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

Caibidil 17

Concerning the different sunrise and sunset in many countries:


The sun rises earlier in Babylon than in Egypt and (earlier) in Egypt than in France, and, consequently, it sets earlier in Babylon than in Egypt, and earlier in Egypt than in France.105

To explain this clearly, I will place these three countries in a geometrical figure as an example of the rising and setting of the sun in the other countries of the world, but I suppose a space of six hours to be between Babylon and Egypt and between it again and France, and in the same way again between the other countries. Then I will describe a perfect circle in the form of the earth, and a circle larger than that outside it representing the orbit of the sun, and will place the letter A for Babylon and B for Egypt and C for Africa, and I will write D for the sunrise of Babylon and E for its noon and F for the sunset, and in the sameway, B for the sunrise in Egypt and F for its noon, and G for its sunset, and in the same way again, F for the sunrise of Africa and G for its noon and D for its sunset.


Thus, I declare when the sun rises at point D, it is visible to the inhabitants of Babylon, and is concealed from the inhabitants of Egypt until it reaches point E which is noon in Babylon, and sunrise in Egypt, and midnight in Africa, for then the sun rises with reference to the Egyptians, yet it is invisible to the inhabitants of Africa106 until it reaches point F, which is the end of the day in Babylon, and noon in Egypt, and sunrise in Africa, because day begins then with reference to the people of Africa, and it is evening with the Egyptians and midnight with the people of Babylon. The Africans behold it until it reaches point D which with them is the end of the day and is midnight in Egypt and the beginning of the day in Babylon. And midnight in Egypt and the end of the day in Africa are at one point. At another point is the beginning of the day in Egypt and midday in Babylon and midnight in Africa. In the same way, at one point is the beginning of the day in Africa, and midday in Egypt, and the end of the day in Babylon. In the same way again, at one point it is midday in Africa, and the end of the day in Egypt, and midnight at Babylon.

Thus, according to the order of God, when the sun rises in some country in the world, it sets in another, and it is the roundness of the earth that causes that difference of sunset and sunrise in the world.

Here follows the figure I promised to make: