Puslapio vaizdai

the centre. This tendency is stronger or weaker, in proportion to the velocity with which the particle moves. Now, a particle situated near one of the polar circles makes a rotation in the same space of time as a particle of the equator; the latter, therefore, having a much larger circle to describe, travels proportionably faster, so that the centrifugal force is much stronger at the equator than at the polar circle: it gradually decreases as we leave the equator and approach the poles, where, as there is no rotatory motion, it entirely ceases. Even at the equator, however, there is no danger of our being thrown from the earth, the force of gravity being there 288 times greater than the centrifugal force.

Bodies weigh less at the equator than at the poles. There are two causes for this, the diminution of gravity at the equator, it being at a greater distance from the earth's centre than the poles,and the increase of the centrifugal force; which, as it tends to drive bodies from the centre, must necessarily decrease the power of gravity.


We shall now explain the variation of the seasons, and the difference of the length of the days and nights in those seasons-both effects resulting from the same cause. In moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit; in other words, its axis does not move round the sun in an upright position, but slanting or oblique. This you will understand more clearly, if you carry a small globe round a candle which is to represent the sun.You must consider the ecliptic drawn on the small globe as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, shows the degree of obliquity of the axis of the earth in that orbit, which is nearly 23 degrees. The

points in which the ecliptic intersects the equator are called nodes. The globe at A is situated as it is in


the midst of summer, or what is called the summer solstice, which is on the twenty-first of June. The north pole is then inclined towards the sun, and the northern hemisphere enjoys much more of his rays than the southern. The sun now shines over the whole of the north frigid zone, and notwithstanding the earth's diurnal revolution, it will continue to shine upon it as long as it remains in this situation, whilst the south frigid zone is at the same time completely in obscurity.

Let the earth now set off from its position in the summer solstice, and carry it round the sun: observe, that the axis must be always inclined in the same direction, and the north pole point to the same spot in the heavens. There is a fixed star situated near that spot, which is hence called the North Polar star. The earth at B has gone through one quarter of its orbit, and is arrived at that point at which the ecliptic cuts or crosses the equator, and which is called the autumnal equinox. The sun now shines from one pole to the other. At this period of the year, the days and nights are equal in every part of the earth; but the next step she takes in her orbit involves the north pole in total darkness, whilst it illumines that of the south. This change was gradually preparing as the

earth moved from summer to autumn. The instant the earth passes the autumnal equinox, the long night of the north pole commences, and the south pole begins to enjoy the light of the sun. As the earth proceeds in her orbit, the days shorten and the nights lengthen throughout the northern hemisphere, until it arrives at the winter solstice, on the 21st of December, when the north frigid zone is entirely in darkness, and the southern enjoys uninterrupted day-light. Exactly half of the equator, it will be observed, is enlightened in every position, and consequently the day is there always equal to the night.

Observe, that the inhabitants of the torrid zone have much more heat than we have, as the sun's rays fall perpendicularly on them, while they shine obliquely on the temperate, and almost horizontally on the frigid zone; for during their long day, the sun moves round at no great elevation above their horizon, without either rising or setting.

To a person placed in the temperate zone, the sun's rays will shine neither so obliquely as at the poles, nor so vertically as at the equator; but will fall upon him more obliquely in autumn and in winter than in summer. Therefore, the inhabitants of the earth between the polar circles and the equator will not have merely one day and one night in the year, as happens at the pole; nor will they have equal days and equal nights, as at the equator, but their days and nights will vary in length at different times of the year, according as their respective poles incline towards or from the sun, and the difference will be greater in proportion to their distance from the equator.-During the other half of her orbit, the same effect takes place in the Southern hemisphere, as what we have just remarked in the Northern. When the earth arrives at the vernal equinox, D, where the ecliptic again cuts the equator, or the 22d of March, she is situated with respect to the sun, exactly in the same position as in the autumnal equinox; excepting that it is now autumn in the

Southern hemisphere, while it is spring time with us: for the half of the globe, which is enlightened, extends exactly from one pole to the other. On the two days of the equinox the sun is visible at both poles; but only half of it is seen from either, the other half being concealed by the horizon.


Let us now turn our attention to the moon. This satellite revolves round the earth in the space of twentyseven days eight hours, in an orbit nearly coinciding with the plane of the earth's orbit, and accompanies us in our revolution round the sun. Her motion, therefore, is of a complicated nature; for, as the earth advances in her orbit, whilst the moon goes round her, the moon proceeds in a sort of progressive circle.

The moon always presents the same face to us, by which it is evident that she turns but once upon her axis, while she performs a revolution round the earth; so that the inhabitants of the moon have but one day and one night in the course of a lunar month. Since we always see the same hemisphere of the moon, the inhabitants of that hemisphere alone can see the earth. One half of the moon, therefore, enjoys our light every night, while the other half has constantly nights of darkness; and we appear to the inhabitants of the moon under all the changes, or phases, which the moon exhibits to us.

When the moon is in the same direction from us as the sun, we cannot see her, as her dark side is towards us; but her disappearance is of very short duration, and as she advances in her orbit we perceive her under the form of a new moon. When she has gone through quarter of her enlightened towards the earth, and she

one-sixth of her orbit, one hemisphere will be turned

will then appear horned. When she has performed one quarter of her orbit, she shows us one half of her enlightened side. She next appears gibbous; and after that full. As she proceeds in her orbit she becomes again gibbous, and her enlightened hemisphere turns gradually away from us, till she completes her orbit and disappears; and then again resumes her form of a

new moon.

When the moon is full, she is always in opposition to the sun-when a new moon, in conjunction with it. At each of these times, the sun, the moon, and the earth are in the same right line; but in the first case, the earth is between the sun and the moon in the second, the moon is between the sun and the earth. An eclipse can only take place when the sun, moon, and earth are in a straight line, or nearly so. When the moon passes between the sun and the earth, she intercepts his rays, or in other words, casts a shadow on the earth: this is an eclipse of the sun, and it continues whilst the shadow is passing over us. When, on the contrary, the earth is between the sun and the moon, it is we, who intercept the sun's rays, and cast a shadow on the moon: she then disappears from our view, and is eclipsed.

Why, it may be asked, have we not a solar and a lunar eclipse every month? Because the planes of the orbits of the earth and moon do not exactly coincide, but cross or intersect each other; and the moon generally passes either on one side or the other, when she is in conjunction with, or in opposition to, the sun; and therefore does not intercept the sun's rays, or produce an eclipse: for this can only take place, when the earth and moon are in conjunction near those parts of their orbits which cross each other (called the nodes of their orbits,) because it is then only that they are both in the same plane, and in a right line with the A partial eclipse takes place when the moon, in passing by the earth, does not entirely escape her shadow. When the eclipse happens precisely at the


« AnkstesnisTęsti »