Synesius, On an Astrolabe 4
The text presented here, a letter that accompagnied the gift of an astrolabe, was sent to Pylaemenes, an important military leader whom Synesius had met in Constantinople. It is a brief essay in which Synesius advises politicians to study the sciences (which the author, as always, calls "philosophy"). Synesius also mentions several improvements to the instrument; the model sent to Pylaemenes appears to have been some sort of prototype.
The text is offered in the translation by A. Fitzgerald.
On an Astrolabe 19-26
[19] Now the question of the projection we thought worthy of study for its own sake. We worked it out and elaborated a treatise and studded it thickly with the necessary abundance and variety of theorems. Then we made haste to translate our conclusions into a material form, and finally executed a most fair image of the cosmic advance.
[20] This very manner of approaching the problem gives us the means of cutting a flat surface and the even cavity into identical divisions. And as we think that any sort of hollow is more nearly allied to the completely spherical, we have hollowed the breadth by pressing it in, and have turned it in, and have turned our attention to the rest in such wise that the image side of the instrument may remind the intelligent observer of the reality. For those stars which are distinguished by six dimensions, we have placed therein, preserving their relative shapes. And of the circles, some we drew round, and some so as to intersect the others. Then we divided them by degrees, making the division lines of each five degrees larger than the lines that divide each separate degree; we also enlarged the inscriptions indicating the numbers at these lines, and what is below is on silver, the black giving the appearance of a book.
[21] But they were not all cut in the same line, either individually or with each other. Some are cut equal in size, others irregularly and unequally in appearance, but in calculation uniform and equal. This was a matter of necessity, so that the different figures should agree. On this account also the largest circles which are drawn through the poles and the signs of the tropics, although they remain circles in calculation, have become straight lines by the change in the method of view.{{I.e., by the projection of the three-dimensional celestial sphere on a two-dimensional disk.} Thus the Antarctic circle has been inscribed greater than the greatest, and the relative distances of the stars have been lengthened to the plan of projection.
[22] As to the epigrams, they are of solid gold, and we have carved them and inserted them under the Antarctic circle in such places as are free from stars.note The second of the two, the one in four lines, is ancient, and contains a general eulogy of astronomy:
[23] Mortal I am, well I wot, and ephemeral; yet when I thread my
way through the crowded stars, hemming me this way and that,
no more do I touch with my feet the dark earth, but in union eternal
dwell I with Zeus the most high: mine is ambrosial food.note
[24] But that which takes precedence of this, the stanza of eight lines, was written by the one who constructed the work itself, by me, and it contains a summary and general view of those things which are seen thereon. The description is forceful, composed with more science than nicety of expression, for it seeks to tell the astronomer alone what advantage he can get from the instrument.
[25] It professes to show the places of the stars, not their positions in respect to the zodiac, but in respect to the equator. For it has been shown in my work that it is impossible to take the positions in reference to the former. And it says that the obliquities have been represented, I mean of the parts of the zodiac with respect to the parts of the equator, and for all of them the ascensions also. That is to say, that as many divisions as there are on the zodiac, there are so many in the equator, and the same equator is divided accordingly. And this is the poem - let it be down for such as may read it later, since for you it is enough that it lies on the tablet:
[26] Wisdom has found a path to the heavens - O mighty marvel! -
and intelligence has come from these heavenly beings.
Behold! It has ordered the curved form of the globe
and it has cut the equal circles with unequal spacings.
See the constellations all the way to the rim whereon the Titan,note
holding his kingdom, metes out day and night.
Accept thou the slantings of the zodiac, nor let escape thee
those famous centers of the noontide assemblages.