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.
This is, in some degree, necessary. For in proportion as a planet is further from the Sun, it receives less light and heat, which deficiency is, in part, made up by a more frequent exposition to his rays.
§ 25. The Sun himself is also known to turn regularly on his Axis, and to complete one whole rotation in about Twentysix of our days. This we have been able to perceive from the spots which have been discovered on its surface, and which gradually move toward and disappear on one side, when in a short time after they appear again on the other.
§ 26. The Moon, and the Satellites of the other Planets have no rotary motion; but have always the same side turned towards their Planets. Thus the moon keeps constantly the same side turned towards the Earth; but her monthly motion round the Earth (Lesson III, § 17) is equal to a rotation on her axis; because by this means every part of her is, at least, once every Twentyseven days turned toward the Sun; as you may see from the Moon’s phases, represented on plate [VII]. A day in the Moon, therefore, is equal to Twentyseven of our days; because the Moon moves in Twentyseven of our days round the Earth, which is equal to turning once on her axis.
RECAPITULATION OF LESSON IV.
QUESTIONS.