from Jupiter and
from one another. On the 8th of January, however, when, from some cause
or other,[12] he had been led to observe the stars again, he found a
very different arrangement of them: all the three were on the west side
of Jupiter, _nearer one another than before_, and almost at equal
distances. Though he had not turned his attention to the extraordinary
fact of the mutual approach of the stars, yet he began to consider how
Jupiter could be found to the east of the three stars, when but the day
before he had been to the west of two of them. The only explanation
which he could give of this fact was, that the motion of Jupiter was
_direct_, contrary to astronomical calculations, and that he had got
before these two stars by his own motion.

[12] Nescio quo fato ductus.

In this dilemma between the testimony of his senses and the results of
calculation, he waited for the following night with the utmost anxiety;
but his hopes were disappointed, for the heavens were wholly veiled in
clouds. On the 10th, two only of the stars appeared, and both on the
east of the planet. As it was obviously impossible that Jupiter could
have advanced from west to east on the 8th of January, and from east to
west on the 10th, Galileo was forced to conclude that the phenomenon
which he had observed arose from the motion of the stars, and he set
himself to observe diligently their change of place. On the 11th, there
were still only two stars, and both to the east of Jupiter; but the more
eastern star was now _twice as large as the other one_, though on the
preceding night they had been perfectly equal. This fact threw a new
light upon Galileo's difficulties, and he immediately drew the
conclusion, which he considered to be indubitable, "_that there were in
the heavens three stars which revolved round Jupiter, in the same manner
as Venus and Mercury revolve round the sun_." On the 12th of January, he
again observed them in new positions, and of different magnitudes; and,
on the 13th, he discovered a fourth star, which completed the _four_
secondary planets with which Jupiter is surrounded.

Galileo continued his observations on these bodies every clear night
till the 22d of March, and studied their motions in reference to fixed
stars that were at the same time within the field of his telescope.
Having thus clearly established that the four new stars were satellites
or moons, which revolved round Jupiter in the same manner as the