Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/843

the innermost parts of the successive loops (as, for instance, when Saturn was as where the date 1710 is placed on his loops), the planet was always opposite the sun. This exact agreement between the times when the planet and sun were in conjunction or in opposition, and the tracing out of the apparent planetary loops, should have suggested, it would seem, a connection between the sun and planets ; for we see from fig. 17, that if the earth were the centre of the sun s motion, and each of the three planets had its looped path wherein to travel around the earth, there could be imagined no reason why the planet s motion round its loops should synchronise with the sun s motion on his

nearly circular path.

This view should have been confirmed by the apparent motions of two other planets, Venus and Mercury, which were found to remain always within a certain apparent distance from the sun, never being seen on the part of the sky opposite to him. Venus, the brighter, was observed to have the greater range on either side of the sun, moving from about 46 on the east of the sun, when she is seen as the Evening Star, to about the same distance on the west of the sun, when she is seen as the Morning Star ; while Mercury s greatest range on either side of the sun is more variable, being sometimes as great as 27, and at other times not greater than 18. So far as the motion of these planets on the star-sphere could be traced (by using such an instrument as in fig. 9 to determine their right ascension and declination), they appeared to follow looped paths, somewhat like the outer planets; but the nearer and farther parts of the successive loops xvere both lost to view, the two planets being always too near the .sun s place in the heavens to be visible when tracing those parts of their paths.

The five planets were found to travel always within a certain range on either side of the ecliptic, Venus, which has the greatest range, being sometimes as far as 9 north, or 9 south of the equator. A zone, or band, having the ecliptic for its central circle, and bounded by circles 9 north and south of the ecliptic, so as to be 18 wide, came thus to be regarded as a sort of celestial roadway, outside of which the planets were never seen. This zone was called the zodiac, and it was probably in connection with the planetary rather than the solar motions that the zodiacal constellations were originally formed.

Considering the observed relation between the motions of all the five planets and those of the sun, it is remark able that any of the astronomers of old time should have regarded the earth as the common centre of solar and planetary movement. It is true that, by supposing each planet to travel around a centre which itself travelled round the earth, the looped paths of the planets might be explained ; of this no further evidence need be given than fig. 17, where it is manifest that the loops are such as might be traced by bodies moving round small circles, these being carried round the central earth in large circles. Yet the synchronism between all the movements in these small circles and the sun s supposed motion round the earth, was left wholly unexplained by that theory. If the Ptolemaic system could have been really presented as it is commonly pictured (fig. 18), it would have compared not unfavourably with the simplicity of the modern system. But the circles depicted in this figure for the paths of Saturn, Jupiter, Mars, Venus, and Mercury, represent only the paths of moving centres, round which those bodies were supposed to travel, each in a circle of its own, and all synchronising in these subordinate movements with the .sun circling in his special orbit ; while the movements of the moon, travelling in a different circle, neither corre sponded in any way with those of the sun, nor, like his, secmod to influence any of the planetary motions.

Fig. 18.—Ptolemaic System.

Fig. 19.—Tychonic System.

Far more reasonable was the ancient Egyptian system,

by some described as identical with, but in any case closely resembling in essentials, the system of Tycho Brahe, shown in fig. 19. Here the earth is the centre of the motions of system, the sun and moon, but all the planets circle around the sun, Venus and Mercury moving in orbits passing between the earth and sun, while Mars, Jupiter, and Saturn move in orbits passing outside the earth. All the observed movements, and all the peculiarities of the observed rela tions, were fully explained by this system. Indeed, a little consideration will show that the geometrical relations are precisely the same for such a system as is depicted in fig. 19, as they would be in the system pictured by remov ing the circle showing the sun s orbit in fig. 19, and substituting a circle of equal size around the sun as centre, and passing therefore through the centre of the earth. It is not too much to say that Tycho s system is not only fully equal to the Copernican in its fitness to explain the observed relations, but that, until the law of gravitation had been established, the arguments for the Tychonic system, modified so as to correspond to Kepler s discovery of the shapes of the different orbits, were almost equal in weight to those used by the disciples of Copernicus. The slight advantage of the Copernican system in point of simplicity was counterbalanced by the difficulty of accepting, in those days, the belief that the stars lie at so inconceivably vast a distance that the motion of the arth in an enormous orbit around the sun (for the sun

was known even then to lie many millions of miles from