first astronomerwho pointed out the superior importance of eclipses of the satellites ascompared with other phenomena. He also calculated tables of theirmotions, but they are said to have been so very inaccurate, that in a few ryears they even ceased to represent the configurations of the differentbodies. In 1666 Borelli attempted to establish a theory of the satellites,

* The preceding configurations are derived from those given by the illustrious dis-coverer in the Sidereus Nuncius .