This is one "Reason Why," assigned by some observers. But there is also another reason to be found in the fact that, when the sun or moon is near the horizon, we view it through a _greater depth of atmosphere_ than we do when at the meridian. (_See_ Fig. 13.) A straight line passed upward through the air, would not be so long as that which passes to S. Consequently, as the air is generally impregnated with moisture, at the time when these effects are observed, the rays of light are caused to diverge more, and the disc of the sun or moon _appears magnified_. Probably both of these reasons contribute to the effect. This latter reason also explains why the disc of the sun or moon may sometimes appear _oval_ in shape, the lower stratum of air being more loaded with moisture than that through which we view the upper part of the disc. 526. _Why do our shadows lengthen as the sun goes down?_ Because light travels only in _straight lines_, and as the sun descends, the direction of his rays becomes more _oblique_, thereby causing longer shadows. 527. _What is the cause of the optical illusions frequently observed in nature?_ There are various kinds of natural optical illusions:-- The _mirage_, in which landscapes are seen reflected in burning sands. The _fata morgana_, in which two or three reflections of objects occur at the same time. The _ærial spectra_, or ærial reflections, &c. [Illustration: Fig. 16.--ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.] The optical illusions above enumerated owe their origin to various atmospheric conditions, in which _refractions_ and _reflections_ are _multiplied_ by the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree. [Verse: "In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."--MATT. XVI.] 528. _Why do we have twilight mornings and evenings?_ Because the coming and the departing rays of the sun are _refracted_ and _reflected_ by the upper portions of the atmosphere. (_See_ Fig. 13.) 529. _How long before the sun appears above the horizon does the reflection of his light reach us?_ The time _varies_ with the refracting and reflecting power of the atmosphere, from _twenty minutes_ to _sixty minutes_. But the sun's position is usually _eighteen degrees_ below the horizon when twilight begins or ends. 530. _Why is the sky blue?_ The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a _polarised_ condition of light which imparts to vision the sensation of a _delicate blue_. (_See_ 549.) 531. _Why do the clouds appear white?_