ASTRONOMY. 



a half of a bright star in the constellation of the 

 Lesser Bear. Its motion will bring it gradually 

 nearer until it is within half a degree of that star, 

 -after which it will recede from it. Its course may 

 be traced by drawing a circle on a celestial globe 

 round the north pole of the ecliptic, at the distance 

 of 23^. The celestial pole describes this circle in 

 25,868 years. Twelve thousand years hence, it will 

 be near one of the brightest stars in the heavens, 

 called a Lyrse, which will then be the pole-star. 



Stability of the System. It is natural to inquire 

 whether the numerous perturbations which all the 

 bodies are subject to, are such as in the long-run 

 to overthrow the present arrangements of the 

 system. Now, so far as has yet been positively 

 ascertained, the total effect of all the mutual dis- 

 turbances has no such tendency. Though there 

 are secular variations that may go on increasing 

 for thousands of years, it has been shewn that they 

 will decrease continually for periods of like dura- 

 tion, and the limits within which this secular 

 oscillation is confined are in all cases extremely 

 narrow. 



Two causes are pointed to as likely to produce 

 permanent changes in the planetary motions. I. 

 It is generally held by philosophers, that space, 

 instead of being perfectly empty, is everywhere 

 filled with an exceedingly thin medium, which they 

 call ether. Now, however slight the resistance this 

 may offer to bodies moving in it, yet, if it exist at 

 all, it must tell in the end, and will have the effect 

 of contracting the orbits of the planets, and bring- 

 ing them nearer and nearer to the sun. Such an 

 effect would be most powerful in the case of light 

 bodies like comets ; and, accordingly, some of the 

 short-period comets have been observed to return 

 to the sun in a shorter period each successive 

 revolution. In the case of the planets, no such 

 effect has yet been appreciable, owing to their 

 much greater mass. 2. The tidal wave moves 

 westward on the earth's surface, contrary to the 

 direction of the earth's daily motion; it is thus 

 believed to act like a friction-break on a wheel, 

 rendering the daily rotation slower, although by a 

 quantity so small, that it has not yet been ascer- 

 tained with certainty. 



SIDEREAL ASTRONOMY THE FIXED 

 STARS. 



The first thing that strikes us about the stars is 

 their difference as to brightness. They can be 

 classified according to this feature. The most 

 brilliant are said to be of the first magnitude; the 

 next of the second ; and so on. The smallest stars 

 visible to the naked eye are of the sixth or seventh 

 magnitude. The number of stars visible in one 

 hemisphere may be about 2000, making in all 

 4000. About twenty-four are reckoned of the first 

 magnitude. By using telescopes, a vast mass of 

 new stars come into view, which are reckoned as 

 far as the seventeenth magnitude; the numbers 

 and closeness increasing with every increase of 

 the telescope's power. 



From the earliest times, the stars have been 

 divided into groups called constellations, which 

 were named from fancied resemblances to animals 

 or other figures. Thus, a group in the northern part 

 -of the sky is called Ursa Major, or the Greater Bear. 

 From the figure of the seven more conspicuous 



stars, it is sometimes called the Plough. Another 

 well-marked group is called after the mythical 

 personage Orion. Individual stars are indicated 

 by the letters of the Greek alphabet or by numbers, 

 as n Ursa Majoris (in the tip of the tail of the 

 Greater Bear), 24 Comas. Some remarkable stars 

 have names, as Aldebaran ( Tauri), Sirius (in the 

 nose of Canis Major). By means of a celestial 

 globe, or a set of star-maps, the more conspicuous 

 constellations may be soon recognised. 



The stars are believed to be so many suns, 

 shining by their own light, and being each per- 

 haps the centre of a system of planets, the abodes 

 of sentient and intelligent existence. Their im- 

 mense distance reduces them all equally to mere 

 points of light ; in the most powerful telescopes 

 they shew no disk, and differ from one another, 

 not in magnitude, properly speaking, but in bril- 

 liancy. Notwithstanding this seeming inaccessi- 

 bility to observation, the spectroscope makes 

 known to us with almost certainty not a few facts 

 regarding their physical constitution. They have 

 white hot cloudy photospheres like the sun, and 

 contain pretty much the same substances; thus 

 Sirius has been ascertained to contain sodium, 

 magnesium, iron, and hydrogen. 



For a long time the distances of the fixed stars 

 were believed to be immeasurable. More refined 

 modes of observation have recently detected the 

 parallax of several stars, and determined positively 

 how far off they are. To state these distances in 

 miles conveys no idea. It is better to take some 

 large unit, such as the distance light travels in a 

 second, which is 1 86,000 miles ; we can then give 

 the distance of a star in the time its light takes to 

 reach us. The sun's distance is sometimes taken 

 as a standard. The nearest distance yet measured 

 is that of a fine double star in the southern hemi- 

 sphere (a. Centauri), calculated at 224,000 distances 

 of the sun, which it takes light three and a half 

 years to traverse. ' From the measurements al- 

 ready made, we may say that, on the average, 

 light requires fifteen and a half years to reach us 

 from a star of the first magnitude, twenty-eight 

 years from a star of the second, forty-three years 

 from a star of the third, and so on, until, for stars 

 of the twelfth magnitude, the time required is 3500 

 years.' Lockyer's Astronomy. 



Variable Stars. Some stars undergo periodical 

 increase and diminution of lustre, and are known 

 as variable stars. There are several instances 

 also on record of stars which have come into sight 

 for a time, and then gradually vanished, to which 

 the name of temporary or new stars has been 

 given ; the latter phenomenon, however, is no 

 doubt only an extreme case of the former, the 

 period being long, and the diminution of lustre 

 excessive. The star Omicron, in Cetus, is a re- 

 markable instance of a variable star. It goes 

 through a series of variations in a period of about 

 330 days. 



The analogy of our sun is thought to afford 

 an explanation of this phenomenon. The sun is 

 clearly a variable star, his light and heat varying 

 with the increase and diminution of the spots on 

 his surface, which follow, as we have seen, a 

 period of ten years. The observations of Balfour 

 Stewart and others go far to prove that the fre- 

 quency of sun-spots is regulated by the position 

 of the nearer planets. Mr Stewart holds that ' the 

 approach of a planet to the sun is favourable to 



