810

Retreat of the Line of Nodes.

the perigee and the apogee. The effect of this disturbance is represented as making the moon's ellipse wheel round in the same direction as the moon herself revolves. The extent of the disturbance is such that the orbit performs an entire revolution in nine years, but not at a steady pace. This is the revolution of the line of apses. The inequality is termed Evection. In amount it is such that the moon's place may be affected to the extent of nearly three diameters (1° 20′).

The eccentricity of the moon's orbit is itself disturbed at the same time, being sometimes above, and sometimes below the average, to the extent of one-fifth of the whole. The mean distance of the moon from the earth is not disturbed from the same cause.

The foregoing inequalities would all occur although the sun, earth, and moon were always in the same plane; that is, if the moon's orbit round the earth coincided with the earth's orbit, or the ecliptic. But as the the moon's orbit is inclined to the ecliptic at or about an angle of 5°, the sun exerts a new kind of disturbance-it pulls the moon, as it were, back to the plane of the ecliptic; the effect being to accelerate its crossing the plane, or to make it cross on each occasion sooner than it ought to do, that is, before it has got round an entire half-circle from the last crossing. This is represented as making a revolution backwards of the line of nodes. The retreat of that line is at such a pace that it completes its backward circle in about nineteen years. The great importance of this period consists in its determining the recurrence of eclipses in a regular series.

While the line of nodes has its regular backward course, the inclination of the orbit is varied to the extent of about 8'.

The Tides.

1044. The nature of the tides, as a matter of fact, is well known. All over the ocean the water rises and falls on the coasts twice in rather more than twenty-four hours. Kepler observed that there was a coincidence between the tidal periods and the positions of the moon, and Newton supplied the mechanical explanation.

In the phenomenon of the tides there is this paradoxical fact, that while on the side of the earth where the moon is seen, the water rises towards her, making there a flood tide of many feet in depth, on the opposite or distant side of the earth, away from the

moon, the water similarly rises to an equal height. The fact is pictured in the adjoining diagram. M is the moon, B A the earth, and the protuberances of the water at A and B are the flood-tides in both situations at the same time. Thus, although the moon appears over any part of the earth only once in twenty-four hours, the floodtide occurs there twice. The explanation of the tide at B on the side of the earth towards the moon offers no difficulty, and the tide at A on the distant side of the earth is the effect of the centrifugal force of the water at A not fully balanced by the attraction of the mocn, which is nearly 8000 miles more distant from A than from B.

A

M

E

Fig. 320.

The tide on the near side, therefore, may be called the centripetal tide, and the other the centrifugal. It is also to be remarked that when there is an accumulation of water or flood-tide on two opposite sides of the earth, there must be a deficiency of water, or ebb-tide, on the intermediate parts. It was explained in a former page (116) that when two mutually-attracting or cohering bodies are revolving round each other they both really are revolving round the common centre of gravity of the two, here marked C.

The sun also produces tidal disturbance in the waters of this globe, but not nearly so great as that made by the moon, because of the vastly greater distance of the sun. When, however, the two influences either join or are in direct opposition, the stronger effects are produced called the spring-tides and the neap-tides.

If the earth were all covered with ocean, the two tidal waves would rise and subside uniformly all round it, but owing to the irregular forms of the land and of the sea-bottom, the simple progress is prevented. A striking phenomenon connected with the tides is, that when the great tidal wave enters a broad inlet or bay which gradually contracts towards the inner end, as in the Bristol Channel, such wave becomes higher as it is compressed by the narrowing channel-in some places to the depth of nearly 100 feet

812 Observations showing the Precession of the Equinoxes.

as in the Bay of Fundy, in North America. Into the mouths of the Ganges, below Calcutta, the tidal wave often enters with steep front and great violence, forming what is called the bore, and causing great commotion among the boats and shipping in the river. In the silence of the night sometimes a person may hear the distant sound of the incoming flood, and may feel surprised to reflect that the apparently small moon, looking down from a calm sky, is really the cause of the uproar in the gulfs and rivers.

Precession of the Equinoxes.

1045. It has been seen that the changes of the seasons are due to the fact that the earth does not spin upright; its equator (the plane of its rotation) is not the same as the ecliptic, the plane of its revolution. The two are inclined at an angle of 231°, called the angle of the obliquity of the ecliptic. The points where the circle of the ecliptic is cut by the plane of the earth's equator are of great importance in astronomy. They are the places where the sun lies at the time of the equinoxes. They are also the land-marks for determining the localities of the stars, as regards their east and west positions (longitude when measured in the ecliptic, right ascension when measured in the equator). The starting-point is the sun's place at the March or vernal equinox. Now, a comparison of ancient observations shows that the measured places of the stars have undergone a steady change; their longitudes, or distances from the spring equinox, have all increased. Since the time of Hipparchus, about two thousand years, the increase has amounted to about a twelfth part of a circle, or a sign of the zodiac. It amounts to 50 seconds of a degree annually, or a degree in 711 years.

The effect would obviously arise, supposing the equinoctial points were to retreat at the rate indicated the stars are where they were before, but the point of reckoning is moving backwards, or from east to west. This fact is called the Precession, or anticipation of the Equinoxes. The equinox does not fall at the same solar position one year as it did the previous year, but is in advance, according to the rate above given. The nodes, or crossing points, are not fixed, but shifting, and make a circuit of the entire heavens in about 26,000 years.

1046. The fact had long been known, but there was no explana

Cause of Precession.--Nutation.

813

tion given of it, until Newton's discovery of gravitation, of which it is one of the many far-reaching consequences. It is a disturbance of the earth's rotation by the combined attraction of the sun and the moon, directed upon the equatorial bulging.

If the earth were perfectly round, precession would not happen. It is the ring of solid matter that surrounds the equator that is disturbed, as if it were a satellite going round the earth in an orbit inclined to the ecliptic. The disturbance is of the same character as that particular disturbance of the moon's orbit arising from its inclination, and has the same result, namely, a retreat of the nodes, or points of intersection of the two planes.

If the earth had no rotation, and if it were, nevertheless, bulged at the equator, the attraction of the sun and moon upon this bulging would reduce the inclination of the two planes; the equator would become parallel to the ecliptic. But this effect is not produced upon a rotating body; the inclination of the axis is not disturbed; just as a spinning-top does not fall on its side, although it spins slantingly. The real effect is best conceived by the supposition of a revolving satellite in an inclined orbit, as the moon. Such a body under a disturbing influence, so directed that part of it is perpendicular to the plane of the ecliptic, is brought precipitately down to the ecliptic, instead of pursuing its course until it reaches the exact point of the previous crossing.

1047. Superadded to the precession, is a secondary disturbance, called the nutation of the earth's axis, which causes, in the motion of the nodes, a fluctuation having a short period. As the great disturbing agent in precession is the moon, and as her tendency is to make the nodes retreat in her own plane, and not in the ecliptic, the actual precession is a combined phenomenon; it is governed in part by the retreat of the moon's plane upon the ecliptic; and as this follows a period of nearly nineteen years, the deviation induced by it will have the same period. It was observed by Bradley that the declinations (distances from the celestial equator) of the stars increased for nine years, and then decreased for nine years following, the maximum being 18 seconds of a degree. This, therefore, constitutes a farther correction to be made to the observed places of the stars, in order to obtain their real places.

814

The Members of the Solar System.

SECTION V.

The earth is the third, as to distance from the sun and as to length of year, of a series of like globes, called PLANETS, which are very various in size, some larger, some smaller, than the earth. There also revolve round the sun other masses, very peculiar in their character, called COMETS.

1048. The adjoining diagram will give a general notion of the arrangement of sun and planets, which constitute what is called the SOLAR SYSTEM. It represents what an eye would see if looking down on the system from a great elevation on its north side. The central figure marks the place of the sun, and the numbered circles around it indicate the paths or orbits of the different planets moving in the direction shown by the arrows, as they would appear if every planet left a line of light where it passed along. The nine chief planets have their orbits here numbered. The following fanciful names were early given to them from the heathen mythology, and although without distinct meaning, are still retained :— 1, Mercury; 2, Venus; 3, the Earth; 4, Mars; 5, a crowd of smaller bodies called planetoids, like parts of a large planet not yet cohering; 6, Jupiter; 7, Saturn; 8, Uranus; 9, Neptune.

And

All these bodies are in form globular, proving that the law of gravitation, which accounts for the form of the earth, as explained in a former section, is active throughout universal nature. they are all restrained in their orbits by the same balance of centripetal and centrifugal forces which rules elsewhere.

1049. The Sun, as already stated, is the only self-luminous body in the system, and, therefore, the most important in it. The sun's presence makes day, its absence night; its more direct beams make summer, its less direct make winter; and on its numerous influences depend the life and well-being of the animal and vegetable creation.

Thinkers have speculated on the probable mode of origin of this glorious system, inquiring whether it may have come into existence at once, nearly as we now see it, by the will of its Divine Author, or whether, like all the subordinate objects, of which men may witness the beginning, it was built up gradually by successive additions of minute particles, in obedience to laws which Divine Wisdom had ordained.

The most majestic oak now standing in any forest was once only a small acorn or seed; the whale, the elephant, the lion, and even