THE 



I.KSSONS IN ASTRONOMY. IX. 



THE SOLAR SYSTEM COHPARATIVE SIZES AND DISTANCES 

 THE SUN VULCAN. 



THE student will by this timo have become acquainted with 

 many of the more important phenomena of the heavens : wo 

 will, therefore, proceed now to notice in detail the principal 

 l';i<-t-t relating to those of tho heavenly bodies which are our nearest 

 neighbours in space, and 

 which belong to tho same 

 system, or family group, as 

 does the earth. 



The following is a list 

 of the principal bodies in 

 thi-! group : The Sun, 

 which 13 the common cen- 

 tre round which they all 

 revolve ; Vulcan, Mercury, 

 and Venus, which are dis- 

 tinguished as the i. 

 planets, their orbits being 

 included within that of tho 

 earth ; tho Earth ; and the 

 .superior planets, Mars, the 



minor planets, or asteroids, r *8 



Jupiter, Saturn, Uranus, 



and Neptune. Several of these have satellites, or secondary 

 planets, revolving around them ; and there are also several 

 comets which are included as regular members of our system. 

 These will be enumerated hereafter. 



As we inquire more particularly into the movements of these 

 bodies, we see many striking points of similarity. They all 

 move round tho sun in tho same direction, 

 .and in elliptical paths of no great eccen- 

 tricity. They are all likewise opaque 

 bodies like the earth, shining only by re- 

 flected light ; and all rotate on their axes, 

 so as to produce the changes of day and 

 night. Their orbits, too, are all inclined 

 "to the plane of the ecliptic. 



Orreries are frequently constructed, in 

 "which the different planets are represented 

 by different-sized balls moving at various 

 distances round a central one. It 

 is, however, quite impossible to 

 make these on a scale at all true 

 to nature. Fig. 15 illustrates 

 roughly their comparative sizes. 

 The following, however, is a plan 

 for obtaining a tolerably correct 

 idea of their comparative distances 

 and magnitudes, and tho relative 

 dimensions of their orbits : 



Select a large clear space, and 

 place at one side a ball about two feet in 

 diameter to represent the sun ; Vulcan will 

 then be represented by a small pin's head 27 

 feet from the globe; Mercury by a mustard-seed 82 feet 

 distant; Venus by a pea at a distance of 142 feet ; the Earth by 

 a slightly larger pea at a distance of 215 feet ; Mars by a largo 

 pin's head at a distance of 327 feet; the minor planets by grains 

 of sand between 500 and GOO feet distant. An orange, about -\ 

 inches in diameter, and 1,120 feet distant, will then represent 

 Jupiter ; one about two inches in diameter, distant two-fifths of 



105 N.P- 



i a mile, will stand for Saturn ; a full-sUed cherry, three-quarters 

 of a mile distant, for Uranus ; and a plum, a mile and a quarter 

 off, for Neptune. On this scale the distance of the nearest 

 : fixed star would be about 7,500 miles. 



As the sun is by far tho largest of these bodies, we will 



treat of it first, and the question that immediately occurs to us 



\Vhat is the distance of this body ? The accurate solution 



, of this question is one of the moat important problems in 



astronomy, as this distance 

 is taken as a measure for 

 Hlrimning the distances 

 and magnitudes of most 

 other heavenly bodies. The 

 principle of the problem 

 can easily be understood, 

 though, of course, there 

 are many difficulties in the 

 carrying of it out. Sup- 

 pose an observer, situated 

 on the line B c (Fig. 16), 

 wishes to ascertain the dis- 

 tance of an inaccessible 

 object A; let AC be the 

 visual ray by which it i4 

 seen at c ; at right angles 

 to this lay off another line, 

 B c, and from B measure accurately the angle c B A. Wo 

 know then the distance B c, and the measure of the angles 

 at B and c ; it is easy, therefore, to calculate the angle BAC 

 and the length of AC. As will at once be seen, the longer BC 

 is, the larger will the angle B A c be, and therefore the less the 

 risk of error in measuring it. When this angle is very small, an 

 exceedingly minute error produces a great 

 difference in the calculated length of c A. 



Now, in the practical application of this 

 principle, the utmost base-line that can 

 be obtained is the earth's diameter ; and 

 this is so small in comparison with the 

 distance of the sun that the angle BAC 

 becomes too minute to be measured di- 

 rect ly with a sufficient degree of accu- 

 racy. We are enabled, however, in an 

 indirect way, to measure it, and thus solve 

 the problem. The planet Venua 

 travels round the sun in an orbit 

 within that of the earth, and 

 hence, at certain intervals, passes 

 between the earth and the sun, 

 and produces what is called a 

 transit of the planet. On these 

 occasions it is seen as a black 

 spot on the bright disc of the sun. 

 and by means of observations 

 taken at that moment the re- 

 quired angle may be measured. 



Fig. 17 will render the mode of proceeding 

 more clear. A B represents a base-line on 

 the earth's surface, and c D the sun, E being the planet 

 Venus when passing between the two. To an observer at 

 B it will appear to travel across the sun's disc along 

 the line H K, while to one situated at A it will pass along 

 F a. Now if Venus were midway between the earth and tha 

 sun, no advantage would be gained, as the angle M B L would 

 then be equal at A L B. The planet's distance from the sun is 



