###### Corpus of Electronic Texts Edition

##### An Irish Astronomical Tract (Author: [unknown])

### Caibidil 35

## Ad haec indicanda
geometrica sunt.

To pursue this study, it is necessary to obtain geometrical arguments, in
which we can believe without doubting. I will make then a figure of the
earth, and I will place E in the centre of it, and I will describe another
circle from the north of it to the south, and draw a straight line from the
Arctic Pole to the Antarctic Pole through the earth and through the orbit of
the earth, and place A at the top of the firmament, and B in the north pole
of the circle, and C down below, and D in the south pole.

Therefore, whosoever being in position E, should take the astrolabe in
his hand (for with it will be obtained full certain knowledge of this
matter), and placing his face along the middle line of the astrolabe which
he holds suspended by a thread from his thumb, and beholding the Arctic Pole
through the two holes of its two surfaces, would find that pole level with
the earth; and if you travel three score six and two-thirds of a mile from E to B and
then place the astrolabe opposite the Arctic Pole, and look through it as you
did before, you would find it to be six degrees in height over the earth

p.135

and one of the three hundred and sixty degrees of the astrolabe proves it to
be thus.
Again, if you move another three score six and two-third miles from that
towards B, and place the astrolabe opposite the same pole, and look as
before, you will find two degrees in height overhead, and so on, always,
from E to B, for every three score six and two-third miles until one would
reach B, one would find the same pole increasing in height by one degree.
The amount of all those miles put together in accordance with the amount of
the three hundred and sixty degrees which are in the circumference of the
sphere of the earth, make 24,000 miles, which is the circumference
measurement, including the orbit of the water and of the land. And the **alkoterra**, i.e., the diameter of the earth, is eight
thousand miles, and, accordingly, it is four thousand miles to the centre of
the earth, and for every mile of those *to the centre*
there ought to be three and one seventh miles *of circumference*.