148 
Mathemati- such constellations in the zodiac, and it is from them 
cal Geogra- that the signs of the ecliptic take their names. The 
phy-_ stars in-each constellation are distinguished by the let- 
ters of the Greek alphabet, the first letter denoting the 
largest star of the constellation, whatever may be its 
Unformed magnitude. Stars not included in any of the constel- 
stars. lations, are called Unformed Stars. 
Planets. The planets, as we have already observed, are bodies 
which, besides their diurnal revolution, have also a mo- 
tion of their own in a direction contrary to the other. 
They are e/evenin number, and are found to revolve about 
the sun as a centre, in different periods, and at diffe- 
rent distances. These eleven are bea te Primary 
Primary Planets, to distinguish them from the Secondary or Sa- 
and second- {el/jtes, with which some of them are accompanied, and 
pr f — to which they serve as centres of revolution. . The path 
of a planet. which a planet describes about its centre is called its 
Orbit. The primary planets are written and expressed 
as follows: 
% 9 © 3 3 
Mercury = Venus. Earth. Mars, Juno. 
Pri Week os cihgtl or ahve inctiaasl 
Ceres. Pallas. Jupiter. Saturn. Georgium Sidus, Herschel, or Uranus. 
Vesta. 
For the distances of the planets from the sun, the pe- 
riods of their revolutions, magnitudes, &c. see AsTRO- 
nomy Indew. 
Comets. Comets are luminous bodies, which appear in the hea- 
vens only occasionally, and for a limited period, gene- 
rally consisting of a nucleus surrounded by a luminous 
vapour, sometimes shooting out into a long train or éail. 
Comets, besides the diurnal revolution common to all 
the heavenly bodies, have also a motion peculiar to 
themselves. In some, this motion is in the same direc- 
tion with that of the planets, and in others it is in the 
contrary direction. See Astronomy and Comets. 
Secr. III. Of the Earth, or Terrestrial Sphere. 
Revolution Ir all the inequalities of the earth’s surface were re- 
of the moved, it would appear to its inhabitants to be a circu- 
earth the Jar plane, fixed in the centre of the celestial sphere. 
cause of the Th d a *n thi 
apparent ere are two eceptions, owever, In this appearance. 
revolution ‘The earth is not a plane, neither is it at rest. Its true 
of the sun figure, as we have already shown, is spherical, and it is 
| jer continually in motion, revolving both daily on its own 
1 ere. 
axis, and annually in an orbit round the sun. It must 
be obvious, on a moment's reflection, that the apparent 
diurnal motion of the heavenly bodies may be produ- 
ced, either by the revolution of the celestial sphere from 
east to west, or by the rotation of the earth on its axis 
from west to east. Of the two explanations of the phe- 
nomenon, therefore, we are bound to adopt that which 
is liable to fewest objections, and this will be found to 
be the rotation of the earth. The revolution of the hea- 
vens, indeed, presents difficulties both physical and me- 
chanical, which it is impossible to remove, while the 
other hypothesis is perfectly simple in itself, and in uni- 
son with the other phenomena of the universe. The 
same observations will apply, with perhaps still more 
force, to the.annual revolution of the earth round the 
sun, which is the true cause of the sun’s apparent mo- 
tion in the heavens from west to east. The earth, 
therefore, is to be ranked among the planets, revolving 
about the sun in a year from west to east, having the 
ecliptic for its orbit, and accompanied by the moon asa 
satellite or secondary planet. But though we have thus 
noticed the real motions of the earth, as the causes of 
GEOGRAPHY. 
the apparent revolution of the sun and the celestial Math 
sphere, we shall not, in the remaining part of the pre- “! © 
sent article, always confine ourselves to this view of the —? 
subject. In the solution of geographical problems, it is 
often much simpler to consider the apparent revolution 
of the celestial sphere, and the motion of the sun inthe 
ecliptic, as real, than as produced by the combined mo- 
tions of the earth, while the solution is the same in both 
cases. We shall not hesitate, therefore, to assume the 
first of these, whenever by doing so we can render the 
subject more intelligible or concise. : 
It is obvious from Plate CCLXV. Fig. 1. that the Circles of 
planes of all great circles of the sileaial sphere form, the terre 
by their inibersectiahe sii eerie of 9 earth, cor- 
responding great circles on latter. ushr, zn, | 
pen @q, &c. on the earth, correspond to HR, ZN, PS, CLE 
/EQ, &c. in the heavens. This transference of the cir- Fig. 1. | 
cles from one sphere to the other, may be still more 
clearly understood, by conceiving the celestial sphere 
uniformly contracted in its dimensions, without any 
change in the relative position of its parts, till it be 
just sufficient to cover the terrestrial sphere, By this 
method, the small as well as the great circles of the hea- 
vens may be transferred to the same positions on the 
earth ; thus TL would coincide with ¢/, and ED with | 
ed. ; eae i 
The points p and s on the earth, immediately under Poles. 
the poles of the world, are called the north and south : 
les of the earth ; aq the equator or equinoctial ; hr or B1*tor~ 
iR e rational horizon, to distinguish it from H/R’, Rational 
the limit of an observer's vision at z, and which is called horizon 
the sensible horizon ; ps and hzrn meridians, or circles Sensi 
of longitude ; and tl, ec, parallels of latitude. In gene- horiz 
ral, all parallels of declination on the celestial sphere Meric 
become els of latitude on the earth, retainin be , 
ever their proper or individual names. Thus the tro= jjtitude, 
pic of Cancer in the celestial sphere, is a paraltel of de- . 
clination 23° 28” to the north of the equinoctial, and on 
the earth it is a parallel of latitude at the same distance 
from the equator. Sede 
Besides the terms already defined in the account of 
the celestial sphere, there are others peculiar to the . 
earth, which require to be explained. 
The first meridian of any country, in modern systems First m 
of phy, is the meridian passing through the ca- “aD — 
pital of that country, from which the position of other 
meridians is determined. The ancients chose for their 
first meridian that of the Fortunate Isles, which they 
conceived to be the limit of the habitable world. In 
later times, the meridian passing through Ferro, one of 
the Canary Islands, and nearly the same with that of 
the ancients, was used as the first meridian by geogra- 
phers of many countries; but now every nation ge- 
nerally reckons from the meridian of its own metropolis. 
The longitude of any place on the earth is its dis- Terr 
tance from the first meridian, measured on the equator; engi 
or it isthe arch of the equator intercepted between the 
first meridian and the meridian of the place. It is cal- 
led east or west longitude, according as the place lies 
to the east or west of the first meridian. Longitudeon —  _ 
the earth corresponds, not to longitude, but to right — 
ascension, .on the celestial sphere. As the longitude of 
places on the earth is reckoned in two directions, it 
never can exceed 180°, whereas the right ascension of a 
heavenly body may be any thing betweeen 0 and 360°, 
being always reckoned in one direction only, viz. in the 
order of the signs. , : 
The difference of longitude between two places, is the Peal 
distance between them measured on the equator ; or it is tad, 
the arch of the equator intercepted between the meridians 
