time, it is close to the sun, so its light is faint, and the redness of the sun’s rays partly impedes its visibility. As it descends towards the horizon, it moves farther from the sun, strengthening the condition that favours visibility, while the impediment remains. Its light thus increases, and it moves farther from the sun’s rays. If someone supposes that it cannot be seen at sunset or immediately afterwards, it may nevertheless be seen later, even as it descends in the west.

If he claims that its state can be precisely determined from the time the sun sets until the crescent itself sets, he can only determine the number of those degrees, since it remains above the horizon by the measure of the distance between them. As for the amount of light it gains and the extent to which the rays impeding its visibility diminish—for these are what make sighting possible—he can never determine these according to a single fixed rule under which sighting is always possible or always impossible. Nor is this, in itself, something subject to precise determination, especially when the sun is … (1).

The fifth factor:

The clarity or haziness of the atmosphere. I do not mean the presence of something that prevents sighting, such as clouds or the haze rising from smoke and vapours. Rather, I mean that, when atmospheric conditions permit it to be seen, some conditions make sighting easier than others: when the atmosphere is entirely clear of haze, as it is in winter after rainfall in open country where there is no vapour, unlike when the atmosphere contains vapour such that it is not possible