Corpus of Electronic Texts Edition
An Irish Astronomical Tract (Author: [unknown])

Caibidil 16

Qui perfecte circulos lineasque etcetera.

Whoever could perfectly understand the orbits, lines and points of the firmament, would understand without doubt the nature of the whole firmament, and the proper way to understand it is to consider its form and shape as it is in itself, and ponder it carefully from the inside in your reason and mind. The position of the first orbit of the firmament is as follows:—From the eastern point to the central upper point above the earth, and from that to the western point, and from that to the central lower point beneath the earth, and from that again to the eastern point whence it began at first. That circle is called orientalis and occidentalis,

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i.e., the eastern orbit or the western orbit, and is also cal1ed the orbit of the straight line, because when the sun is in that straight line, day and night are equal in the countries of the whole world. The situation of the second orbit is from the point of the Arctic Pole to the upper point of the firmament, and from that to the point of the Antarctic Pole and from that to the central lower point of the firmament beneath the earth, and from that to the point of the Arctic Pole whence it previously began. There are three other names which philosophers apply to that orbit—septentrionalis, australis and meridionalis—the northern orbit, or the southern orbit, or the orbit of the middle of the day. The situation of the third orbit is from the eastern point of the firmament to the point of the Antarctic Pole, and from that to the western point of the firmament, and from that to the point of the Arctic Pole, and from that to the eastern point of the firmament. This orbit is called circulus terminorum or circulus signorum —the orbit of the termini, or the orbit of the signs.

This is the position of the first of the three lines of the firmament, from the eastern point of the firmament through the middle point of the earth, to the western point of the firmament. The second line from the central upper point of the firmament above the earth through the middle point of the earth, to the central lower point of the firmament beneath the earth. The third line from the Arctic Pole through the middle point of the earth to the point of the Antarctic Pole.

Here below are the seven points of the firmament—six of them in the six places where the three orbits I mentioned cross each other, and where the six ends of the three lines I mentioned are. The seventh point is the centre of the earth, which is the centre of the whole universe. The first of these points is situated

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in the east of the firmament in the place where circulus terminorum and circulus orientalis or occidentalis cross each other. The second point in the centre top zenith of the firmament, over the earth, in the place where circulus orientalis or occidentalis and circulus septentrionalis or australis cross each other. The third point, in the west of the firmament, in the place where circulus orientalis or occidentalis and circulus terminorum cross each other. The fourth point, in the centre bottom of the firmament, below the earth in the place where the circulus septentrionalis or australis and circulus orientalis or occidentalis cross each other. The fifth point, in the north of the firmament, in the place where the circulus septentrionalis or australis and the circulus terminorum cross each other. The sixth point, in the south of the firmament, in the place where circulus terminorum or circulus septentrionalis or australis cross each other. The seventh point, as I remarked, is the centre of the earth which is the centre of the whole universe, where the three lines I mentioned cross each other; and that is the situation and description of the three orbits and the three lines of the seven points which I mentioned above. Whoever could understand them perfectly would understand the nature of the whole firmament.