Traduzione di N. M. SWERDLOW, The University of Chicago
Testo inglese
[On the Sun]
The Sun has two orbs concentric to the world, of which the higher has its poles
under the poles of the zodiac of the eighth sphere, and [its] ecliptic under the ecliptic
of the eighth sphere. In the concavity of this orb is a small circle with a pole outside
the ecliptic just mentioned. But the solar body is fixed in the lower orb.
Now the higher orb moves uniformly about its poles by about 0;59,8° in each
natural day. And the lower orb is so arranged that in the convexity of it there is a point
invariably remaining directly at the centre of the solar body, and also another point
distant from the first by a quadrant of a great circle. One of these points moves uniformly
in the circumference of the small circle mentioned before, completing one
revolution in the same time as the higher orb. However, the other end of the quadrant,
which is at the Sun, always adheres to the ecliptic designated in the higher orb.
In order that this will more easily be clear, a representation should be drawn in
which I shall now call the points by letters of the alphabet. And later when the opportunity
is more favourable, I shall assign to the individual points their names.
I shall also complete a new work in four treatises. In the first of these I shall present
the old theory of eccentrics and epicycles demolished by strong reasons and observations
that will be made. In the second I shall clearly set out a theory of concentric orbs
by which all inequalities of the motions can be saved. And in the third I shall confirm
what is in the second by geometrical proofs. The fourth will contain the way by which
the motions can be computed and tables [with] new epoch positions established for
them.
But where have I wandered? Now returning, I shall describe the ecliptic of the
higher orb, which let be ABCD, one pole of which let be point Z, from which let
descend a quadrant of a great circle ZA. In this quadrant let a point E be designated,
about which let the small circle KFHG be described. Then in the convexity of the
lower orb, let a quadrant of a great circle be estimated, one end of which let adhere to
the circumference of the small circle just mentioned, which point let letter K represent.
And let the other end of this quadrant, [point B], which is directly at the centre of
the solar body, always adhere to the ecliptic of the higher orb.
Accordingly, every point of the ecliptic ABCD moves uniformly around the centre
of the world, and likewise the small circle moves [uniformly around the centre of the
world]. And if the quadrant KB in the convexity of the lower orb always, as was
mentioned, adhered to the ecliptic in the same points, the Sun would have no inequality
in its motion, for the centre of the Sun would always be found directly at the point
B that is moved uniformly. But it is not so, rather, the end of the quadrant which
adheres to the circumference of the small circle moves in the circumference of the
small circle [from K] towards point F, and draws with it the other end of the quadrant,
which is at the Sun, yet adhering to the ecliptic.
Understand, therefore, that an arc, [as ZFN, ZHKA or ZGO], passing through the
pole of the ecliptic [Z] and the end of the quadrant [at the small circle, as F, H, G or K,
extended] as far as the ecliptic, [as to N, A or O], likewise moves in accordance with
the motion of the end [at the small circle]. Accordingly, as much as is the portion of
the ecliptic which two arcs contain, one of which [ZEA] passes through the pole of the
ecliptic and the pole of the small circle, and the other, [as ZFN, ZHKA or ZGO],
through the pole of the ecliptic and the end of the quadrant [at the small circle extended
to the ecliptic], so much is also the portion [of the ecliptic] which is found
between point B and the end of the quadrant adhering to the ecliptic, [as L or M].
And thus when the revolving end of the quadrant arrives at point F, the point, I say,
in which arc ZN is tangent to the small circle, there the portion of the ecliptic [NA] of
which I have spoken is the greatest, and hence the other, adhering end of the quadrant
draws the farthest away from B, [namely to L]. But afterwards this portion gradually
decreases until it becomes zero, namely, when the revolving end of the quadrant is in
point H of the small circle. When, however, the revolving end of the quadrant recedes
from point H, once again the portion of the ecliptic contained between the two arcs
increases until the revolving end of the quadrant arrives at the other point of tangency,
which is G. Then once again the portion of this kind [AO] is greatest, which finally, on
account of the motion of the revolving end, decreases continuously until an entire
revolution in the small circle is completed.
And thus this is the disposition of the motions of the Sun. Now I come to the
description of the technical terms.
The line of the mean motion of the Sun is the line which, proceeding from the
centre of the world through point B, is extended as far as the zodiac.
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Traduzione italiana
di G.Ferrero, Università di Genova
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Copyright 2001 Giovanni Ferrero