Galilei, Galileo - Discourse concerning the natation of bodies

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 Authors
Obſervations of
the Solar Spots.

Continuall obſervation hath at laſt aſſured me that theſe Spots are
matters contiguous to the Body of the Sun, there continually produced
in great number, and afterwards diſſolved, ſome in a ſhorter, ſome in a
longer time, and to be by the Converſion or Revolution of the Sun in it
ſelfe, which in a Lunar Moneth, or thereabouts, finiſheth its Period,
caried about in a Circle, an accident great of it ſelfe, and greater for
its Conſequences.

The occaſion in
ducing the Au
thor to write
this Treatiſe.

As to the other particular in the next place. ^{*} Many cauſes have
moved me to write the preſent Tract, the ſubject whereof, is the
Diſpute which I held ſome dayes ſince, with ſome learned men of
this City, about which, as your Highneſſe knows, have followed
many Diſcourſes: The principall of which Cauſes hath been the
Intimation of your Highneſſe, having commended to me Writing,
as a ſingular means to make true known from falſe, reall from appa
rent Reaſons, farr better than by Diſputing vocally, where the
one or the other, or very often both the Diſputants, through too

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