Chapter IV: Part 4

“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.