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

Caibidil 26

The characteristic of the light of the moon and of the constellations:


I declare that the moon and the stars appear within the same time and season, bright in one country, and dark in another. The reason why the people of one country see them with their light, is because the night is on their side at the same time. The daylight prevents the people of the other country from seeing them, yet it is not the daylight that causes that, but the weakness of men's sight. Thus the moon and the stars are concealed from the country in which the sun is visible during that period, and when it the sun is concealed, it renders them visible. And thus is fire and every other light-giving object; though they are far from you at night their light is prodigious and though they were near you in the day their light is not great. Then if you mean to see the stars by day, go in the morning or evening into a dark deep pit, and look up,


and you will see them clearly over your head at the top of the firmament. Also you will see them clearly at the time of an eclipse of the sun137.

Now I will describe a circle for the orbit of the fixed stars in which I will place a diagram of many of the great stars. Inside of it, I will make another sphere for the sphere of the sun, in which I will place the sun itself, and inside of that I will make another sphere for the sphere of the moon, in which I will place the moon itself at the end of the thirtieth day from the beginning of the month. I will make a fourth sphere within these, and write I in the centre of it, and A in the east of it, and B at the top of it, and C in the west of it, and D at the bottom of it, and let four cities be represented by the four small circles on the orbit of the earth and four letters which mark these four cities; E the eastern city and F the upper one, and G the western city, and H the lower one; and thus when the sun sets, its light leaves city F and the beginning of night approaches it, then it is midnight in the city E, and at that time the inhabitants of the other city behold the stars with their full light and the moon rising in the east, half illuminated, and when the moon reaches city H it is then at the top of the firmament, and when the sun rises in the east of that city the inhabitants of the city cannot see the moon or the stars. When the sun arrives over city G, it is at the top of its orbit, and then it is mid-day, and the moon is there in the western half of the world over city H, and the stars do not show their light to the inhabitants of city G138.