I teach you to preserve your health; and in this you will succed
better in proportion as you shun physicians, because their medicines
are the work of alchemists.

[Footnote: This passage is written on the back of the drawing Pl.
CVIII. Compare also No. 1184.]

_XV_.

_Astronomy_.

_Ever since the publication by Venturi in_ 1797 _and Libri in_ 1840
_of some few passages of Leonardo's astronomical notes, scientific
astronomers have frequently expressed the opinion, that they must
have been based on very important discoveries, and that the great
painter also deserved a conspicuous place in the history of this
science. In the passages here printed, a connected view is given of
his astronomical studies as they lie scattered through the
manuscripts, which have come down to us. Unlike his other purely
scientific labours, Leonardo devotes here a good deal of attention
to the opinions of the ancients, though he does not follow the
practice universal in his day of relying on them as authorities; he
only quotes them, as we shall see, in order to refute their
arguments. His researches throughout have the stamp of independent
thought. There is nothing in these writings to lead us to suppose
that they were merely an epitome of the general learning common to
the astronomers of the period. As early as in the XIVth century
there were chairs of astronomy in the universities of Padua and
Bologna, but so late as during the entire XVIth century Astronomy
and Astrology were still closely allied._

_It is impossible now to decide whether Leonardo, when living in
Florence, became acquainted in his youth with the doctrines of Paolo
Toscanelli the great astronomer and mathematician (died_ 1482_), of
whose influence and teaching but little is now known, beyond the
fact that he advised and encouraged Columbus to carry out his
project of sailing round the world. His name is nowhere mentioned by
Leonardo, and from the dates of the manuscripts from which the texts
on astronomy are taken, it seems highly probable that Leonardo
devoted his attention to astronomical studies less in his youth than
in his later years. It was evidently his purpose to treat of
Astronomy in a connected form and in a separate work (see the
beginning of Nos._ 866 _and_ 892_; compare also No._ 1167_). It is
quite in accordance with his general scientific thoroughness that he
should propose to write a special treatise on Optics as an
introduction to Astronomy (see Nos._ 867 _and_ 877_). Some of the
chapters belonging to this Section bear the title "Prospettiva"
_(see Nos._ 869 _and_ 870_), this being the term universally applied
at the time to Optics as well as Perspective (see Vol. I, p._ 10,
_note to No._ 13, _l._ 10_)_.

_At the beginning of the XVIth century the Ptolemaic theory of the
universe was still universally accepted as the true one, and
Leonardo conceives of the earth as fixed, with the moon and sun
revolving round it, as they are represented in the diagram to No._
897. _He does not go into any theory of the motions of the planets;
with regard to these and the fixed stars he only investigates the
phenomena of their luminosity. The spherical form of the earth he
takes for granted as an axiom from the first, and he anticipates
Newton by pointing out the universality of Gravitation not merely in
the earth, but even in the moon. Although his acute research into
the nature of the moon's light and the spots on the moon did not
bring to light many results of lasting importance beyond making it
evident that they were a refutation of the errors of his
contemporaries, they contain various explanations of facts which
modern science need not modify in any essential point, and
discoveries which history has hitherto assigned to a very much later
date_.

_The ingenious theory by which he tries to explain the nature of
what is known as earth shine, the reflection of the sun's rays by
the earth towards the moon, saying that it is a peculiar refraction,
originating in the innumerable curved surfaces of the waves of the
sea may be regarded as absurd; but it must not be forgotten that he
had no means of detecting the fundamental error on which he based
it, namely: the assumption that the moon was at a relatively short
distance from the earth. So long as the motion of the earth round
the sun remained unknown, it was of course impossible to form any
estimate of the moon's distance from the earth by a calculation of
its parallax_.

_Before the discovery of the telescope accurate astronomical
observations were only possible to a very limited extent. It would
appear however from certain passages in the notes here printed for
the first time, that Leonardo was in a position to study the spots
in the moon more closely than he could have done with the unaided
eye. So far as can be gathered from the mysterious language in which
the description of his instrument is wrapped, he made use of
magnifying glasses; these do not however seem to have been
constructed like a telescope--telescopes were first made about_
1600. _As LIBRI pointed out_ (Histoire des Sciences mathematiques
III, 101) _Fracastoro of Verona_ (1473-1553) _succeeded in
magnifying the moon's face by an arrangement of lenses (compare No._
910, _note), and this gives probability to Leonardo's invention at a
not much earlier date._

I.

THE EARTH AS A PLANET.

The earth's place in the universe (857. 858).