The Borghesi Astronomical Clock in the Museum of History and Technology: Contributions from the Museum of History and Technology, Paper 35Bedini, Silvio A.
History
The Borghesi Astronomical Clock in the Museum of History and Technology: Contributions from the Museum of History and Technology, Paper 35
Bedini, Silvio A.
Astronomical clocks
But whether the eclipse takes place in the head or in the tail of the
dragon, or whether it is north or south, is indicated by the small disk
of the sun attached to one of the two pointers hidden by the segments of
the circle. If, at that time, the little disk shall be found in the head
of the dragon inscribed on the plane of the dial, then the sun has been
snatched from the earth and ingloriously entombed, as it were, in the
huge jaw of the dragon. Then, ... the heavens themselves will lend aid
to the woeful pomp of the senseless funeral in full darkness by suddenly
lighting the unhappy lamps of the fixed stars. However, if the little
disk occupies the tail of the dragon on the mechanism, then the sun in
the heavens also, as if freed from the toils of the immense dragon's
tail, will emerge without difficulty.
The center of the eclipse will traverse the hemisphere of the earth
north of the solar path, always nearer to the pole of the ecliptic, in
proportion to the inclination of the disk to the north. On the other
hand, if the little disk inclines to the left semicircle, then the
people south of the solar path will enjoy the spectacle of the total
central eclipse.
But if the little disk remains neutral (inclining neither way) and
remains halfway between the two sections of the circle, then the
greatest solar eclipse will take place at the equator and those who live
near the poles of the ecliptic will not enjoy a trace of that eclipse.
This is because the half of the equatorial diameter enormously
outmeasures even the greatest apparent semidiameters of the sun and of
the moon, even taking as a norm the smallest horizontal parallax of the
moon.
What has been said about the true new moon is to be understood also,
proportionately, about the true full moon. For when, with respect to the
equations of the centers, the moon shall be distant on the mechanism by
a full semicircle from the sun (also in the heavens it will be truly in
opposition to the sun) there will be a true full moon. Likewise, the
moon in the heavens will be in eclipse if, at the time of opposition,
the pointers of the little index (which we mentioned before) situated
below the center of the clock are so far away from the belly of the
dragon that they are forced to lie under the two smaller segments of the
circle which, in all full moons, are always to be moved from the index
of the synodic moon to the region of that little index. As a matter of
fact, the closer the little pointers approach to the middle of the
segments, the more obscured it will be.
You will know, furthermore, that the eclipse of the moon occurs in the
head of the dragon if the disk of the little moon, attached to the other
point of the little index, is raised to the head of the dragon;
conversely, when the little disk of the moon inclines to the tail, the
eclipse is taking place in the tail of the dragon.
Public-domain text, read in full here on John Shaqi.
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