_Galileo announces his discoveries in Enigmas--Discovers the
Crescent of Venus--the Ring of Saturn--the Spots on the
Sun--Similar Observations made in England by Harriot--Claims of
Fabricius and Scheiner to the discovery of the Solar
Spots--Galileo's Letters to Velser on the claims of Scheiner--His
residence at the Villa of Salviati--Composes his work on Floating
Bodies, which involves him in new controversies._
The great success which attended the first telescopic observations of
Galileo, induced him to apply his best instruments to the other planets
of our system. The attempts which had been made to deprive him of the
honour of some of his discoveries, combined, probably, with a desire to
repeat his observations with better telescopes, led him to announce his
discoveries under the veil of an enigma, and to invite astronomers to
declare, within a given time, if they had observed any new phenomena in
the heavens.
Before the close of 1610, Galileo excited the curiosity of astronomers
by the publication of his first enigma. Kepler and others tried in vain
to decipher it; but in consequence of the Emperor Rodolph requesting a
solution of the puzzle, Galileo sent him the following clue:--
"Altissimam planetam tergeminam observavi."
I have observed that the most remote planet is triple.
In explaining more fully the nature of his observation, Galileo remarked
that Saturn was not a single star, but three together, nearly touching
one another. He described them as having no relative motion, and as
having the form of three o's, namely, oOo, the central one being larger
than those on each side of it.
Although Galileo had announced that nothing new appeared in the other
planets, yet he soon communicated to the world another discovery of no
slight interest. The enigmatical letters in which it was concealed
formed the following sentence:--
"Cynthiae figuras aemulatur mater Amorum."
Venus rivals the phases of the moon.
Hitherto, Galileo had observed Venus when her disc was largely
illuminated; but having directed his telescope to her when she was not
far removed from the sun, he saw her in the form of a crescent,
resembling exactly the moon at the same elongation. He continued to
observe her night after night, during the whole time that she could be
seen in the course of her revolution round the sun, and he found that
she exhibited the very same phases which resulted from her motion round
that luminary.
Galileo had long contemplated a visit to the metropolis of Italy, and he
accordingly carried his intentions into effect in the early part of the
year 1611. Here he was received with that distinction which was due to
his great talents and his extended reputation. Princes, Cardinals, and
Prelates hastened to do him honour; and even those who discredited his
discoveries, and dreaded their results, vied with the true friends of
science in their anxiety to see the intellectual wonder of the age.
In order to show the new celestial phenomena to his friends at Rome,
Galileo took with him his best telescope; and as he had discovered the
spots on the sun's surface in October or November 1610, or even
earlier,[16] he had the gratification of exhibiting them to his admiring
disciples. He accordingly erected his telescope in the Quirinal garden,
belonging to Cardinal Bandini; and in April 1611 he shewed them to his
friends in many of their most interesting variations. From their change
of position on the sun's disc, Galileo at first inferred, either that
the sun revolved about an axis, or that other planets, like Venus and
Mercury, revolved so near the sun as to appear like black spots when
they were opposite to his disc. Upon continuing his observations,
however, he saw reason to abandon this hasty opinion. He found that the
spots must be in contact with the surface of the sun,--that their
figures were irregular,--that they had different degrees of
darkness,--that one spot would often divide itself into three or
four,--that three or four spots would often unite themselves into
one,--and that all the spots revolved regularly with the sun, which
appeared to complete its revolution in about twenty-eight days.
[16] Professor Rigaud is of opinion that Galileo had discovered the
solar spots at an earlier period than eighteen months before May
1612.
Previous to the invention of the telescope, spots had been more than
once seen on the sun's disc with the unassisted eye. But even if these
were of the same character as those which Galileo and others observed,
we cannot consider them as anticipations of their discovery by the
telescope. As the telescope was now in the possession of several
astronomers, Galileo began to have many rivals in discovery; but
notwithstanding the claims of Harriot, Fabricius, and Scheiner, it is
now placed beyond the reach of doubt that he was the first discoverer of
the solar spots. From the communication which I received in 1822 from
the late Dr Robertson, of Oxford,[17] it appeared that Thomas Harriot
had observed the solar spots on the 8th of December 1610; but his
manuscripts, in Lord Egremont's possession,[18] incontestably prove that
his regular observations on the spots did not commence till December 1,
1611, although he had seen the spots at the date above mentioned, and
that they were continued till the 18th of January 1613. The
observations which he has recorded are 199 in number, and the accounts
of them are accompanied with rough drawings representing the number,
position, and magnitude of the spots.[19] In the observation of Harriot,
made on the 8th December 1610, before he knew of Galileo's discovery, he
saw three spots on the sun, which he has represented in a diagram. The
sun was then 7 deg. or 8 deg. high, and there was a frost and a mist, which no
doubt acted as a darkening glass. Harriot does not apply the name of
spots to what he noticed in this observation, and he does not enumerate
it among the 199 observations above mentioned. Professor Rigaud[20]
considers it "a misapplication of terms to call such an observation a
discovery;" but, with all the respect which we feel for the candour of
this remark, we are disposed to confer on Harriot the merit of an
original discoverer of the spots on the sun.
[17] See page 40.
[18] These interesting MSS. I have had the good fortune of seeing
in the possession of my much valued friend, the late Professor
Rigaud of Oxford.
[19] Edin. Phil. Journ. 1822, vol. vi. p. 317. See Rigaud's Life of
Bradley, Supplement, p. 31.
[20] Id. It., p. 37, 38.
Another candidate for the honour of discovering the spots of the sun,
was John Fabricius, who undoubtedly saw them previous to June 1611. The
dedication of the work[21] in which he has recorded his observation,
bears the date of the 13th of June 1611; and it is obvious, from the
work itself, that he had seen the spots about the end of the year 1610;
but as there is no proof that he saw them before October, we are
compelled to assign the priority of the discovery to the Italian
astronomer.
[21] Joh. Fabricii Phrysii de Maculis in Sole observatis, et
apparente earum cum Sole conversione, Narratio. Wittemb. 1611.
The claim of Scheiner, professor of mathematics at Ingolstadt, is more
intimately connected with the history of Galileo. This learned
astronomer having, early in 1611, turned his telescope to the sun,
necessarily discovered the spots which at that time covered his disc.
Light flying clouds happened, at the time, to weaken the intensity of
his light, so that he was able to show the spots to his pupils. These
observations were not published till January 1612; and they appeared in
the form of three letters, addressed to Mark Velser, one of the
magistrates of Augsburg, under the signature of _Appelles post Tabulam_.
Scheiner, who, many years afterwards, published an elaborate work on
the subject, adopted the same idea which had at first occurred to
Galileo--that the spots were the dark sides of planets revolving round
and near the sun.[22]
[22] It does not appear from the history of solar observations at
what time, and by whom, coloured glasses were first introduced for
permitting the eye to look at the sun with impunity. Fabricius was
obviously quite ignorant of the use of coloured glasses. He
observed the sun when he was in the horizon, and when his
brilliancy was impaired by the interposition of thin clouds and
floating vapours; and he advises those who may repeat his
observations to admit at first to the eye a small portion of the
sun's light, till it is gradually accustomed to its full splendour.
When the sun's altitude became considerable, Fabricius gave up his
observations, which he often continued so long that he was scarcely
able, for two days together, to see objects with their usual
distinctness. Fabricius speaks of observing the sun by admitting
his rays through a small _hole_ into a dark room, and receiving his
image on paper; but he says nothing about a lens or a telescope
being applied to the hole; and he does not say that he saw the
spots of the sun in this way. Harriot also viewed the solar spots
when the sun was near the horizon, or was visible through "thick
layer and thin cloudes," or through thin mist. On December 21,
1611, at a quarter past 2 P.M., he observed the spots when the sky
was perfectly clear, but his "sight was after dim for an houre."
Scheiner, in his "Appelles post Tabulam," describes four different
ways of viewing the spots; one of which is by the _interposition of
blue or green glasses_. His first method was to observe the sun
near the horizon; the second was to view him through a transparent
cloud; the third was to look at him through his telescope with a
blue or a green glass of a proper thickness, and plane on both
sides, or to use a thin blue glass when the sun was covered with a
thin vapour or cloud; and the fourth method was to begin and
observe the sun at his margin, till the eye gradually reached the
middle of his disc.
On the publication of Scheiner's letters, Velser transmitted a copy of
them to his friend Galileo, with the request that he would favour him
with his opinion of the new phenomena. After some delay, Galileo
addressed three letters to Velser, in which he combated the opinions of
Scheiner on the cause of the spots. The first of these letters was dated
the 4th of May 1612;[23] but though the controversy was carried on in
the language of mutual respect and esteem, it put an end to the
friendship which had existed between the two astronomers. In these
letters Galileo showed that the spots often dispersed like vapours or
clouds; that they sometimes had a duration of only one or two days, and
at other times of thirty or forty days; that they contracted in their
breadth when they approached the sun's limb, without any diminution of
their length; that they describe circles parallel to each other; that
the monthly rotation of the sun again brings the same spots into view;
and that they are seldom seen at a greater distance than 30 deg. from the
sun's equator. Galileo likewise discovered on the sun's disc _faculae_,
or _luculi_, as they were called, which differ in no respect from the
common ones but in their being brighter than the rest of the sun's
surface.[24]
[23] The original of this letter is in the British Museum.
[24] See Istoria e Dimonstrazioni, intorno alle macchie solare.
_Roma_, 1616. See Opere di Galileo, vol, v., p. 131-293.
In the last of the letters which our author addressed to Velser, and
which was written in December 1612, he recurs to his former discovery of
the elongated shape, or rather the triple structure, of Saturn. The
singular figure which he had observed in this planet had entirely
disappeared; and he evidently announces the fact to Velser, lest it
should be used by his enemies to discredit the accuracy of his
observations. "Looking on Saturn," says he, "within these few days, I
found it solitary, without the assistance of its accustomed stars, and,
in short, perfectly round and defined like Jupiter; and such it still
remains. Now, what can be said of so strange a metamorphosis? Are the
two smaller stars consumed like the spots on the sun? Have they suddenly
vanished and fled? or has Saturn devoured his own children? or was the
appearance indeed fraud and illusion, with which the glasses have for so
long a time mocked me, and so many others who have often observed with
me? Now, perhaps, the time is come to revive the withering hopes of
those who, guided by more profound contemplations, have followed all the
fallacies of the new observations, and recognised their impossibilities.
I cannot resolve what to say in a chance so strange, so new, and so
unexpected; the shortness of the time, the unexampled occurrence, the
weakness of my intellect, and the terror of being mistaken, have greatly
confounded me." Although Galileo struggled to obtain a solution of this
mystery, yet he had not the good fortune to succeed. He imagined that
the two smaller stars would reappear, in consequence of the supposed
revolution of the planet round its axis; but the discovery of the ring
of Saturn, and of the obliquity of its plane to the ecliptic, was
necessary to explain the phenomena which were so perplexing to our
author.
The ill health to which Galileo was occasionally subject, and the belief
that the air of Florence was prejudicial to his complaints, induced him
to spend much of his time at Selve, the villa of his friend Salviati.
This eminent individual had ever been the warmest friend of Galileo, and
seems to have delighted in drawing round him the scientific genius of
the age. He was a member of the celebrated Lyncaean Society, founded by
Prince Frederigo Cesi; and though he is not known as the author of any
important discovery, yet he has earned, by his liberality to science, a
glorious name, which will be indissolubly united with the immortal
destiny of Galileo.
The subject of floating bridges having been discussed at one of the
scientific parties which had assembled at the house of Salviati, a
difference of opinion arose respecting the influence of the shape of
bodies on their disposition to float or to sink in a fluid. Contrary to
the general opinion, Galileo undertook to prove that it depended on
other causes; and he was thus led to compose his discourse on floating
bodies,[25] which was published in 1612, and dedicated to Cosmo de
Medici. This work contains many ingenious experiments, and much acute
reasoning in support of the true principles of hydrostatics; and it is
now chiefly remarkable as a specimen of the sagacity and intellectual
power of its author. Like all his other works, it encountered the most
violent opposition; and Galileo was more than once summoned into the
field to repel the aggressions of his ignorant and presumptuous
opponents. The first attack upon it was made by Ptolemy Nozzolini, in a
letter to Marzemedici, Archbishop of Florence;[26] and to this Galileo
replied in a letter addressed to his antagonist.[27] A more elaborate
examination of it was published by Lodovico delle Colombe, and another
by M. Vincenzo di Grazia. To these attacks, a minute and overwhelming
answer was printed in the name of Benedetti Castelli, the friend and
pupil of Galileo; but it was discovered, some years after Galileo's
death, that he was himself the author of this work.[28]
[25] Discorso intorno alle cose che stanno in su l'acqua, o che in
quella si muovono. Opere di Galileo, vol. ii. pp. 165-311.
[26] Opere di Galileo, vol. ii. pp. 355-367.
[27] Ibid. 367-390.
[28] These three treatises occupy the whole of the third volume of
the Opere di Galileo.