924 



ASTRONOMY. 



[TUX 8m. 



en , Turing also represent* 1. Group of stun in Aquarius. 

 Mart, as toen Aug. 10, 1830, by Sir John Ht'nchel 

 at Slough. 3. Group of stars in tho constellation of 

 Hercules. 4. Great Comet, M neon Sept 10. 1811. 

 6. Groups in Cancer. 6. A star in tho middle of tlio 

 elliptical nebula, 7. Comet of 1811. 8. Nebula in Urea 

 JInjor. 9. Bright elliptical nebula in Sagittarius. 10. 

 Three stars in Auriga 11. View of Saturn with hU 

 rings. 12. Nebula in Gemini. 13, 14. Nebula in An- 

 dromeda. 15. Nebula in Monoceros. 16. The Comet of 

 1810. 17. Nebula in Orion. 18. Streaks in Jupiter, 

 observed Sept. 23, 1832. 19. Group of stars in Cancer. 



The earth is represented on the first day of each of the 

 twelve months of the year, the solar distances correspond- 

 ing to these twelve positions, and the shape of the earth's 

 orbit. The deeper circle surrounding the pole at a short 

 distance, is intended to represent the parallel of latitude 

 of Europe, or the hour circle of that place divided into 

 twenty-four hours. Although at the end of December the 

 earth is nearest the sun, yet, at that time in the northern 

 hemisphere, the heat is less than at any other. The 

 reason of this lies in the fact of the short days and long 

 nights, as well as that the sun's rays fall very obliquely on 

 the earth, traversing a longer path through the atmo- 

 sphere, and consequently losing much of their heating 

 power. At the beginning of July, on the contrary, 

 although then the earth is at its greatest distance, the 

 temperature of the northern hemisphere is greatest, on 

 a-count of the long days and short nights, and the great 

 altitude of the sun at noon. It must not be forgotten, 

 however, that owing to the precession of the equinoxes, 

 these signs no longer correspond to the constellations of 

 the same name, so that now the sign Pisces corresponds 

 to the constellation Aries, the sign Aries to the constella- 

 tion Taurus, <bc. It is further evident, that if the earth 

 at the beginning of spring, summer, autumn, and winter, 

 should be in the signs Aries, Cancer, Libra, and Capri- 

 coruns, respectively, then the sun, as being always 

 directly opposite in the ecliptic, will be in the signs Libra, 

 Capricornus, Aries, and Cancer. 



An atmosphere the region of the winds, lightning, and 

 meteoric confiscations upon which respiring beings de- 



pend for vitality, surrounds our 







, nml in ono of it 

 most important attri 

 hute ; elm-fly ini 

 ing to the astronomer, 

 on account <>f its effect 

 in displaying to us tho 

 heavenly bodies, and dif- 

 fusing the rays of li^lit 

 in every direction around 

 us. The extent of this 

 wonderful and benign 

 envelope is not precisely 

 known ; but its density 

 diminishes as we ascend 

 from tho surface, and, 

 at a very inconsiderable 

 elevation, it becomes so 

 rare as to interfere with the functions of existence. The 

 diagram represents the engirdling atmosphere of our 

 planet, various strata of air resting upon it, the upper 

 pressing upon the lower, and causing the interior to be 

 more dense than the exterior strata. * 



The density of the earth is 4Ag that of water, so that our 

 globe would counterpoise 4^ globes of the same size, com- 

 posed of materials of the same specific gravity as water. 

 Yet, abandoned to the solar attraction, it would require 

 64 days, 13 hours, to fall upon the sun. 



The varying intensity of the force of gravity at the 

 surface of the earth, as shown by the unequal action of 

 the pendulum, which vibrates slower at the equator than 

 in other places, is as 1 at the equator to 1 T J T at the poles. 

 A body, therefore, weighing 194 pounds at the equator, 

 would weigh 195 pounds at the north pole. 



The place of the earth in the system is a favoured one, 

 where nearly all the planets are visible to the naked eye. 

 While they appear in our heavens, Jupiter, Venus, and 

 occasionally Mars, shining with great splendour, our 

 planet may be presumed to return the compliment, exhi- 

 biting to Venus, at the time of her inferior conjunction, 

 when she is nearest to us, a full orb resplendent through 

 her whole night. The apparent size of the earth will be 

 greater than that of Venus to us. 



CHAPTER III. 



ON THE SUN. 



Tan stupendous luminary, to which we are indebted 

 for many of the blessings we enjoy the source of light 

 and heat, and which also contributes materially to the 

 development of vegetation will now claim our attention. 

 It also serves as a standard for the regulation of our 

 calendar. The orbit which it appears to describe about 

 the earth fixes our year, whilst its displacements in the 

 ecliptic regulate our seasons. A little attention will 

 show us, that its motions in the heavens from day to 

 day differ from that of a fixed star. We have only to 

 mount firmly any line of sight, properly protected by 

 coloured glaives from the glare of sun-light, and we can 

 readily determine, by means of a watch or chronometer, 

 that the interval between the successive returns of the 

 inn will be different at different times of the year ; 

 whilst the invariable constancy of the returns of the 

 fixed stan, on the other hand, impresses us at once with 

 the notion that the earth rotates on its axis. The ap- 

 parent path of the sun, however, in the heavens is not 

 so easily traced as that of the moon or planets, whose 

 poniti.ro can be readily compared with the fixed stars. 

 fot any accurate investigations, we are obliged to make 

 use of a transit instrument and mural circle, which will 

 be described hereafter. By these instruments we find 

 two elemnnU neoemary to define it* path, namely, its 

 right ascension and declination. Its diameter can also 

 b readily known. We thus find, that, in addition to 

 iU diurnal motion, it partakes of a proper motion always 



in the same direction from west to east, and that it is 

 six months above, and six months below the equator. 

 Its path in the heavens is termed the ecliptic, and p. 

 through the twelve constellations. Its orbit is inclined 

 to the equator by an angle of 23 27', termed the obll / 

 of the ecliptic. The intersections of its orbit with the 

 equator occur at two points, termed the vernal and autum- 

 nal equinoxes. The points at which the greatest and least 

 declinations of the sun take place, are called the solstices. 

 MOTION or THE SCN IN ITS ORBIT. In order to obtain 

 the accurate path of the sun in the ecliptic, it will be 

 necessary to convert the right ascension and declination, 

 obtained by means of the transit instrument and mural 

 circle, into longitudes, which can be readily done, having 

 given the obliquity of the ecliptic. It will then be easily 

 seen that the arc described in the space of a day is not 

 uniform, varying in amount at different parts of his orbit. 

 The maximum change occurs on the 1st of January, 

 when it amounts to 61' 10", and which gradually 

 diminishes to the 1st of July, when the value is 57' 112' , 

 after which ite motion is ULjain quicker. Tho average 

 velocity of 69/ 11" takes place at the commencement of 

 April and October. The observed diameter obtained by 

 means of the passage of his eastern and western limli 

 over the meridian, or the vertical diameter HMMtmd l>y 

 the mural circle, will also exhibit fluctuations ; the 

 maximum occurring at the time of its greatest angular 

 BM ante, Mechanical rhiloiophy, Pniumatia, p. 770. 



