Book on the Universe

of the fourth month, and the shortest at the beginning of the tenth. The oblique circle deviates from the equator, and the shortest days from the average days, by exactly 23 degrees. Thus, the two smallest circles near the poles are distant from the pole of the world, or the equator, by the same amount as the oblique circle is distant from it. They are nothing other than a sign of the revolution of the pole of the oblique circle itself.

The meridian circle is the greatest, divided into 360 degrees. It passes through the poles of the world and the middle of the sky, and it can be placed anywhere at the halfway point between the sunrise and the sunset. When the Sun reaches this point, it is midday in the daytime, and midnight at night. It is divided into four sections of 90 degrees starting from the horizon, which is also called the boundary, because it separates the visible part of the sky from the invisible part. This also divides the equator into four