VIII 



After having completed briefly the description of 

 the dial and the indices and their motions, I have 

 not without reason delayed in satisfying the desires 

 of many who wish to learn at least the method by 

 which, from this mechanism, may be calculated the 

 true times of new and full moons, and their ecliptics. 

 In order to make these matters clearer, it is necessary 

 that they be explained here at greater length. 



With the indices, then, adjusted astronomically- 

 a;eographically to the longitude of any given region, 

 and to the mean time whether past, present or future, 

 and assuming the clock to be in normal operation 

 (as at present it has been for a whole year and more), 

 then the moon will be in conjunction with the sun in 

 the heavens. When the equations on the mechanism 

 are examined, the sun and moon shall be found to 

 be in the same degree of longitude, and in the same 

 part of a degree. There will also be an ecliptic new 

 moon that is in conjunction with a solar eclipse, or 

 rather with a terrestrial eclipse. This will occur if, 

 at that time, both apexes of the first index, located 

 below the center of the clock, are hidden by the 

 two segments of the circle extending from the center 

 of the mechanism through the lowest index. 



And the eclipse will be greater and greater and, 

 consequently, visible in more regions of the earth, 

 the more deeply the two pointers, indicating the 

 distance of the sun from its apogee, are hidden in the 

 center of the segments. 



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 v\ ill 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. 



And, accordingly, when you observe the little moon 

 of the index inclined to one or other section of the 

 circle, so also in the heavens, the eclipse of the moon 

 is only partial and the northern or the southern part 

 of the moon is illuminated. 



The current time will indicate whether the lunar 

 eclipse is visible or not. As the new moon ecliptic 

 falls during the day, the eclipse will not be visible, 

 since the earth denies a sight of the moon which is 

 below the horizon. But, conversely, if there are no 

 clouds, the eclipse will be visible anywhere, if the 



74 



Bl I I I TIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



