Part 5

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. 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.

To what may the double motion of the Planets’ progression and rotary be compared?

What would be the case, if the Earth was always to keep the same relative position with regard to the Sun? What is the advantage derived from the rotation of the Earth on its Axis. Would the Earth be habitable without it or not?

What would be the case, if from the darkness of night we were at once exposed to the luminous rays of the Sun? Could our eyes endure the brightness of noon without a gradual transition from darkness to light?

How do the rays of the Sun strike us in winter? What season, therefore, is it? How do the rays of the Sun strike us in summer? What season, therefore, is summer with regard to temperature? How are the rays of the Sun striking us from spring till mid-summer? How, from mid-summer till winter? What similar change of temperature do we experience every day from Sun-rise till noon, and from noon till evening or Sun-set?

Why is this, in some degree, necessary?

FOOTNOTES:

The propriety of this comparison becomes still more evident, when we reflect that the Axis of the top is generally somewhat inclined to the plane.

If the terms perpendicular and oblique, should not be perfectly understood by the pupils, it will be easy for the teacher to explain their meaning.

If the Earth requires 365 days to travel round the Sun; and each day has twentyfour hours; then the Earth will, during the whole of this time, turn Three Hundred and Sixtyfive times on its axis.

LESSON V.

OF THE APPEARANCE AND PECULIARITIES OF THE MOON AND SOME OF THE PLANETS WHEN VIEWED THROUGH A TELESCOPE.

§ 27. Next to the Sun there is no heavenly body so interesting to us as the Moon. When viewed through a good Telescope she has nearly the same appearance as in Figure I, Plate X. The bright parts are supposed to be lofty mountains and tracts of land; (which is evident also from the shadow which they cast) and the dark spots are supposed to be valleys and caverns. Many of the mountains of the Moon are higher than the largest mountains on the Earth. Some of them are volcanos, and their eruptions have been distinctly observed by many distinguished Philosophers. Some of the caverns are ascertained to have a depth of many miles and a width of almost Three miles.—No water has as yet been discovered in the Moon. Hence if she is inhabited, as we have reason to believe, her inhabitants must be very differently constructed from ourselves.

§ 28. Among the Planets Venus is by far the most beautiful in appearance. She is known also by the name of the Morning and Evening Star. Her light is so bright that she is often seen at Noon. When viewed through a good telescope she exhibits phases similar to those of the Moon (Lesson III, § 17), which proves her spherical form (Lesson I, § 3). The mountains in Venus have been calculated to be at least Six times as high as those on our Earth. Her Atmosphere is only half as dense as ours.

§ 29. Mars appears in many respects similar to our Earth. His light is red and changeable; his surface exhibits black changeable spots (see Figure II, Plate X). Some philosophers pretend to have noticed a region of ice on his poles. His atmosphere is twice as dense as ours.

N^o. X. ]

§ 30. Jupiter, viewed through a telescope, exhibits a surface covered with stripes. These are supposed to be clouds. A representation of them is given in Fig. III, Plate X. His light is very white and subject to but little variation. His atmosphere is nearly Twentyseven times denser (thicker) than ours.

§ 31. Very remarkable, as we have already observed, is the Planet Saturn, on account of its luminous ring. Viewed through a telescope it has the appearance, represented in Figure IV. It is highly probable that to the inhabitants of that Planet, this ring has an entirely different appearance from what it has to us. It appears to be a solid opaque mass, and is probably inhabited like the Planet, which it constantly accompanies on its journey round the Sun. Saturn’s atmosphere is nearly Ninety times denser than that of our Earth. No mountains have as yet been discovered on its surface.