On the Apparent Magnitude of the Sun when Low and High

I would contribute nothing concerning the affection: a medical or philosophical term for a temporary state or condition of the eye. Therefore, distinguished man, allow me to transcribe here the article in which I contended to address both causes. After certain preliminary matters were assumed, my words were as follows:

First, it seems possible to say that the low Sun appears larger to the eye than when it rises higher because, while it is near the horizon, there is a long series of vapors and small particles. These particles blunt the rays of the Sun so that the eye squints less. The pupil, as if in shadow, is much more dilated than when the Sun is high and few vapors are intercepted. When the Sun is high, it shines so brightly that the pupil becomes most contracted when looking at it. Indeed, this seems to be the reason why the visible species: the image or form of an object that travels from the source to the observer proceeding from the Sun and entering through the dilated pupil into the Retina: the light-sensitive layer at the back of the eye occupies a larger space on it. Consequently, it creates a larger appearance than when it enters through a contracted pupil.

Second, it seems possible to say that a larger shadow is cast by the Sun when it is in that position because the image of the Sun actually decreases due to the particles of vapor cutting off its rays. Meanwhile, neither the shadow nor the pinnacidium: a sighting vane on an astronomical instrument, being inanimate things, are subject to the same conditions as the organ of sight. Indeed, if you look at the border between light and shadow, you can observe that the light there is very weak. This is because that part of the space is only illuminated by the outer edge of the Sun. The light is stronger where it is further away because that space is reached by a more abundant portion of the Sun. Finally, the light further away is most splendid because that space receives light from the entire Sun, since nothing opaque hinders it.

Furthermore, if it happens that the rays, which are emitted almost individually from the very outer edge of the Sun