Plato and other Philosophers said, indeed, that our sight was not because the visible came into the eye, but because the visual virtue went out to the visible form. And this opinion is confuted by the Philosopher in that book of his on Sense and Sensation. Having thus considered this law of vision, one can easily perceive how, although the star is always in one way bright, clear, and resplendent, and receives no change whatever except that of local movement, as is proved in that book on Heaven and the World, yet from many causes it may appear dim and obscure; since it may appear thus on account of the medium, the atmosphere, that changes continually. This medium changes from light to darkness, according to the presence or absence of the Sun; and during the presence of the Sun the medium, which is transparent, is so full of light that it overpowers the star, and therefore it no longer appears brilliant. This medium also changes from rare to dense, from dry to moist, because of the vapours of the Earth which rise continually. The medium, thus changed, changes by its density the image of the star, which passes through it, makes it appear dim, and by its moisture or dryness changes it in colour. In like manner it may thus appear through the visual organ, that is, the eye, which on account of some infirmity, or because of fatigue, is changed into some degree of dimness or into some degree of weakness. So it happens very often, owing to the membrane of the pupil becoming suffused with blood, on account of some corruption produced by weakness, that things all appear of a red colour; and therefore the star appears so coloured. And owing to the sight being weakened, there results in it some dispersion of the spirit, so that things do not appear united, but scattered, almost in the same way as our writing does on a wet piece of paper. And this is the reason why many persons, when they wish to read, remove the paper to some distance from the eyes, in order that the image thereof may come within the eye more easily and more subtly, and thereby the lettering is left impressed on the sight more distinctly and connectedly. For like reason the star also may appear blurred; and I had experience of this in the same year in which this Song was born, for, by trying the eyes very much in the labour of reading, the visual spirits were so weakened that the stars all appeared to me to be blurred by some white mist: and by means of long repose in shady and cool places, and by cooling the ball of the eye with spring water, I re-united the scattered powers, which I restored to their former good condition.
And thus, for the reasons mentioned above, there are many visible causes why the star can appear to us different to what it really is.