§ 22. The rotation of the Earth on its Axis is also the cause of the _Rising_ and _Setting of the Sun_. For no portion of our Earth is at once turned toward or from the Sun; but moves toward or from it by degrees (as you may see by slowly turning a ball near the flame of a candle). This gradual motion of each portion of the Earth’s surface toward or from the Sun, makes the Sun himself appear to us as rising and going down; while, in fact, we ourselves are turning towards, or receding from him. This is a kind of deception similar to that which you experience when slowly gliding down a river; when the objects on shore have the appearance of receding from you, while in fact, it is you, yourself, who are travelling away from them. [Illustration: _N^o. IX._ ] The gradual rise and setting of the Sun is another excellent provision of nature. Were we from the darkness of night at _once_ exposed to the luminous rays of the Sun, it would dazzle our eyes and render them unfit to distinguish a single object.—It is only by a gradual transition from darkness to light that we are able to accustom our eyes to the brilliancy of noon. § 23. There is another peculiarity in the situation of our Earth with regard to the Sun, which you have not yet learned. The Earth’s Axis is not even (parallel) with that of the Sun; but is somewhat inclined towards it, as represented in the last Diagram. To this is owing the _Change of the Seasons_. For on account of the inclination of the Earth’s Axis, the Sun’s rays fall, sometimes nearly perpendicular upon us, while at other times they are striking us more obliquely[5]. This is the principal cause of those changes of temperature which we are in the habit of distinguishing by the names, _Spring_, _Summer_, _Autumn_ and _Winter_. In winter the Sun’s rays strike us most obliquely; it is therefore the coldest season of the year. In summer they are most perpendicular;—Summer therefore is the hottest season. Spring and autumn are standing in the middle between these two. From spring till mid-summer the Sun’s rays are striking us more and more perpendicularly; from mid-summer till winter more and more obliquely. A similar change of temperature is felt every day from Sun-rise (when the Sun-beams are most oblique) till noon, (when they are most perpendicular); and from noon again towards evening or Sun-set, when they are again oblique. § 24. It has been mentioned (Lesson IV, § 20), that each Planet in our Solar System is regularly turning on its Axis. But all of them do not perform this rotation with equal _velocity_. The Planets which are farther from the Sun are turning quicker than those which are near him. Jupiter, for instance, turns on its Axis twice as fast as our Earth. The nights in Jupiter, therefore, do not last half as long as ours.