ASTRONOMY. 



parallax, or displacement. The annexed figure re- 

 presents the effect 



* of parallax on a 

 heavenly body C. 



* V is its true place, 

 A or place in the 



sky as seen from 

 the earth's centre ; 

 T is its place as 

 seen from the sur- 

 face at A, and the 

 arc TV, or the 

 angle ACE, is the 

 amount of the 



* horizontal paral- 

 lax. 



The distances of the heavenly bodies, even the 

 nearest of them, are so great, that even when the 

 two stations are taken as wide apart as possible 

 that is, from the opposite sides of the earth, where 

 the distance between them is equal to the earth's 

 -diameter, or nearly 8000 miles the angle of paral- 

 lax ACB, or the displacement on the sky TS, is 

 very small. In the case of the moon, the nearest 

 -of the heavenly bodies, it is nearly two degrees ; 

 and by means of it the distance of the moon is 

 found to be about 239,000 miles, or about sixty 

 times the half-diameter or radius of the earth. 

 When the sun is observed from opposite sides 

 of the earth, he suffers a displacement of only 

 seventeen seconds of a degree. On so small an 

 angle, a slight error of observation materially 

 affects the result ; and therefore other expedients 

 were had recourse to, to determine the sun's dis- 

 tance with greater certainty and exactness, which 

 may be stated in round numbers as 92,500,000 

 jniies. When the distance of the earth from the 

 sun was once known, a new and vastly extended 

 base was obtained for the trigonometrical survey 

 of the heavens. As the earth travels in its annual 

 circuit round the sun at the distance of 92,500,000 

 miles, by observing a heavenly body at two differ- 

 ent times, at an interval of six months, we in effect 

 observe it from two stations removed from each 

 other by the whole breadth of the earth's orbit, 

 or nearly 185,000,000 miles. The displacement 

 produced in this way is called annual parallax. 

 Even this enormous base becomes as nothing 

 when applied to measure the distances of the fixed 

 stars ; by far the greater number shewing no 

 sensible parallax. 



Knowing the distances, we can find the real 

 sizes of bodies from their apparent size, or from 

 their dimensions, taken by an angular instrument. 

 If an object be a mile off, and if its breadth make 

 an angle of i at that distance, its lineal breadth 

 is determined by these two quantities. Thus the 

 moon, being about 239,000 miles from the earth, 

 and having an angular breadth in the sky of 

 somewhat more than half a degree, its actual 

 breadth must be about 2000 miles. The accurate 

 estimate is 2153 miles. The sun has almost 

 the same apparent size in the sky as the moon ; 

 but being at nearly 400 times the distance of the 

 moon, it must have nearly 400 times the breadth 

 of the moon to appear equally large. The diam- 

 eters of the planets are determined in the same 

 way. 



The solid contents or volumes of the several 

 bodies are found from their diameters, by a simple 

 rule of geometry. To state the number of cubic 



miles that each contains, gives little real infor- 

 mation, the important thing being their relative 

 magnitudes. Now, it is important to bear in 

 mind that the magnitudes or bulks are not in the 

 simple proportion of the diameters, but as the 

 cubes of the diameters. Thus, the diameter of the 

 sun being about 107 times that of the earth, his 

 volume or bulk will be 1,200,000 (107 cubed) times 

 greater. 



To speak of weighing such stupendous masses 

 as the sun, moon, and stars, seems at first sight 

 extravagant ; yet it is by no means one of the 

 most difficult problems, as will be explained under 

 Physical Astronomy. 



Diameter of the Earth. In measuring the dis- 

 tances of the heavenly bodies, the diameter of 

 the earth is taken as a known base to start from ; 

 but how is this got at? If at any place we 

 observe the height of a star on the meridian say 

 the pole-star and after travelling directly north 

 for a considerable distance, observe the height of 

 the same star, it will be found to be greater. 

 This arises from the round form of the earth ; and 

 it is easily shewn that, if the star has risen one 

 degree higher in the heavens, the observer must 

 have moved north one degree. We are thus 

 enabled to fix two stations on a meridian, the dis- 

 tance between which shall be exactly one degree, 

 or the -sfajth part of the whole circle or circum- 

 ference of the earth. When the length of such a 

 line is measured, it is found to be a little less than 

 seventy English miles. This, multiplied by 360, 

 gives the whole circumference in round numbers 

 at 25,000 miles, and the diameter at 8000. 



SEPARATE MEMBERS OF THE SYSTEM. 



The Sun ('3). The apparent or angular breadth 

 of the sun, at the mean distance of the earth, is 

 slightly more than 32'. The real diameter of the 

 sun is 853,380 miles, which, as already stated, is 

 107 times that of the earth, so that in volume or 

 bulk the sun is equal to 1,200,000 earths. 



That the sun is a ball or globe is evident from 

 its always appearing round, while we know, at the 

 same time, that it turns or rotates on an axis. 

 The fact of its rotation, and the time it occupies, 

 are inferred from observing the motion of the dark 

 spots which are seen at times on its surface : the 

 period of rotation is ascertained to be a little over 

 25 days. 



A solar spot presents the appearance of a black 

 irregular patch, called the umbra, surrounded by 

 a less dark fringe, called the penumbra. Spots 

 appear and disappear very irregularly, some lasting 

 only a day, others for weeks and even months. 

 Sometimes few or no spots are to be seen, at other 

 times they appear in profusion ; and these alter- 

 nations are observed to come regularly round in 

 periods of about ten years. Individual spots have 

 been seen to attain the enormous breadth of 

 50,000 miles, covering an area of five times the 

 surface of the earth. They are inferred, with 

 almost certainty, to be hollows. 



Around the spots, and on other places, there 

 are often masses brighter than the general surface, 

 which are called facula, or torches. The general 

 surface itself is not uniform, but appears to be 

 coarsely mottled, and to be made up of bright 

 roundish patches, with soft edges, sprinkled irregu- 

 larly on a less luminous background. The move- 



