from the Sun, not only does its orb show itself to our sight on the
side where it is furnished with shining horns, but a slight and faint
circumference is also seen to mark out the circle of the dark part,
that part, namely, which is turned away from the Sun, and to separate
it from the darker background of the sky. But if we examine the matter
more closely, we shall see that not only is the extreme edge of the
part in shadow shining with a faint brightness, but that the entire
face of the Moon, that side, that is, which does not feel the Sun’s
glare, is illuminated with a pale light of considerable brightness.
At the first glance only a fine circumference appears shining, on
account of the darker part of the sky adjacent to it; whereas, on the
contrary, the rest of the surface appears dark, on account of the
contiguity of the shining horns, which destroys the clearness of our
sight. But if any one chooses such a position for himself, that by the
interposition of a roof, or a chimney, or some other object between
his sight and the Moon, but at a considerable distance from his eye,
the shining horns are hidden, and the rest of the Moon’s orb is left
exposed to his view, then he will find that this tract of the Moon
also, although deprived of sunlight, gleams with considerable light,
and particularly so if the gloom of the night has already deepened
through the absence of the Sun; for with a darker background the same
light appears brighter. Moreover, it is found that this secondary
brightness of the Moon, as I may call it, is greater in proportion as
the Moon is less distant from the Sun; for it abates more and more in
proportion to the Moon’s distance from that body, so much so that after
the first quarter, and before the end of the second, it is found to be
weak and very faint, although it be observed in a darker sky; whereas,
at an angular distance of 60° or less, even during twilight, it is
wonderfully bright, so bright indeed that, with the help of a good
telescope, the great spots may be distinguished in it.
This strange brightness has afforded no small perplexity to
philosophical minds; and some have published one thing, some another,
as the cause to be alleged for it. Some have said that it is the
inherent and natural brightness of the Moon; some that it is imparted
to that body by the planet Venus; or, as others maintain, by all
the stars; while some have said that it comes from the Sun, whose
rays, they say, find a way through the solid mass of the Moon. But
statements of this kind are disproved without much difficulty, and
convincingly demonstrated to be false. For if this kind of light were
the Moon’s own, or were contributed by the stars, the Moon would retain
it, particularly in eclipses, and would show it then, when left in
an unusually dark sky, but this is contrary to experience. For the
brightness which is seen on the Moon in eclipses is far less intense,
being somewhat reddish, and almost copper-coloured, whereas this is
brighter and whiter; besides, the brightness seen during an eclipse is
changeable and shifting, for it wanders over the face of the Moon, so
that that part which is near the circumference of the circle of shadow
thrown by the Earth is bright, but the rest of the Moon is always seen
to be dark. From which circumstance we understand without hesitation
that this brightness is due to the proximity of the Sun’s rays coming
into contact with some denser region which surrounds the Moon as an
envelope; owing to which contact a sort of dawn-light is diffused over
the neighbouring regions of the Moon, just as the twilight spreads in
the morning and evening on the Earth:[11] but I will treat more fully
of this matter in my book upon the _System of the Universe_.[12]
[11] The illumination of the Moon in eclipses, noticed by Galileo,
is now referred to the refraction of the sunlight by the earth’s
atmosphere, and the reddish colour of the light is explained by
Herschel (_Outlines of Astronomy_, ch. vii.) to be due to the
absorption of the violet and blue rays by the aqueous vapour of the
Earth’s atmosphere. The idea of a sensible lunar atmosphere is not
in accordance with the observed phenomena of the occultations of
stars.
[12] Galileo’s _Systema Mundi_. Owing to the violent opposition
provoked by the discussion of the discoveries of Galileo, and
their bearing on the Copernican system of astronomy, Galileo seems
to have found it necessary to delay the publication of this work
until 1632, when, believing himself safe under the friendship and
patronage of Pope Urban VIII. and others in power at Rome, he
at length published it. Urban, however, now turned against him,
denounced the book and its author, and summoned him to Rome, where
the well-known incidents of his trial and condemnation took place.
Again, to assert that this sort of light is imparted to the Moon by the
planet Venus is so childish as to be undeserving of an answer; for who
is so ignorant as not to understand that at conjunction and within an
angular distance of 60° it is quite impossible for the part of the Moon
turned away from the Sun to be seen by the planet Venus?
But that this light is derived from the Sun penetrating with its light
the solid mass of the Moon, and rendering it luminous, is equally
untenable. For then this light would never lessen, since the hemisphere
of the Moon is always illumined by the Sun, except at the moment of
a lunar eclipse, yet really it quickly decreases while the Moon is
drawing near to the end of her first quarter, and when she has passed
her first quarter it becomes quite dull. Since, therefore, this
kind of secondary brightness is not inherent and the Moon’s own, nor
borrowed from any of the stars, nor from the Sun, and since there now
remains in the whole universe no other body whatever except the Earth,
what, pray, must we conclude? What must we assert? Shall we assert that
the body of the Moon, or some other dark and sunless orb, receives
light from the Earth? Why should it not be the Moon? And most certainly
it is. The Earth, with fair and grateful exchange, pays back to the
Moon an illumination like that which it receives from the Moon nearly
the whole time during the darkest gloom of night. Let me explain the
matter more clearly. At conjunction, when the Moon occupies a position
between the Sun and the Earth, the Moon is illuminated by the Sun’s
rays on her half towards the Sun which is turned away from the Earth,
and the other half, with which she regards the Earth, is covered with
darkness, and so in no degree illumines the Earth’s surface. When the
Moon has slightly separated from the Sun, straightway she is partly
illumined on the half directed towards us; she turns towards us a
slender silvery crescent, and slightly illumines the Earth; the Sun’s
illumination increases upon the Moon as she approaches her first
quarter, and the reflexion of that light increases on the Earth; the
brightness in the Moon next extends beyond the semicircle, and our
nights grow brighter; at length the entire face of the Moon looking
towards the Earth is irradiated with the most intense brightness by
the Sun, which happens when the Sun and Moon are on opposite sides of
the Earth; then far and wide the surface of the Earth shines with the
flood of moonlight; after this the Moon, now waning, sends out less
powerful beams, and the Earth is illumined less powerfully; at length
the Moon draws near her first position of conjunction with the Sun, and
forthwith black night invades the Earth. In such a cycle the moonlight
gives us each month alternations of brighter and fainter illumination.
But the benefit of her light to the Earth is balanced and repaid by the
benefit of the light of the Earth to her; for while the Moon is found
near the Sun about the time of conjunction, she has in front of her
the entire surface of that hemisphere of the Earth which is exposed to
the Sun, and vividly illumined with his beams, and so receives light
reflected from the Earth. Owing to such reflexion, the hemisphere
of the Moon nearer to us, though deprived of sunlight, appears of
considerable brightness. Again, when removed from the Sun through
a quadrant, the Moon sees only one-half of the Earth’s hemisphere
illuminated, namely the western half, for the other, the eastern, is
covered with the shades of night; the Moon is, therefore, less brightly
enlightened by the Earth, and accordingly that secondary light appears
fainter to us. But if you imagine the Moon to be set on the opposite
side of the Earth to the Sun, she will see the hemisphere of the Earth,
now between the Moon and the Sun, quite dark, and steeped in the gloom
of night; if, therefore, an eclipse should accompany such a position
of the Moon, she will receive no light at all, being deprived of the
illumination of the Sun and Earth together. In any other position, with
regard to the Earth and the Sun, the Moon receives more or less light
by reflexion from the Earth, according as she sees a greater or smaller
portion of the hemisphere of the Earth illuminated by the Sun; for such
a law is observed between these two orbs, that at whatever times the
Earth is most brightly enlightened by the Moon, at those times, on the
contrary, the Moon is least enlightened by the Earth; and contrariwise.
Let these few words on this subject suffice in this place; for I will
consider it more fully in my _System of the Universe_, where, by very
many arguments and experimental proofs, there is shown to be a very
strong reflexion of the Sun’s light from the Earth, for the benefit of
those who urge that the Earth must be separated from the starry host,
chiefly for the reason that it has neither motion nor light, for I will
prove that the Earth has motion, and surpasses the Moon in brightness,
and is not the place where the dull refuse of the universe has settled
down; and I will support my demonstration by a thousand arguments taken
from natural phenomena.
[Sidenote: Stars. Their appearance in the telescope.]
Hitherto I have spoken of the observations which I have made concerning
the Moon’s body; now I will briefly announce the phenomena which have
been, as yet, seen by me with reference to the Fixed Stars. And first
of all the following fact is worthy of consideration:—The stars, fixed
as well as erratic, when seen with a telescope, by no means appear to
be increased in magnitude in the same proportion as other objects,
and the Moon herself, gain increase of size; but in the case of the
stars such increase appears much less, so that you may consider that
a telescope, which (for the sake of illustration) is powerful enough
to magnify other objects a hundred times, will scarcely render the
stars magnified four or five times. But the reason of this is as
follows:—When stars are viewed with our natural eyesight they do not
present themselves to us of their bare, real size, but beaming with a
certain vividness, and fringed with sparkling rays, especially when
the night is far advanced; and from this circumstance they appear much
larger than they would if they were stripped of those adventitious
fringes, for the angle which they subtend at the eye is determined
not by the primary disc of the star, but by the brightness which so
widely surrounds it. Perhaps you will understand this most clearly
from the well-known circumstance that when stars rise just at sunset,
in the beginning of twilight, they appear very small, although they
may be stars of the first magnitude; and even the planet Venus itself,
on any occasion when it may present itself to view in broad daylight,
is so small to see that it scarcely seems to equal a star of the last
magnitude. It is different in the case of other objects, and even of
the Moon, which, whether viewed in the light of midday or in the depth
of night, always appears of the same size. We conclude therefore that
the stars are seen at midnight in uncurtailed glory, but their fringes
are of such a nature that the daylight can cut them off, and not only
daylight, but any slight cloud which may be interposed between a star
and the eye of the observer. A dark veil or coloured glass has the
same effect, for, upon placing them before the eye between it and
the stars, all the blaze that surrounds them leaves them at once. A
telescope also accomplishes the same result, for it removes from the
stars their adventitious and accidental splendours before it enlarges
their true discs (if indeed they are of that shape), and so they seem
less magnified than other objects, for a star of the fifth or sixth
magnitude seen through a telescope is shown as of the first magnitude
only.
The difference between the appearance of the planets and the fixed
stars seems also deserving of notice. The planets present their discs
perfectly round, just as if described with a pair of compasses,
and appear as so many little moons, completely illuminated and of
a globular shape; but the fixed stars do not look to the naked eye
bounded by a circular circumference, but rather like blazes of
light, shooting out beams on all sides and very sparkling, and with
a telescope they appear of the same shape as when they are viewed by
simply looking at them, but so much larger that a star of the fifth or
sixth magnitude seems to equal Sirius, the largest of all the fixed
stars.[13]
[13] The immense distance of stars makes it impossible for them to
be magnified by any telescope, however powerful; the apparent or
spurious disc is an optical effect, which depends on the telescope
used, and is smallest with the largest aperture.
[Illustration: _Orion’s Belt and Sword; 83 Stars_]
[Illustration: _Pleiades; 36 Stars_
_Galileo: “Sidereus Nuncius.”_]
[Sidenote: Telescopic Stars: their infinite multitude. As examples,
Orion’s Belt and Sword and the Pleiades are described as seen by
Galileo.]
But beyond the stars of the sixth magnitude you will behold through
the telescope a host of other stars, which escape the unassisted
sight, so numerous as to be almost beyond belief, for you may see more
than six other differences of magnitude, and the largest of these,
which I may call stars of the seventh magnitude, or of the first
magnitude of invisible stars, appear with the aid of the telescope
larger and brighter than stars of the second magnitude seen with the
unassisted sight. But in order that you may see one or two proofs of
the inconceivable manner in which they are crowded together, I have
determined to make out a case against two star-clusters, that from them
as a specimen you may decide about the rest.
As my first example I had determined to depict the entire constellation
of Orion, but I was overwhelmed by the vast quantity of stars and
by want of time, and so I have deferred attempting this to another
occasion, for there are adjacent to, or scattered among, the old stars
more than five hundred new stars within the limits of one or two
degrees. For this reason I have selected the three stars in Orion’s
Belt and the six in his Sword, which have been long well-known groups,
and I have added eighty other stars recently discovered in their
vicinity, and I have preserved as exactly as possible the intervals
between them. The well-known or old stars, for the sake of distinction,
I have depicted of larger size, and I have outlined them with a double
line; the others, invisible to the naked eye, I have marked smaller and
with one line only. I have also preserved the differences of magnitude
as much as I could.
As a second example I have depicted the six stars of the constellation
Taurus, called the Pleiades (I say _six_ intentionally, since the
seventh is scarcely ever visible), a group of stars which is enclosed
in the heavens within very narrow precincts. Near these there lie
more than forty others invisible to the naked eye, no one of which is
much more than half a degree off any of the aforesaid six; of these
I have noticed only thirty-six in my diagram. I have preserved their
intervals, magnitudes, and the distinction between the old and the new
stars, just as in the case of the constellation Orion.
[Sidenote: The Milky Way consists entirely of stars in countless
numbers and of various magnitudes.]
The next object which I have observed is the essence or substance of
the Milky Way. By the aid of a telescope any one may behold this in a
manner which so distinctly appeals to the senses that all the disputes
which have tormented philosophers through so many ages are exploded at
once by the irrefragable evidence of our eyes, and we are freed from
wordy disputes upon this subject, for the Galaxy is nothing else but a
mass of innumerable stars planted together in clusters. Upon whatever
part of it you direct the telescope straightway a vast crowd of
stars presents itself to view; many of them are tolerably large
and extremely bright, but the number of small ones is quite beyond
determination.
[Illustration: _Star-cluster in Orion’s Head_]
[Illustration: _Star-cluster of Praesepe in Cancer_
_Galileo: “Sidereus Nuncius,” Venice, 1610._]
[Sidenote: Nebulæ resolved into clusters of stars: as examples, the
nebulæ in Orion’s Head and Præsepe.]
And whereas that milky brightness, like the brightness of a white
cloud, is not only to be seen in the Milky Way, but several spots of
a similar colour shine faintly here and there in the heavens, if you
turn the telescope upon any of them you will find a cluster of stars
packed close together. Further—and you will be more surprised at
this,—the stars which have been called by every one of the astronomers
up to this day _nebulous_, are groups of small stars set thick together
in a wonderful way, and although each one of them on account of its
smallness, or its immense distance from us, escapes our sight, from
the commingling of their rays there arises that brightness which has
hitherto been believed to be the denser part of the heavens, able to
reflect the rays of the stars or the Sun.
I have observed some of these, and I wish to subjoin the star-clusters
of two of these nebulæ. First, you have a diagram of the nebula called
that of Orion’s Head, in which I have counted twenty-one stars.
The second cluster contains the nebula called Præsepe, which is not one
star only, but a mass of more than forty small stars. I have noticed
thirty-six stars, besides the Aselli, arranged in the order of the
accompanying diagram.
[Sidenote: Discovery of Jupiter’s satellites, Jan. 7, 1610: record of
Galileo’s observations during two months.]
I have now finished my brief account of the observations which I have
thus far made with regard to the Moon, the Fixed Stars, and the Galaxy.
There remains the matter, which seems to me to deserve to be considered
the most important in this work, namely, that I should disclose and
publish to the world the occasion of discovering and observing four
PLANETS, never seen from the very beginning of the world up to our
own times, their positions, and the observations made during the last
two months about their movements and their changes of magnitude; and
I summon all astronomers to apply themselves to examine and determine
their periodic times, which it has not been permitted me to achieve up
to this day, owing to the restriction of my time. I give them warning
however again, so that they may not approach such an inquiry to no
purpose, that they will want a very accurate telescope, and such as I
have described in the beginning of this account.
On the 7th day of January in the present year, 1610, in the first[14]
hour of the following night, when I was viewing the constellations of
the heavens through a telescope, the planet Jupiter presented itself to
my view, and as I had prepared for myself a very excellent instrument,
I noticed a circumstance which I had never been able to notice before,
owing to want of power in my other telescope, namely, that three little
stars, small but very bright, were near the planet; and although I
believed them to belong to the number of the fixed stars, yet they made
me somewhat wonder, because they seemed to be arranged exactly in a
straight line, parallel to the ecliptic,[15] and to be brighter than
the rest of the stars, equal to them in magnitude. The position of them
with reference to one another and to Jupiter was as follows (Fig. 1).
[14] The times of Galileo’s observations are to be understood as
reckoned from sunset.
[15] The satellites of Jupiter revolve in planes very nearly,
although not exactly, coincident with that of the equator of the
planet, which is inclined 3° 5´ 30´´ to the orbit of the planet,
and the plane of the orbit is inclined 1° 18´ 51´´ to the ecliptic.
On the east side there were two stars, and a single one towards the
west. The star which was furthest towards the east, and the western
star, appeared rather larger than the third.
I scarcely troubled at all about the distance between them and Jupiter,
for, as I have already said, at first I believed them to be fixed
stars; but when on January 8th, led by some fatality, I turned again to
look at the same part of the heavens, I found a very different state
of things, for there were three little stars all west of Jupiter, and
nearer together than on the previous night, and they were separated
from one another by equal intervals, as the accompanying illustration
(Fig. 2) shows.
At this point, although I had not turned my thoughts at all upon the
approximation of the stars to one another, yet my surprise began to
be excited, how Jupiter could one day be found to the east of all the
aforesaid fixed stars when the day before it had been west of two of
them; and forthwith I became afraid lest the planet might have moved
differently from the calculation of astronomers, and so had passed
those stars by its own proper motion. I therefore waited for the next
night with the most intense longing, but I was disappointed of my hope,
for the sky was covered with clouds in every direction.
But on January 10th the stars appeared in the following position with
regard to Jupiter; there were two only, and both on the east side of
Jupiter, the third, as I thought, being hidden by the planet (Fig. 3).
They were situated just as before, exactly in the same straight line
with Jupiter, and along the Zodiac.
When I had seen these phenomena, as I knew that corresponding
changes of position could not by any means belong to Jupiter, and as,
moreover, I perceived that the stars which I saw had been always the
same, for there were no others either in front or behind, within a
great distance, along the Zodiac,—at length, changing from doubt into
surprise, I discovered that the interchange of position which I saw
belonged not to Jupiter, but to the stars to which my attention had
been drawn, and I thought therefore that they ought to be observed
henceforward with more attention and precision.
Accordingly, on January 11th I saw an arrangement of the following
kind (Fig. 4), namely, only two stars to the east of Jupiter, the
nearer of which was distant from Jupiter three times as far as from
the star further to the east; and the star furthest to the east was
nearly twice as large as the other one; whereas on the previous night
they had appeared nearly of equal magnitude. I therefore concluded,
and decided unhesitatingly, that there are three stars in the heavens
moving about Jupiter, as Venus and Mercury round the Sun; which
at length was established as clear as daylight by numerous other
subsequent observations. These observations also established that there
are not only three, but four, erratic sidereal bodies performing their
revolutions round Jupiter, observations of whose changes of position
made with more exactness on succeeding nights the following account
will supply. I have measured also the intervals between them with the
telescope in the manner already explained. Besides this, I have given
the times of observation, especially when several were made in the
same night, for the revolutions of these planets are so swift that an
observer may generally get differences of position every hour.
Jan. 12.—At the first hour of the next night I saw these heavenly
bodies arranged in this manner (Fig. 5). The satellite[16] furthest to
the east was greater than the satellite furthest to the west; but both
were very conspicuous and bright; the distance of each one from Jupiter
was two minutes. A third satellite, certainly not in view before, began
to appear at the third hour; it nearly touched Jupiter on the east
side, and was exceedingly small. They were all arranged in the same
straight line, along the ecliptic.
[16] Galileo continues to call these bodies _stars_, perhaps
meaning “Medicean stars,” throughout the description of their
configurations, but as he had now detected their nature, it is more
convenient to call them _satellites_, the term introduced by Kepler.
Jan. 13.—For the first time four satellites were in view in the
following position with regard to Jupiter (Fig. 6).
There were three to the west, and one to the east; they made a straight
line nearly, but the middle satellite of those to the west deviated a
little from the straight line towards the north. The satellite furthest
to the east was at a distance of 2´ from Jupiter; there were intervals
of 1´ only between Jupiter and the nearest satellite, and between the
satellites themselves, west of Jupiter. All the satellites appeared
of the same size, and though small they were very brilliant, and far
outshone the fixed stars of the same magnitude.
Jan. 14.—The weather was cloudy.