CHAMBERS'S INFORMATION FOR THE PEOPLE. 



seems to be their common centre. A star lying 

 in it would be perfectly at rest, and stars near it 

 describe very small circles, or move within narrow 

 limits. This point of rest is situated in the north 

 quarter of the heavens, more than half-way up 

 from the sky-line, towards the summit. It is called 

 the north celestial pole, or the north pole of the 

 heavens, and is the most important point in the 

 sky for astronomical references. No visible star 

 is actually residing in it ; but a very bright star 

 lies near it, called for that reason the pole-star. 



By observing the movements of stars that rise 

 in the south-east, and seeing their visible paths 

 becoming shorter and shorter, as they lie more 

 southward, we cannot help inferring that there is 

 a large region of stars describing circles wholly 

 out of sight, and that there is a point beneath, 

 which is the centre of these circles, corresponding 

 to the centre of the northern heavens. This point 

 is called the south celestial pole. An imaginary 

 line drawn from pole to pole, or from the elevated 

 north resting-point to the concealed south resting- 

 point, is called the axis of the heavens, or the axis 

 of the -world. Owing to one pole being a con- 

 siderable way up in the sky, and the other far 

 down out of sight, it is evident that the great 

 whirl of the starry sphere goes on in a slanting 

 direction. 



Besides the poles and axis of the heavens, there 

 are many other imaginary points, lines, and circles, 

 which it is convenient to suppose drawn among 

 the stars, and which of course we can actually 

 draw in all figures and sketches of the starry 

 heavens. The most important of these may be 

 here explained. 



Equator, Horizon, Zenith, Meridian. The ce- 

 lestial equator, so called because it cuts the starry 

 sphere into two equal halves, is an imaginary ! 

 circle passing round the heavens midway between 

 the two poles. The two halves into which it divides 

 the sphere of stars form the northern and southern 

 hemispheres. 



The horizon, or the line where the earth seems 

 to meet the sky, makes another division of the 

 starry sphere, into the visible hemisphere, or stars 

 above the horizon; and the invisible, or stars below 

 the horizon. The point of the heavens directly 

 over our heads, or the very summit of our sky, is 

 called the zenith. The point that we should see 

 directly beneath our feet, if the earth could be 

 seen through, is called the nadir. 



Another artificial circle supposed to be drawn 

 in the heavens, is the meridian. Like the equator 

 and the horizon, it divides the starry sphere into 

 two equal half-spheres, but in a different direction 

 from either of those circles. In fact, it is so drawn 

 as to be perpendicular 

 to both. It passes 

 through the north point 

 of the horizon, then up- 

 ward through the north 

 . celestial pole, through 

 I the zenith, down through 

 the south point of the ' 

 horizon, through the 

 south celestial pole 

 beneath, and finally 

 through the nadir : so 

 that, in the first place, it lies north and south ; 

 and in the second place, it stands upright. Thus, 

 if in the figure, P/ be the great axis of the heavens 



terminating in the north and south celestial poles, 

 EQ the celestial equator, NS the horizon, and N 

 and S the north and south points of it, Z the zenith, 

 and D the nadir ; then the enclosing circle of the 

 figure NPZES/DQ is the meridian. 



It is evident from this description, that when 

 the sun or the moon passes the meridian, or lies 

 due south, it has reached its greatest height, and 

 will immediately commence to descend. Hence, 

 also, we can determine the meridian line in the 

 heavens, by observing where a star is when it 

 ceases to ascend and begins to descend ; which 

 of course gives us the direction of the north and 

 south points, and serves the same end as the 

 mariner's compass. It is evident, also, that the 

 middle point between a body's rising and setting 

 is its meridian passage. 



Fixed Stars and Wanderers. Although the 

 general starry sphere turns round in one great 

 mass, so that each star is always in the same 

 place among the other stars, it was early observed 

 that there were exceptions to the common move- 

 ment ; and that a small number, besides going 

 round the sky daily, shift about among the others. 

 In opposition to these erratic or wandering 

 bodies, the general multitude that kept their places 

 were called fixed stars. 



The moon, for example, is soon observed to be 

 an erratic body. One night we see it near one 

 star, and the next night it is at a considerable 

 distance to the east of it ; even in a few hours 

 there is a visible change of position. In the course 

 of a month, we find it has gone through a complete 

 circle from west to east among the stars, and come 

 back nearly to the same place. 



The sun has a similar motion, though it is 

 slower, and also less easily marked, because his 

 light prevents the stars from being seen at the 

 same time. If we could see a star any night 

 setting at the same instant as the sun, we should 

 find that the next night it would set four minutes 

 before the sun, the sun having in the meantime 

 moved backward a short distance. 



Ecliptic, Latitude, Longitude, Declination, &c. 

 When this motion of the sun among the stars 

 is observed for a length of time, it is found that it 

 forms a circuit, bringing him back, in the course 

 of a year, to the same position. The path, how- 

 ever, which he travels, is not exactly east and west, 

 or parallel to the equator. If this were the case, 

 the sun would rise and set always in the same 

 points of the horizon. But we see that on the 

 2 ist of March, for instance, he rises exactly in the 

 east ; for the next three months the point of his 

 rising is more and more north of east, till the 

 2 ist of June, when it begins to recede, and again 

 reaches the east point on the 2 ist of September ; 

 and during the winter months it makes a similar 

 approach to and retreat from the south. The 

 amount of divergence either way is about a fourth 

 of the distance between the east point and the 

 north or south point. From this, and other appear- 

 ances, it is evident that the annual path described 

 by the sun among the stars runs slanting across 

 the equator. This is an important circle in astro- 

 nomy, and is called the Ecliptic; the angle which 

 it makes with the equator is called its obliquity, 

 and on this depends the variety of the seasons. 

 Its amount is about a fourth of a quarter circle 

 (23 280. 



The points where the celestial equator and the 



