o understand how Jupiter
could be east of the three stars, when on the preceding night he was
observed to the west of two of them. Galileo was unable to reconcile the
altered positions of those bodies with the apparent motion of Jupiter
among the fixed stars as indicated by the astronomical tables. The next
opportunity he had of observing them was on the 10th, when two stars
only were visible, and they were to the east of the planet. As it was
impossible for Jupiter to move from west to east on January 8 and from
east to west on the 10th, he concluded that it was the motion of the
stars and not that of Jupiter which accounted for the observed
phenomena. Galileo watched the stars attentively on successive evenings
and discovered a fourth, and on observing how they changed their
positions relatively to each other he soon arrived at the conclusion
that the stars were four moons which revolved round Jupiter after the
manner in which the Moon revolves round the Earth. Having assured
himself that the four new stars were four moons that with periodical
regularity circled round the great planet, Galileo named them the
Medicean Stars in honour of his patron, Cosmo de' Medici, Grand Duke of
Tuscany. He also published an essay entitled 'Nuncius Sidereus,' or the
'Sidereal Messenger,' which contained an account of this important
discovery.

The announcement of Galileo's discovery of the four satellites of
Jupiter created a profound sensation, and its significance became at
once apparent. Aristotelians and Ptolemaists received the information
with much disfavour and incredulity, and many persons positively refused
to believe Galileo, whom they accused of inventing fables. On the other
hand, the upholders of the Copernican theory hailed it with
satisfaction, as it declared that Jupiter with his four moons
constituted a system of greater magnitude and importance than that of
our globe with her single satellite, and that consequently the Earth
could not be regarded as the centre of the universe.

When Kepler heard of this remarkable discovery, he wrote to Galileo and
expressed himself in the following characteristic manner: 'I was sitting
idle at home thinking of you, most excellent Galileo, and your letters,
when the news was brought me of the discovery of four planets by the
help of the double eye-glass. Wachenfels stopped his carriage at my door
to tell me, when such a fit of wonder seized me at a report which seemed
so very absur