The sidereal messenger of Galileo Galilei : $b and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveriesGalilei, Galileo
History
The sidereal messenger of Galileo Galilei : $b and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries
Galilei, Galileo
Astronomy -- Early works to 1800; Jupiter (Planet) -- Satellites
“Cioè, che Venere imita le figure della Luna. Osservai tre notti
sono l’ecclisse, nella quale non vi è cosa notabile, solo si vede
il taglio dell’ombra indistinto, confuso e come annebbiato,
e questo per derivare essa ombra dalla Terra lontanissima da
essa Luna. Voleva scrivere altri particolari, ma essendo stato
trattenuto molto da alcuni gentiluomini, ed essendo l’ora
tardissima, son forzato a finire. Favoriscami salutare in mio nome
i SS. Keplero, Asdale e Segheti, ed a Vostra Signoria Illustrissima
con ogni reverenza bacio le mani, e dal Signore Dio gli prego
felicità. Di Firenze il primo di Gennaio 1610. Ab Incarnatione.
“Di Vostra Signoria Illustrissima e Reverendissima Servidore
obbligatissimo.
Galileo Galilei.”
Such is Galileo’s letter; but let me give you the substance of it:—
“It is time for me to disclose the method of reading the letters
which some weeks since I sent you as an anagram. It is time now, I
mean, after I have become quite certain about the matter, so much
so that I have no longer even a shadow of doubt. You must know then
that about three months ago, when the star of Venus could be seen,
I began to look at it through a telescope with great attention,
so that I might grasp with my physical senses an idea which I was
entertaining as certain. At first then you must know the planet
Venus appeared of a perfectly circular form, accurately so, and
bounded by a distinct edge, but very small; this figure Venus kept
until it began to approach its greatest distance from the sun,
and meanwhile the apparent size of its orb kept on increasing.
From that time it began to lose its roundness on the eastern side,
which was turned away from the sun, and in a few days it contracted
its visible portion into an exact semicircle; that figure lasted
without the smallest alteration until it began to return towards
the sun where it leaves the tangent drawn to its epicycle.[27]
At this time it loses the semicircular form more and more, and
keeps on diminishing that figure until its conjunction, when it
will wane to a very thin crescent. After completing its passage
past the sun, it will appear to us, at its appearance as a morning
star, as only sickle-shaped, turning a very thin crescent away from
the sun; afterwards the crescent will fill up more and more until
the planet reaches its greatest distance from the sun, in which
position it will appear semicircular, and that figure will last
for many days without appreciable variation. Then by degrees, from
being semicircular it will change to a full orb, and will keep that
perfectly circular figure for several months; but at this instant
the diameter of the orb of Venus is about five times as large as
that which it showed at its first appearance as an evening star.
Public-domain text, read in full here on John Shaqi.
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