make a Mercury’s magic-wand to cross the liquid ether with, and, like
Lucian,[28] lead a colony to the uninhabited evening star, allured by
the sweetness of the place? or shall we make it a Cupid’s arrow, which,
entering by our eyes, has pierced our inmost mind, and fired us with
a love of Venus? For what language is too strong for the marvellous
beauty of this orb, if, having no light of its own, it can attain
simply by the borrowed light of the sun to such splendour, as Jupiter
has not, nor the moon, though enjoying a proximity to the sun as close
as Venus; for if the moon’s light be compared with the light of Venus,
it will be seen to be certainly greater on account of the apparent
magnitude of the moon, but, in comparison with the light of Venus,
dull, dead, and leaden. O truly golden Venus! Will any one doubt any
more that the whole orb of Venus is wrought most smoothly out of pure
unalloyed gold, since its surface, when only placed in the sunlight,
reflects a splendour so intense! And here let me add my experiments
about the alteration of the light of Venus on blinking the eye, which
I have examined in the part of my Astronomy which treats of Optics.
Reasoning will be able to conclude nothing else but this, that the orb
of Venus turns on its own axis with an exceedingly swift rotation,
displaying one after another different parts of its surface which are
more or less capable of retaining the sun’s light.[29]
[28] Lucian, _Ver. Hist._ i. 12.
[29] The first scientific determination of the period of the
rotation of Venus was made by Dominique Cassini in 1666, from
observations of spots on the planet, and concluded to be about 24
hours; but in 1726 Bianchini deduced a period of 24 d. 8 h. from
similar observations. The true period is considered to be 23 h. 21
m., determined by Schroeter by a series of observations lasting
from 1788 to 1793 on the periodicity of the deformation of the
horns of Venus.—(ARAGO, _Astronomie Populaire_, 1854.)
Kepler’s statements can only be regarded as anticipations of
phenomena not yet actually observed.
But enough of my own conclusions. Let us now hear as an epilogue
Galileo’s conclusions built up out of all the observations which he
has made with his telescope, and announced from time to time. Thus he
writes once more:—
[Sidenote: Galileo’s conclusions with regard to the inherent nature of
the brightness of the stars.]
“Illustrissimo e Reverendissimo Signore mio colendissimo.
“Ho ricevuto gusto, e contento particolarissimo nella lettura
dell’ ultima di Vostra Signoria Illustrissima e Reverendissima
delli 7 stante, ed in particolare in quella parte dove ella
m’accenna la favorevole inclinazione dell’ Illustriss. Sig.
Cons. Wackher, verso di me, la quale io infinitamente stimo,
ed apprezzo; e poichè quella ha principalmente origine dall’
aver io incontrate osservazioni necessariamente dimostranti
conclusioni per avanti tenute vere da sua Signoria Illustrissima
per confermarmi maggiormente il possesso di grazia tanto pregiata
da me, prego Vostra Signoria Illustrissima e Reverendissima a
fargli intendere per mia parte come conforme alla credenza di
Sua Signoria Illustrissima ho dimostrazione certa, che siccome
tutti i Pianeti ricevono il lume dal Sole, essendo per se stessi
tenebrosi, e opachi; così le Stelle fisse risplendono per lor
natura, non bisognose dell’ illustrazione de’ raggi solari, li
quali, Dio sa, se arrivino a tanta altezza, più di quello, che
arrivi a noi il lume di una di esse fisse. Il principal fondamento
del mio discorso è nell’ osservare io molto evidentemente con
gli occhiali che quei Pianeti di mano in mano, che si trovano
più vicini a noi, o al Sole, ricevono maggiore splendore, e più
illustremente ce lo riverberano; e perciò Marte perigeo, e a noi
vicinissimo si vede assai più splendido, che Giove; benchè a
quello di mole assai inferiore; e difficilmente se gli può coll’
occhiale levare quella irradiazione, che impedisce il vedere il suo
disco terminato, e rotondo; il che in Giove non accade, vedendosi
esquisitamente circolare. Saturno poi per la sua gran lontananza
si vede esattamente terminato, sì la Stella maggiore di mezzo,
come le due piccole laterali; ed appare il suo lume languido, ed
abbacinato e senza niuna irradiazione, che impedisca il distinguere
i suoi tre piccoli globi terminatissimi. Ora poichè apertamente
veggiamo, che il Sole molto splendidamente illustra Marte vicino,
e che molto più languido è il lume di Giove (sebbene senza lo
strumento appare assai chiaro, il che accade per la grandezza, e
candore della Stella) languidissimo, e fosco quello di Saturno,
come molto più lontano, quali doveriano apparirci le Stelle
fisse lontane indicibilmente più di Saturno, quando il lume loro
derivasse dal Sole? Certamente debolissime, torbide e smorte. Ma
tutto l’opposito si vede, perocchè se rimireremo per esempio il
Cane, incontreremo un fulgore vivissimo, che quasi ci toglie la
vista, con una vibrazione di raggi tanto fiera, e possente, che
in comparazione di quello rimangono i Pianeti, e dico Giove e
Venere stessa, come un impurissimo vetro appresso un limpidissimo
e finissimo diamante. E benchè il disco di esso Cane apparisca non
maggiore della cinquantesima parte di quello di Giove, tuttavia la
sua irradiazione è grande, e fiera in modo, che l’istesso globo
tra i proprii crini s’implica, e quasi si perde, e con qualche
difficoltà si distingue; dove che Giove (e molto più Saturno)
si vedono e terminati, e di una luce languida, e per così dire
quieta. E per tanto io stimo, che bene filosoferemo, referendo la
causa della scintillazione delle Stelle fisse, al vibrare, che elle
fanno dello splendore proprio e nativo dall’intima loro sustanza;
dove che nella superficie de’ Pianeti termina più presto, e si
finisce l’illuminazione, che dal Sole deriva, e si parte. Se io
sentirò qualche particolare questione ricevuta dal medesimo Sig.
Wackher, non resterò d’affaticarmivi intorno, per dimostrarmi,
quale io sono desiderosissimo di servire un tanto Signore, e non
già con isperanza di aggiungere al termine conseguito dal suo
discorso, perchè benissimo comprendo, che a quanto sia passato
per lo finissimo cribro del giudizio di esso, e del Sig. Keplero,
non si può aggiungere di squisitezza; nè io pretenderei altro,
che col dubitare, e mal filosofare, eccitar loro al ritrovamento
di nuove sottigliezze. Gl’ingegni singolari, che in gran numero
fioriscono nell’Alemagna, mi hanno lungo tempo tenuto in desiderio
di vederla, il qual desiderio ora si raddoppia per la nuova grazia
dell’Illustrissimo Sig. Wackher, la quale mi farebbe divenir
grande ogni picciola occasione, che mi si presentasse. Ma ho di
soverchio occupata Vostra Signoria Illustrissima e Reverendissima.
Degnisi per fine di offerirmi e dedicarmi devotissimo servidore
all’Illustrissimo Sig. Wackher, salutando anco caramente il Sig.
Keplero, ed a lei con ogni riverenza bacio le mani, e dal Signore
Dio le prego somma felicità.
“[Di Firenze li 26] di Marzo 1611. Di Vostra Signoria Illustrissima
e Reverendissima obbligatissimo Servidore,
GALILEO GALILEI.”
When translated, the meaning is as follows:—
“Your last letter has exceedingly pleased me, especially that part
which assures me of the friendly feeling entertained towards me by
the most illustrious Imperial Counsellor, Wagher, which I for my
part highly appreciate. And since the cause of this friendliness
is, that I have incontestably demonstrated by some observations of
mine certain conclusions which he had long held as true, I wish
to confirm my possession of favour, which I value so much, and
accordingly I ask you to give him this piece of news from me; that
I have most conclusive arguments ready, showing clearly that, just
as he holds, all the planets receive their light from the sun,
being by constitution bodies dark and devoid of light;[30] but that
the fixed stars shine by their own proper light, not needing to be
illuminated by the sun’s rays, since God knows whether they reach
the very remote region of the fixed stars with intensity even equal
to the small intensity with which the rays of the fixed stars come
down to us.
“My demonstration depends chiefly on this fact, that with the
telescope I have distinctly observed that the planets receive
greater brightness, and reflect it more intensely, in proportion
as each one is nearer to us and to the sun. So Mars in perigee,
that is, when nearest to the earth, greatly surpasses Jupiter in
brightness, although in actual size it is far inferior to Jupiter;
and in consequence it is difficult to receive the effulgence of
this planet in the telescope, for it is so great as to prevent the
eye from being able to distinguish the circular termination of the
planet’s disc. This does not happen in the case of Jupiter, for it
appears quite circular. The next planet, Saturn, on account of its
great distance likewise—for indeed it is the most remote of the
planets,—appears bounded by a well-defined edge, both the greater
orb in the middle and the two small orbs at its sides. Indeed,
it appears to shine with a faint and delicate light, without any
effulgence to prevent an observer recognising the well-defined
termination of its three orbs. Since, then, we see that Mars, the
nearest of the three, is illumined by the sun with very great
splendour, and that the light of Jupiter, at a greater distance,
is much more faint (although without the use of an instrument it
appears tolerably bright, which is due to the size and brilliancy
of its body), and that the light of Saturn, at the greatest
distance, is most faint, and almost watery, of what kind, do you
think, would appear the light of the fixed stars, which are at an
immeasurable distance further from the sun than Saturn, if they
only received light from the sun? Most certainly, extremely feeble,
indistinct, and pallid. And yet we find quite the contrary; for,
let us look with our eyes at the Dog-Star, for example. We shall
encounter a most vivid brilliancy, which almost pricks the eye
with the rapid sparkling of its rays, of such intensity that, in
comparison with it, the planets, even Jupiter, and Venus too, are
as thoroughly outshone as common and bad glass compared with a
highly polished and most sparkling diamond. And although the orb of
the Dog-Star appears no larger than the fiftieth part of Jupiter’s
disc, nevertheless its brilliancy is great and very strong; so that
the form of its disc, which you expect to see, hides itself among
the rays of its own refulgence, envelops itself in them, and almost
disappears; and it is not distinguished without some difficulty
from the rays which surround it. Whereas Jupiter, and still
more Saturn, are seen well defined; and their light is without
intensity, and, if I may say so, quiescent. Wherefore I think that
we shall rightly apply our philosophy if we refer the cause of the
twinkling of the fixed stars to vibrations of a brilliancy, which
is their own, belonging to their constitution, and inherent in
their substance, and say, on the other hand, that the illumination
of the planets, which is derived from the sun, and distributed to
the world, is limited to their surface.”
These are the scientific conclusions in Galileo’s letter; the rest I
omit.
[30] Proctor (_Other Worlds than Ours_, 1875) has given some
reasons for believing that Jupiter and Saturn shine in part with
their own light, owing to their great internal heat.
You see then, studious reader, how the subtle mind of Galileo, in my
opinion the first philosopher of the day, uses this telescope of ours
like a sort of ladder, scales the furthest and loftiest walls of the
visible world, surveys all things with his own eyes, and, from the
position he has gained, darts the glances of his most acute intellect
upon these petty abodes of ours—the planetary spheres I mean,—and
compares with keenest reasoning the distant with the near, the lofty
with the deep.
VALE ET DEUM IN OPERIBUS SUIS CELEBRARE NUNQUAM DESINE.
KEPLER, _Narratio_.
Edinburgh University Press:
THOMAS AND ARCHIBALD CONSTABLE, PRINTERS TO HER MAJESTY.