Solar System


In the above picture we have represented the planets of the Solar System as we should see them from the earth if the human eye could grasp a space of such immensity. The spectator is supposed to be standing on the earth, and the moon is in the foreground, 240,000 miles away. The planets are in their order outward from the sun, and vary in distance from 40,000,000 miles, in the case of Mars, to 2,700,000,000 miles in the case of Neptune. From the bottom upward, the planets are Mercury, Venus, Mars, Jupiter, Saturn and its rings, Uranus and Neptune.

This is the name given to that collection of worlds which, with the sun as a centre, circulate at different distances from and around it. There have at present been discovered eight large planets and thirty-four smaller ones called "asteroids" or "planetoids," the larger planets revolving in the following order from the sun or centre:—

1.Mercury. 5.Jupiter.
2.Venus. 6.Saturn.
3.The Earth.           7.Uranus.
4.Mars. 8.Neptune.

Diversities of the Planets

In illustration of the great diversity of the physical peculiarities and probable condition of the planets, Sir John Herschel describes the intensity of solar radiation as nearly seven times greater on Mercury than on the earth, and on Uranus 330 times less; the proportion between the two extremes being that of upwards of 2000 to 1. Let any one figure to himself, (adds Sir John,) the condition of our globe were the sun to be septupled, to say nothing of the greater ratio; or were it diminished to a seventh, or to a 300th of its actual power! Again, the intensity of gravity, or its efficacy in counteracting muscular power and repressing animal activity, on Jupiter is nearly two-and-a-half times that on the earth; on Mars not more than one-half; on the moon one-sixth; and on the smaller planets probably not more than one-twentieth; giving a scale of which the extremes are in the proportion of sixty to one. Lastly, the density of Saturn hardly exceeds one-eighth of the mean density of the earth, so that it must consist of materials not much heavier than cork.

Jupiter is eleven times, Saturn ten times, Uranus five times, and Neptune nearly six times, the diameter of our earth.

These four bodies revolve in space at such distances from the sun, that if it were possible to start thence for each in succession, and to travel at the railway speed of 33 miles per hour, the traveller would reach

Jupiter in 1712 years
Saturn 3113
Uranus 6226
Neptune 9685

If, therefore, a person had commenced his journey at the period of the Christian era, he would now have to travel nearly 1300 years before he would arrive at the planet Saturn; more than 4300 years before he would reach Uranus; and no less than 7800 years before he could reach the orbit of Neptune.

Yet the light which comes to us from these remote confines of the solar system first issued from the sun, and is then reflected from the surface of the planet. When the telescope is turned towards Neptune, the observer's eye sees the object by means of light that issued from the sun eight hours before, and which since then has passed nearly twice through that vast space which railway speed would require almost a century of centuries to accomplish.—Bouvier's Familiar Astronomy.

Velocity of the Solar System

M. F. W. G. Struve gives as the splendid result of the united studies of MM. Argelander, O. Struve, and Peters, grounded on observations made at the three Russian observatories of Dorpat, Abo, and Pulkowa, “that the velocity of the motion of the solar system in space is such that the sun, with all the bodies which depend upon it, advances annually towards the constellation Hercules[1] 1·623 times the radius of the earth's orbit, or 33,550,000 geographical miles. The possible error of this last number amounts to 1,733,000 geographical miles, or to a seventh  of the whole value. We may, then, wager 400,000 to 1 that the sun has a proper progressive motion, and 1 to 1 that it is comprised between the limits of thirty-eight and twenty-nine millions of geographical miles.”

That is, taking 95,000,000 of English miles as the mean radius of the Earth's orbit, we have 95 × 1·623 = 154·185 millions of miles; and consequently,

 English Miles.
The velocity of the Solar System 154,185,000 in the year.
422,424 in a day.
17,601 in an hour.
293 in a minute.
57 in a second.

The Sun and all his planets, primary and secondary, are therefore now in rapid motion round an invisible focus. To that now dark and mysterious centre, from which no ray, however feeble, shines, we may in another age point our telescopes, detecting perchance the great luminary which controls our system and bounds its path: into that vast orbit man, during the whole cycle of his race, may never be allowed to round.—North-British Review, No. 16.

[1] Sir William Herschel ascertained that our solar system is advancing towards the constellation Hercules, or more accurately to a point in space whose right ascension is 245° 52′ 30″, and north polar distance 40° 22′; and that the quantity of this motion is such, that to an astronomer placed in Sirius, our sun would appear to describe an arc of little more than a second  every year.—North-British Review , No. 3.

The earth upon which we live is only one of many worlds that whirl through space. If we are to understand our own world, we must first learn something about the worlds in the skies. These bodies are arranged in groups, or systems, sweeping through circuits that baffle measurement; and such is the magnitude of the boundless space they occupy that our entire solar system is only a point in comparison. To this vast expanse of worlds, and systems and space we give the general name Universe.

The Solar System and Its Members

First in importance to us in this immense space filled with stars is what astronomers call the Solar System, so-called because the sun is its center. It contains the planets, eight in number, of which our earth is one. They have been named after the ancient deities; the two interior ones, Mercury and Venus, and the exterior ones, Mars, Jupiter, Saturn, Uranus, and Neptune; the first three being smaller than our earth, and the remainder a great deal larger.

Mercury and Venus are known to be interior  planets, that is, planets between us and the sun, because they appear to swing on either side of the sun. Mercury very seldom leaves the sun sufficiently to rise so early before the sun, or set so late after him, as to be visible. Venus, however, gets so far away as to be seen long after sunset or before sunrise, and is called the Evening or Morning star, accordingly.

Besides the planets there are other members of the system, namely, comets  and falling stars, which will be mentioned again more fully hereafter. All these bodies form a sort of family, having the sun for their head. The illustrations and drawings on separate pages give a view of the entire system.

Comparative Size. The size of the planets, in general, increases with their distance from the sun. The four composing the first group are all comparatively small, the earth being the largest. Those of the second group are all of great size. Jupiter, the largest, is not less than 1,390 times as large as the earth; but as it is much less dense, the amount of matter it contains is only a trifle more than 337 times that of the earth. All the planets together equal but one seven-hundredth part of the mass of the sun.

The Satellites , except our moon, and the two satellites of Mars, belong wholly to the second group of planets. Jupiter has eight; Saturn eight and several revolving rings; Uranus has four, and possibly more; while Neptune, so far as known with certainty, has but one.

Movements Within the Solar System

Rotary Motion. The sun, all the primary planets, and their satellites, as far as known, rotate from west to east. Each rotation constitutes a day for the rotating body. The central line of rotary motion is called the axis of rotation, and the extremities of the axis are called the Poles.

Revolution Around the Sun. All the primary planets and asteroids revolve around the sun in the direction of their rotation, that is from west to east; and the planes of the orbits in which they revolve coincide very nearly with the plane of the sun's equator. One revolution around the sun constitutes the year of a planet.

All the satellites, except those of Uranus and perhaps Neptune, also revolve from west to east.

Most of the comets revolve around the sun in very irregular and elongated orbits, only a few having their entire orbit within the planetary system. Some so move that after having entered our system and made their circuit around the sun, they seem to leave it, never to return.


Large illustration  (258 kB)

Since the orbits of the planets are in most cases not far removed from the plane of the ecliptic, they are to be seen in a comparatively narrow belt of the heavens called,

The Zodiac. The belt of the sky which occupies 8° on each side of the ecliptic is called the Zodiac, and it is within this belt that the moon and the chief planets confine their movements, as none of their orbits is inclined to that of the earth by more than 8°. The Zodiac, which circles the celestial sphere, is divided into twelve signs each of which occupies 30°, and roughly coincides [15]with a constellation. The following lists give the signs of the Zodiac, with the seasons in which the sun passes through each of them:

Spring: Aries the Ram; Taurus the Bull; Gemini the Twins.

Summer: Cancer the Crab; Leo the Lion; Virgo the Virgin.

Autumn: Libra the Balance; Scorpio the Scorpion; Sagittarius the Archer.

Winter: Capricornus the Goat; Aquarius the Water-bearer; Pisces the Fishes.

Owing to the precession of the equinoxes, the signs of the Zodiac do not now correspond with the constellations of which they bear the names. Thus the sign Aries, in which the sun is seen on March 21st as it passes the vernal equinox, with which the solar year begins, is now in the constellation of Pisces, and in the course of the next 23,000 years it will move steadily backward through the constellations until it returns to the Ram, where it stood when its name was first given to it.

Kepler's Celebrated Laws of Planetary Movements

The laws under which the planets move were discovered through the genius of John Kepler, and are known as Kepler's Laws of Planetary Motion. Kepler derived these laws from observation only, but Newton first explained them by showing that they were the necessary consequences of the laws of motion and the law of universal gravitation.

Kepler's First Law  states: “The earth and the other planets revolve in ellipses with the sun in one focus.”

Kepler's Second Law  states: “The radius vector of each planet moves over equal areas in equal times.”

Kepler's Third Law  states: “The squares of the periodic times of the planets are in proportion to the cubes of their mean distances from the sun.”


The diagram on the top illustrates the ellipse, and explains the first and second laws. The picture-diagram on the bottom illustrates the second law, which is that, as the planet moves round the sun, its radius vector describes equal areas in equal times. That is to say, a planet moves from A to B in the same time as it takes to move from C to D.

These laws cannot be fully understood without some acquaintance with mathematics. They may, however, be briefly explained for the comprehension of the non-mathematical reader. The figure in the diagram  is an ellipse—what is known in popular language as an oval—which is symmetrical about the line AB, known as its major axis. It has two foci, S and S 1. The fundamental law of the ellipse is that if we take any point P on it, and join this point by a straight line to the two foci, then the sum of these two lines SP and S 1 P is always the same—SP + S 1 P = C.

The second law is rather less easy to understand. The radius vector  is the line joining the sun to the planet at any moment; if we suppose the sun to be at the focus S, and P to be the planet, the radius vector at various positions of the planet will be represented by the lines SP, SP 1 , SP 2 , and so on. If the positions P, P 1 , P 2 , and so on, represent those which the planet occupies after equal periods of time—say, once a month—then the sectors of the ellipse bounded by each pair of lines, SP and SP 1 , SP 1  and SP 2 , will be equal. If a planet were to move in a circle round the sun, it is obvious that this law would imply that it moved with a uniform speed; but since the curvature of the ellipse varies in every part of its course, so must the speed of the planet, in order that its radius vector may describe equal areas in equal times. The planet will, in fact, be moving faster when it is near the sun, as at P, than when it is far off from the sun, as at P 2.

The third law shows that there is a definite numerical relation between the motions of all the planets, and that the time which each of them takes to complete its orbit depends upon its distance from the sun.

On his discovery of his third law Kepler had written: “The book is written to be read either now or by posterity—I care not which; it may well wait a century for a reader, as God has waited six thousand years for an observer.” Twelve years after his death, on Christmas Day, 1642, near Grantham, England, the predestined “reader” was born. The inner meaning of Kepler's three laws was brought to light by Isaac Newton.