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

Caibidil 7

44

ARGUMENTUM AD ROTUNDITATEM TERRAE, ET CETERA.

It is a certain indisputable argument to prove the roundness of the earth, that the rivers run and flow over the surface of the earth. Because if the earth were a flat level surface with no convexity on it, as ignorant men have declared, the rain, which comes from the clouds and which is the cause of rivers, would form one large permanent expanse of sea on the surface of the earth, and would not flow from place to place as it does in now. Therefore, since it flows and does not remain in one place, let it be understood for certain that the earth is round and convex.45

Another argument to prove the same thing— If you journeyed from the centre of the world to the North Sea, there would there be discovered to you stars that you never saw in the centre of the world, and some of the Southern stars that you saw in the centre of the world would be concealed from you. So if you made the same journey to the South, there would be discovered to you stars you did not see in the North or centre of the world, and the stars that you saw in these places would be concealed from you. Thus it is certain that it is the convexity of the earth, rising behind you on your journey, that discovers to you the stars before you, and conceals from you the stars behind you.

More on the same subject:—In every place you are throughout the earth, you see some portion of the firmament you did not see anywhere else, and it is proved from that, that the curve around the earth is spherical, and therefore the earth is in the middle of it. Of the same subject still, I add that, on every course which the sun makes around the earth, it illumines the half of the earth, that is exactly opposite it, and it is that light between the sun and the earth


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which is always day; and I say that the other half of the earth is always dark, owing to the shadow of the earth, and it is that dark shadow which is always night. Thus whatever course the sun makes around the earth, day follows it and night flees before it to the other side of the earth. Hence, the people over whom the sun passes, see sun and day, and they who are at the other side of the earth see stars and night, therefore those things can never be seen during the same course, i.e., sun and stars, night and day, because when we have day on the upper part of the globe there is night in the part beneath, and vice versa.46 Consider carefully the expression which I used—'beneath the earth'. For everything that is down underneath the earth, is the earth or one of its parts, and everything that is on every side of the earth, out from the earth, is above. Therefore the earth alone with its parts is below and all the rest of the works of God, on whatever side of the earth they are, are above. Thus on whatever point of the globe people stand, their heads are up and their feet down. Whoever may declare that the earth is a level plane without convexity, whilst sun rises on one side of it and goes down on the other, I say that that cannot possibly be true, and that no reason or argument can be found to prove it; because if this theory were true, the sun would appear small when rising, and according as it mounted higher and approached nearer to us, it would appear larger to us. It is clear to every intelligent person in the world that this is untrue, since we see that it the sun is of equal size in the east and west and north of the world.

From this it is proved that the earth, and the course of the sun around the earth, are spherical, and to make this more clearly understood, I shall make a geometrical figure here below, and first I shall make the round figure of the circle of the globe, and write E in the middle of it—in the centre of it, and around that I shall describe a larger circle than that representing the orbit of the sun and place A in the west and B in the top and C in the east of it, and I shall describe a small circle representing the circular body of the sun, beside each of those letters, and then I shall draw three lines from the centre of the earth to the surrounding circle of the sun, one of those lines to A, the second to B, and the third to C. And it is evident to everyone who considers them that those three lines are even—equal each to each.

Therefore the earth is equidistant from each, wherever the sun is, at its rising or setting, or when it is at the highest point of its course, and thus it is evident that the earth, and the circuit of the sun around the earth, are equidistant.


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Whoever should declare as an argument against this that the sun appears distinctly larger47 when rising or setting than it does at the highest point at mid-day, and that it is understood from this that it is further away at mid-day, than when it is in those other quarters, and that this proves that the earth is a level plane without convexity, I reply to him appropriately, in giving a solution for that argument, that that often happens, but not always, and when it does happen the reason is—when the sun is rising or setting, it draws up the moisture and the rain and black wet vapours rise to a great height between us and it, and then, when we look at the sun, that mist which is seen broadens and amplifies the sphere of vision within it48, therefore, according to the denseness and materiality of that mist, does the sun appear larger through it, than it would appear without that mist being present. As the day advances, and the sun is at the highest point of the firmament with no mist between us and it, then we see it with its own proper size.

The example is clearly illustrated in the case of the naked person under water, because he appears larger to the sight under water than out of water; although there is no proof in that, except the fact of the wet dense water spreading and amplifying the sight, and preventing it from passing directly and naturally towards the person. The same reason is the cause of an object appearing larger and thicker through glass than otherwise. Consequently old people, who are losing their sight so that they cannot read small letters, use glass spectacles49 to magnify the letters they read, and for the same reason the sun appears larger in the early morning and in the evening than at mid-day, as I have mentioned.

If any ignorant person should make the same statement, i.e., that the earth is a level plane and the sun a round orb encircling the earth, and that the people of the world in general can see it at the same time as it would rise in one place, I should say that that was false, if it were stated. To understand it, imagine two cities in your own mind, one in the east and the other in the west of the world,


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and imagine if the earth were a level plane without convexity (as this opinion has hitherto maintained), that the people of the city in the east of the world would see the sun rise in their own proximity as a large mass, and, having traversed its circuit, they would see it setting in the west as a smaller mass. Vice versa, the people of the city in the west of the world would see the sun rise as a small mass, on account of its being distant from them, and set close to them, as a much larger mass on account of its proximity to them. But we have never heard of any such thing. In addition to this we may point out50, the first half of the day would seem shorter to the people of the eastern city than the latter half. In the same manner the latter half of the day would seem shorter to the people of the western city than the first half.

We, and the learned, have an unquestionable proof that the day is of equal length at equal distances from the middle point of the day at those two cities, and in every place in the world, and that it is error and lack of knowledge which caused the other opinion to be upheld. It is clearly proved from that equality of the first and last half of the day that the sun moves in a circular orbit51 about the spherical earth52.

To further illustrate this subject, and to confute that theory, I will make a geometrical figure here below. First I will draw a straight line called the surface of the earth, and above it a circle, which I will call the path of the course of the sun, and I will make a diagram of a city on the eastern end of the line and write the letter A above it, and in the western end of the same line I will make a diagram of another city and write B above it, and C at the point of the rising of the sun, and D at the point of its setting, and E at mid-day of the eastern city, and F at mid-day of the western. Consequently when the sun rises at a point C, and advances to E, the first half of the day in the eastern city is spent, and the second stage, from E to D has not arrived, and again when the sun rises at a point C and advances to F, the first half of the day in the western city is spent and the second stage from F to D has not arrived, accordingly day at each of those cities would have one part longer than the other, because it is much longer from E to D, the


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last part of the day in the eastern city, than from C to E, the first part of the day in the same city. Again, it is much longer from C to F, the first part of the day in the western city, than from F to D, the last part of the same day. And it is evident to everyone on earth who wishes to examine it closely that that is untrue, therefore the statements out of which it arises are erroneous, i.e., that the earth is a level plane without convexity, and therefore, in the interests of truth, it is declared that the earth is convex and spherical.

As a proof that it is true, the sun does not set in the same place in regard to any two cities in world, and, if you change your position, you change the sunset in regard to you. As a proof of that — If you were in the city of Jerusalem, in regard to you, the sun would set in Rome, and if you were there, it would set in the west of France, in regard to you, and on your being there, it would set in the west of Spain in regard to you. After that some place in the Atlantic Ocean would conceal it from you, and if53 sea could support you, and if you could follow the sun, it would change its setting in regard to you every day that you would follow it, until at last it would set in the place in which you saw it rising, when you were in the city of Jerusalem.54

It is certain there is no difference in the sunset, but the convexity of the earth constantly coming between us and it causes it, because if the earth were a level plane according to that theory just mentioned, there would be only one place where the sun would rise in regard to the inhabitants of the world, and one place where it would set. Consequently since there are a number of places where it rises and sets, the earth must be round and not a level plane.

Therefore the Eternal First Cause who ordered it thus, blessed be He forever.55