to see it, as happens in summer because of vapours and smoke; under such conditions it cannot be seen as it can in clear air. As for facing it correctly, knowing where it appears, and similar matters, these are things that an observer can learn or endeavour to determine.
It may therefore be said that these are prerequisites for sighting, like gazing intently westward behind the sun. We have thus not included them among the causes of differences in sighting. We have mentioned only those factors that observers cannot fully encompass: the quality of their eyesight, their numbers, the place and time of observation, and the clarity or haziness of the atmosphere. If visibility is an outcome jointly determined by these factors, none of which enters into the calculator’s calculations, how can he nevertheless make a universal claim that no one can possibly see the crescent when he finds it at seven, eight, or nine degrees? Or how can he state categorically that it will be seen when it is at nine or ten degrees, for example?
This is why you find them disagreeing about the arc of visibility: how high must it be? Some say nine and a half, while others say … (1). They also need to distinguish between summer