Galilei, Galileo, Discourse concerning the natation of bodies, 1663

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and future, eſpecially if the time be for many Moneths or Years; I
am therefore forced, with other Obſervations, and more exact than
the former, and in times more remote from one another, to correct
the Tables of ſuch Motions, and limit them even to the ſhorteſt mo­
ment: for ſuch exactneſſe my firſt Obſervations ſuffice not; not only
in regard of the ſhort intervals of Time, but becauſe I had not as then
found out a way to meaſure the diſtances between the ſaid Planets
by any Inſtrument: I Obſerved ſuch Intervals with ſimple relation
to the Diameter of the Body of Jupiter; taken, as we have ſaid, by
the eye, the which, though they admit not errors of above a Minute,
yet they ſuffice not for the determination of the exact greatneſs of the
Spheres of thoſe Stars.
But now that I have hit upon a way of ta­
king ſuch meaſures without failing, ſcarce in a very few Seconds, I will
continue the obſervation to the very occultation of JVPITER,
which ſhall ſerve to bring us to the perfect knowledge of the Moti­
ons, and Magnitudes of the Orbes of the ſaid Planets, together

alſo with ſome other conſequences thence ariſing.
I adde to theſe
things the obſervation of ſome obſcure Spots, which are diſcover­
ed in the Solar Body, which changing, poſition in that, propounds
to our conſideration a great argument either that the Sun revolves in
it ſelfe, or that perhaps other Starts, in like manner as Venus and
Mercury, revolve about it, inviſible in other times, by reaſon of their
ſmall digreſſions, leſſe than that of Mercury, and only viſible when
they interpoſe between the Sun and our eye, or elſe hint the truth
of both this and that; the certainty of which things ought not to be
contemned, nor omitted.
The occaſion in­
ducing the Au­
thor to write
this Treatiſe.

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