ASTRONOMY. 



ecliptic cross one another are called the equinoctial 

 points, because the sun is in them at the equinoxes. 

 The vernal equinoctial point, called Aries, is used 

 as a fixed mark from which to measure distances 

 on the heavens east and west, just as we measure 

 distances east and west on the earth, or terrestrial 

 longitude, from Greenwich. 



The terms latitude and longitude have different 

 meanings in astronomy and in geography. The 

 latitude of a place on the earth's surface is its 

 distance north or south of the equator ; the lati- 

 tude of a planet or star is its distance from the 

 ecliptic, while its distance from the celestial equator 

 is called declination. The longitude of a place 

 is its distance east or west from the first meridian 

 counted along the earth's equator ; longitude in 

 the heavens is distance eastward from the point 

 Aries, measured in the direction of the ecliptic, 

 and not in that of the equator. The distance of 

 a heavenly body from Aries, measured in the direc- 

 tion of the celestial equator, is called its right 

 ascension. The reason why the ecliptic is chosen 

 to refer the latitudes and longitudes of the heavenly 

 bodies to, is, that it is the only circle in the 

 heavens that approaches to the permanency of the 

 earth's equator ; it maintains almost a fixed posi- 

 tion among the stars, while the celestial equator 

 shifts its position in the course of ages, and thus 

 divides the starry sphere differently at different 

 periods. 



Besides the sun and moon, there are a good 

 many other wandering stars. The ancients dis- 

 covered five of these, and gave them the name of 

 planets, from the Greek word ' to wander.' The 

 rest have been discovered since the invention of 

 the telescope. 



Copernican and Ptolemaic Systems. We have 

 hitherto spoken of the appearances of the heavenly 

 bodies and their motions, without inquiring 

 whether they are as they seem. These appa- 

 rent motions were long considered to be real 

 motions. The earth was believed to be what it 

 seems to be a fixed station, the centre of the 

 universe ; the starry sphere was a solid shell, 

 revolving daily with its thousand fixed fires round 

 the central earth ; while within it were a succes- 

 sion of crystal spheres, carrying the sun, moon 

 and planets, and, while partaking of the common 

 inotion from east to west, having also each a 

 motion of its own. But as observations of the 

 planetary motions were multiplied, it became 

 more and more difficult to account for all the 

 appearances in this way. At last it was found 

 necessary altogether to change the point of view 

 to give up the earth as the centre of all the 

 heavenly motions, and admit that it itself, along 

 with the other planets, is in motion round the sun 

 as a centre. This view, called the Copernican 

 system, from Copernicus, its author, or rathe 

 reviver for it had been held by more than on< 

 ancient philosopher is now universally prevalent 

 the earlier is known as the Ptolemaic system, from 

 Ptolemy, the chief ancient writer on the subjec 

 whose works we possess. 



Instead of attempting to state the argument 

 by which the old opinions are refuted, we wil 

 proceed at once to give an outline of what is now 

 believed to be the arrangement and constitution 

 of the heavenly bodies. The simple way in which 

 the Copernican system accounts for all the appear- 

 ances, is the best argument in its favour. 



THE SOLAR SYSTEM. 



Of the heavenly bodies, those spoken of above 

 s wanderers compose a group altogether apart, 

 ic earth being one of them. The sun, which is 

 astly greater than all the rest put together, forms 

 he centre of the group ; which is hence called the 

 olar system (from sol, the Latin word for ' sun '). 

 Chough the distances of the bodies of this group, 

 rom the central sun and from one another, are 

 normous when measured by ordinary terrestrial 

 tandards, they become as nothing when compared 

 with the space that separates the whole of them 

 rom the nearest of the fixed stars. The solar 

 ystem forms thus a compact inner world, cut off 

 jy an almost immeasurable chasm from the outer 

 universe. It is with this inner world that the 

 astronomer has chiefly to do ; what is known of 

 he fixed stars will be spoken of apart, under the 

 lead of Sidereal Astronomy. 



Around the sun as a centre, the smaller bodies 

 or planets wheel at different distances in paths or 

 orbits of a round form, being what are called 

 ellipses ; these orbits lie pretty nearly in the same 

 jlane, a plane passing through the sun's centre ; 

 ind the motions of the planets are, as a general 

 rule, all in one direction from west to east. 



Some of the planets have other planets moving 

 round them as centres the moon, for instance, 

 round the earth. These are called secondary 

 slanets, moons, or satellites; while those that 

 inove round the sun are called primary planets. 

 The primary planets now known consist ist, of 

 eight larger planets, including the Earth ; their 

 names, in the order of their nearness to the sun, 

 are Mercury, Venus, the Earth, Mars, Jupiter, 

 Saturn, Herschel or Uranus, and Neptune, id, 

 A group of small planets, called sometimes also 

 asteroids, of which more than two hundred are 

 now known. They are situated between Mars 

 and Jupiter. 



The satellites or moons, as yet discovered, 

 number eighteen ; of which the Earth has one, 

 Mars two, Jupiter four, Saturn eight, Uranus four, 

 and Neptune one. In addition to its moons, 

 Saturn is attended by a luminous ring. 



Those singular bodies called comets (Lat. coma, 

 a lock or brush of hair) also belong, many of them 

 at least, to the solar system. And in addition to 

 the above luminous members of the system, it is 

 now becoming apparent that there are multitudes 

 of dark bodies, of various sizes, circling in the 

 spaces between the known planets, some of which 

 become visible to us when they accidentally ente 

 our atmosphere as shooting-stars and other forms 

 of meteors. See METEOROLOGY. 



Such is a brief inventory, as it were, of the 

 furniture of this inner world to which we belong 

 The figure on the following page represents the 

 relative positions of the chief planetary orbits 

 Appended are tables of the diameters, distances,and 

 other numerical particulars of the several bodies. 



Astronomical Mensuration. Though it would 

 be inconsistent with the scope of the present 

 sketch to describe minutely the processes by wnicl 

 these numbers are found, it is necessary to give 

 a general idea of the methods followed, that the 

 reader may be able to conceive the possibilit 

 measuring and weighing objects so completely 

 beyond our reach. ^ 



