Astronomy and General Physics Considered with Reference to Natural Theology

by William Whewell

The earth’s orbit, however, is not exactly a circle. The mark is not precisely a single point, but is a space of the breadth of one thirtieth of the distance from the centre. Still this is much too near an agreement with the circle to be considered as the work of chance. The chances were great against the ball passing so nearly at the same distance, for there were twenty-nine equal spaces through which it might have gone, between the mark and the centre, and an indefinite number outside the mark.

But it is not the earth’s orbit alone which is nearly a circle: the rest of the planets also approach very nearly to that form: Venus more nearly still than the earth: Jupiter, Saturn, and Uranus have a difference of about one tenth, between their greatest and least distances from the sun: Mars has his extreme distances in the proportion of five to six nearly; and Mercury in the proportion of two to three. The last mentioned case is a considerable deviation, and two of the small planets which lie between Mars and Jupiter, namely Juno and Pallas, exhibit an inequality somewhat greater still; but the smallness of these bodies, and other circumstances, make it probable that there may be particular causes for the exception in their case. The orbits of the satellites of the Earth, of Jupiter and of Saturn, are also nearly circular.

Taking the solar system altogether, the regularity of its structure is very remarkable. The diagram which represents the orbits of the planets might have consisted of a number of ovals, narrow and wide in all degrees, intersecting and interfering with each other in all directions. The diagram does consist, as all who have opened a book of astronomy know, of a set of figures which appear at first sight concentric circles, and which are very nearly so; no where approaching to any crossing or interfering, except in the case of the small planets, already noticed as irregular. No one, looking at this common diagram, can believe that the orbits were made to be so nearly circles by chance; any more than he can believe that a target, such as archers are accustomed to shoot at, was painted in concentric circles by the accidental dashes of a brush in the hands of a blind man.

The regularity, then, of the solar system excludes the notion of accident in the arrangement of the orbits of the planets. There must have been an express adjustment to produce this circular character of the orbits. The velocity and direction of the motion of each planet must have been subject to some original regulation; or, as it is often expressed, the projectile force must have been accommodated to the centripetal force. This once done, the motion of each planet, taken by itself, would go on for ever, still retaining its circular character, by the laws of motion.

If some original cause adjusted the orbits of the planets to their circular form and regular arrangement, we can hardly avoid including in our conception of this cause, the intention and will of a Creating Power. We shall consider this argument more fully in a succeeding chapter; only observing here, that the presiding Intelligence, which has selected and combined the properties of the organic creation, so that they correspond so remarkably with the arbitrary quantities of the system of the universe, may readily be conceived also to have selected the arbitrary velocity and direction of each planet’s motion, so that the adjustment should produce a close approximation to a circular motion.

We have argued here only from the regularity of the solar system; from the selection of the single symmetrical case and the rejection of all the unsymmetrical cases. But this subject may be considered in another point of view. The system thus selected is not only regular and symmetrical, but also it is, so far as we can judge, the only one which would answer the purpose of the earth, perhaps of the other planets, as the seat of animal and vegetable life. If the earth’s orbit were more eccentric, as it is called, if for instance the greatest and least distances were as three to one, the inequality of heat at two seasons of the year would be destructive to the existing species of living creatures. A circular, or nearly circular, orbit, is the only case in which we can have a course of seasons such as we have at present, the only case in which the climates of the northern and southern hemispheres are nearly the same; and what is more clearly important, the only case in which the character of the seasons would not vary from century to century. For if the eccentricity of the earth’s orbit were considerable, the difference of heat at different seasons, arising from the different distances of the sun, would be combined with the difference, now the only considerable one, which depends on the position of the earth’s axis. And as by the motion of the perihelion, or place of the nearest distance of the earth to the sun, this nearest distance would fall in different ages at different parts of the year, the whole distribution of heat through the year would thus be gradually subverted. The summer and winter of the tropical year, as we have it now, being combined with the heat and cold of the anomalistic year, a period of different length, the difference of the two seasons might sometimes be neutralized altogether, and at other times exaggerated by the accumulation of the inequalities, so as to be intolerable.

The circular form of the orbit therefore, which, from its unique character, appears to be chosen with some design, from its effects on the seasons appears to be chosen with this design, so apparent in other parts of creation, of securing the welfare of organic life, by a steadfast and regular order of the solar influence upon the planet.

[CHAPTER III.]
The Stability of the Solar System.

There is a consequence resulting from the actual structure of the solar system, which has been brought to light by the investigations of mathematicians concerning the cause and laws of its motions, and which has an important bearing on our argument. It appears that the arrangement which at present obtains is precisely that which is necessary to secure the stability of the system. This point we must endeavour to explain.