The Story of the Solar System

by George F. Chambers

Though the Earth is commonly regarded as a sphere it is not that in reality, because it is not of identical dimensions from east to west and from north to south. It is somewhat flattened at the poles; its polar diameter is less than its equatorial diameter, in the ratio of about 298 to 299, or, expressed in miles, its polar diameter is about 26 miles less than its equatorial diameter. If a globe 3 feet in diameter be taken to represent the Earth, then the polar diameter will, on this scale, be ⅛ inch too long. This flattening of the poles of the Earth finds its counterpart, so far as we know, in most, and probably in all of the planets. It is most considerable and therefore most conspicuous in the case of Jupiter. It ought here to be added that a suspicion exists that the equatorial section of the Earth is not a perfect circle, but that the diameter of the Earth, taken through the points on the equator marked by the meridians 13° 58′ and 193° 58′ east of Greenwich, is one mile longer than the diameter at right angles to these two points.

The science which inquires into matters of this kind, including besides the figure of the Earth, the length of the degree at different latitudes, and the distances of places from one another, alike in angular measure and in time, is called Geodesy; it is, in point of fact, land-surveying on a very large scale, in which instruments and processes of astronomical origin are brought into operation, and in which astronomers are more or less required to take the lead.

Although we all of us now perfectly understand that the Earth is a planet moving round the Sun as a centre, it is, comparatively speaking, but recently that this fact has become generally recognised and understood. It is true that we can discover here and there in ancient writings some trace of the idea, yet it is doubtful whether 2000 years ago more than a few “advanced” thinkers thoroughly and clearly accepted it as a distinct truth. It was much more in consonance with popular thought and the actual appearance of things that the Earth should be the centre round which the Sun revolved and on which the planets depended; and accordingly, sometimes in one shape and sometimes in another, the notion of the Earth being the centre of the universe was generally accepted. The contrary opinion had, however, a few sympathisers. For instance, Aristarchus of Samos, who lived in the third century before the Christian era, supposed, if we may trust the testimony of Archimedes and Plutarch, that the Earth revolved round the Sun; this, however, was regarded as a “heresy,” in respect of which he was accused of “impiety.” Some few years elapsed and a certain Cleanthes of Assos is said by Plutarch to have suggested that the great phenomena of the universe might be explained by assuming that the Earth was endued with a motion of translation round the Sun together with one of rotation on its own axis. The historian states that this idea was so contrary to the received opinions that it was proposed to put Cleanthes on his trial for impiety.

In former times the philosophers who studied the solar system ranged themselves in several “schools of thought,” to use a modern hackneyed phrase. Some upheld the Ptolemaic system, which took its name from a great Egyptian astronomer, Claudius Ptolemy, though it does not appear that he was actually the first to suggest it. The Ptolemaic system regarded the Earth as the centre, with the following bodies, all called planets, revolving round it in the order stated:—the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. It will be observed that there are seven bodies here named, and as seven was regarded as the “number of perfection,” it was in later times considered that only these seven bodies (neither more nor less) could really be the Earth’s celestial attendants. Though Ptolemy was in one sense an Egyptian, there yet prevailed amongst the Egyptians at large another theory slightly different from Ptolemy’s. According to the “Egyptian theory,” Mercury and Venus were regarded as satellites of the Sun, and not as primary planets appurtenant to the Earth.

After Ptolemy’s era many centuries elapsed, during which the whole subject of the solar system lay practically dormant, and it continued so until the revival of learning brought new theorists upon the scene. The most important of these was Copernicus, who, in the sixteenth century, propounded a theory which eventually superseded all others, and, with slight modifications, is the one now accepted. Copernicus placed the Sun in the centre of the system, and treated it as the point around which all the primary planets revolved. So far, so good; but Copernicus went astray on the question of the orbits of the planets. He failed to realise the true character of the curves which they follow and treated these curves as “epicycles,” which word may be described as representing a complicated combination of little circles which taken together form a big one. It was left to Kepler and Newton to settle all such details on a true and firm basis. But before this stage was reached a man of the highest astronomical attainments and practical experience, Tycho Brahe, made shipwreck of his reputation as an astronomer by solemnly reviving the idea of the Earth being the immovable centre of everything. He treated the Moon as revolving round the Earth at no great distance and the Sun as doing the same thing a little farther off; the five planets revolving round the sun as solar satellites. The “Tychonic system,” as it is called, has something in common with the Ptolemaic system without being by any means as logical as the latter. That such far-fetched ideas as Tycho’s should have been palmed off on the world of science so recently as 300 years ago is passing strange; but the explanation appears to be that his action arose out of a misconception of certain passages of Holy Scripture, which seemed irreconcilable with the Copernican theory. It must not be forgotten that Copernicus’s famous book, published in 1543, in which he had announced his views, had been condemned by the Papal “Congregation of the Index;” and therefore Tycho might have had as a further motive a desire to curry favour with the authorities of the Church of Rome, and to gratify his own vanity at the same time.

With these explanations it will no longer be misleading if, for convenience sake, I speak of a certain great circle of the heavens as apparently traversed by the Sun every year, owing to the revolution of our Earth round that body. This circle is called the “Ecliptic,” and its plane is usually employed by astronomers as a fixed plane of reference. It must be distinguished from that other great circle called the “celestial equator,” which is the plane of the Earth’s equator extended towards the stars. The plane of the equator is inclined to the ecliptic at an angle of about 23½°, which angle is known as the “obliquity of the ecliptic.” It is this inclination which gives rise to the seasons which follow one another in succession during our annual journey round the Sun. The two points where the celestial equator and the ecliptic intersect are called the “equinoxes,” of spring or autumn as the case may be; the points midway between these being the “solstices,” of summer or winter as the case may be. These words need but little explanation, at any rate, as regards those persons who are able to trace the Latin origin of the words. “Equinox” is simply the place occupied by the Sun twice every year (namely about March 20 and September 22), when day and night are theoretically equal throughout the world, when also the sun rises exactly in the east and sets exactly in the west. The “solstices” represent the standing still of the sun at the given times and places, and are the neutral points where the Sun attains its greatest northern or southern declination. This usually occurs about June 21 and December 21. It must not be forgotten by the way, that the above application of the words “summer” and “winter” to the solstices is only correct so far as concerns places in northern terrestrial latitudes—Europe and the United States, for instance. In southern terrestrial latitudes—for instance, when speaking of what happens at the Cape of Good Hope and in Australia—the words must be reversed.

We have seen in a previous chapter that whilst the orbits of the planets are nearly true circles, none of them are quite such: and the departure from the truly circular form results in some important consequences. Whilst some of these are too technical to be explained in detail here, one at least must be referred to because of what it involves. Not only is the Earth’s orbit eccentric in form, but its eccentricity varies within narrow limits; and besides this the orbit itself, as a whole, is subject to a periodical shift of place, from the joint effect of all which changes it comes about that our seasons are now of unequal length, the spring and summer quarters of the year unitedly extending to 186 days, whilst the autumn and winter quarters comprise only 178 days. The sun therefore has the chance of shining for a longer absolute period of time over the northern hemisphere than over the southern hemisphere; hence the northern is the warmer of the two hemispheres, because it has a better, because a longer, chance of storing up an accumulation of solar radiant heat. Probably it is one result of this that the north polar regions of the Earth are easier of access than the south polar regions. In the northern hemisphere navigators have reached to 81° of latitude, whereas 71° is the highest limit yet attained in the southern hemisphere. Readers who have studied the history of explorations in the Arctic regions will not need to be reminded of the controversy which has so often arisen respecting the existence or nonexistence of an “Open Polar Sea.”

It has already been hinted that it is not an easy matter to determine, when dealing with the Earth, where astronomy and its allied sciences, geography, geodesy and geology respectively, begin and end. But as certain topics connected with these sciences, such as the rotundity of the Earth and its rotation on its axis, will come more conveniently under consideration in other volumes of this series, I shall pass them over and only treat of a few things which more directly concern the student of nature observing either with or without the assistance of a telescope.

The fact that the Earth is surrounded by a considerable atmosphere largely composed of aqueous vapour has a material bearing on the success or failure of observations made on the Earth of bodies situated at a distance. It may be taken as a general rule that the nearer an observer is to the surface of the sea, or otherwise to the surface of the land at the sea-level, the greater will be the difficulty which will confront him in carrying on astronomical observations. Hence such observations are generally made with unsatisfactory results on the sea coast or on the banks of rivers. An interesting but rather ancient illustration of this last-named fact is to be found in the circumstance that Copernicus, who died at the age of 70, complained in his last moments that much as he had tried he had never succeeded in detecting the planet Mercury, a failure due, as Gassendi supposed, to the vapours prevailing near the horizon at the town of Thorn on the banks of the Vistula where the illustrious philosopher lived.

The phenomena depending on the presence of aqueous vapour in the atmosphere which especially under the notice of the astronomer are Refraction, Twilight, and the Twinkling of the Stars.