330 



DISCOVERY 



foundations for the great modern advances. Yet 

 even Copernicus was not entirely free from the 

 influence of ancient superstition. He kept the perfect 

 curve, the circle, for his planetary orbits, but he 

 stated that the sun was displaced very slightly from 

 the centre ; he also accounted for a few irregularities 

 by retaining one or two epicycles. With the excep- 

 tion of these minor points, however, the Copernican 

 system was much the same as the one we now adopt. 

 Kepler completed the work of his great predecessor 

 some sixty-five years later by showing that the paths 

 of the planets are ellipses, but ellipses that do not 

 depart very greatly from the circular form. 



Let us now pass on to consider how the loops in 

 the planetary' paths can be explained. Suppose the 

 inmost circle represents the orbit of the earth (Fig. 3), 

 and the next one that of one of the outer planets, say 

 Mars. Let the outer circle, shown in part, represent 



the background of the sky, and let s be the sun. 

 At equal intervals of time the earth will be at points 

 marked i, 2, 3, 4, 5, 6, spaced out equally on its path. 

 At the same time Mars will be at corresponding points 

 on its orbit, marked I to 6 on the larger circle. When 

 the earth and Mars are at their points i, Mars will 

 appear to us to be at the corresponding point i on 

 the sky-circle. Similarly for all the other points. 

 The diagram shows us that we get a forward move- 

 ment from I to 3, followed by a backward from 

 3 to 4, and then a forward from 4 to 6, and onwards 

 until the next loop is commenced. If we imagine 

 the circles as lying slightly out of the plane of the 

 paper, and inclined to it at different angles, the path 

 traced out becomes a loop. 



The outer planets alwavs appear brightest when 

 they are in the middle of the backward portion of 

 the loop, for then they are always in a line with the 

 earth and the sun, or exactly opposite the sun as seen 

 by us — a configuration to which we give the technical 

 name " opposition." Their brilliancy varies con- 

 siderably at different oppositions. In the case of the 

 earth and Mars the orbits are comparatively near 



each other at one part, and are much farther away at 

 other parts. WTien an opposition takes place with 

 the planets at the former points, Mars is nearer the 

 earth than it is at any other time, and so appears 

 much brighter. But if the opposition takes place 

 with the earth and Mars at some of the latter points, 

 the distance is so much greater that the maximum 

 brilliancy of the planet is far less than it was in the 

 previous case. 



This variation of brightness is more marked in the 

 case of Mars than in that of any other planet. The best 

 possible oppositions are those that occur at the end 

 of August, when the planet loops its loop under the 

 Great Square of Pegasus. These maximum opposi- 

 tions happen once in about fifteen or seventeen years ; 

 the last was in 1907, and the next will be in 1924, 

 and will be absolutely the most favourable for the two 

 centuries 1800-1999. The planet will then be only 

 34,650,000 miles from the earth. 



The variation in the case of Jupiter is not nearly so 

 much. For this planet the most favourable oppositions 

 occur about October 6, when Jupiter is 369,000,000 

 miles away. The least favourable are those that 

 take place in April, when the distance is increased 

 to 411,000,000 miles. The other visible outer planet, 

 Saturn, is so far away that the variation in its 

 brilliancy is not at all a conspicuous feature of its 

 oppositions ; its brightness is, in fact, controlled by 

 the inclination of the ring system. 



A few words will probably suffice to explain the 

 apparent movements of the inner planets. Mercury 

 and Venus. It wiU be evident from the figure that 

 they can never rise at sunset and remain visible all 

 through the night as do those that lie beyond the orbit 

 of the earth. They appear to vibrate, swinging from 

 side to side of the sun. If the inner circle (Fig. 4) 

 represents the orbit of Venus and the outer that of 

 the earth, it is obvious that the greatest apparent 

 distance of Venus from the sun is given when the line 

 EV, joining the two planets, is a tangent to the orbit 

 of Venus. This planet is visible for some months 

 when it is coming into, or receding from, this position. 

 Mercury, however, is visible to the naked eye for only 

 a few days each year, when it appears like a white 

 or silvery point flashing in the afterglow of sunset or 

 just before the dawn. 



Continufd /rent pa§s 327.] 



cavities as shown in X-ray photographs, Prof. Elliot Smith i-; 

 inclined to agree that it affords evidence of the existence of a 

 primate, in some respects hke the cliimpanzee, but definitely 

 human rather than ape-like and representing a type of tin- 

 human race older than Pilhccanthropus of Java. Dr. Smith 

 Woodward, however, adhered to the opinion he had previously 

 expressed that it was more likely to belong to the genus of 

 extinct primitive bears known as Hyanarctus, and claimed 

 that it was more like the upper molar of a carnivore than of a 

 primate. 



