- Zhe eardinal points of the ecliptic are the two equi- 
* noctial and two solstitial points. |) 
The equinoctial points or equinoxes, are those points 
of the ecliptic where it intercepts the equin viz. 
the beginnin of Aries and the beginning of Libra 
180° distant’ from each other. The sun enters Aries 
about the middle of spring, and Libra about the middle 
of autumn; hence the former is called the vernal, and 
_ the latter the autumnal equinox. : 
The solstitial points or solstices, are those points of 
the ecliptic that are at the test distance from the 
equinoctial, viz. the beginning of Cancer and the be- 
ginning of Capricorn, also 180° distant from each other. 
‘he sun enters the former about the middle of sum- 
mer, andthe latter at the middle of winter ; hence the 
one is called the summer, and the other the winter sol- 
stice. . 
~The equinoctial colure is a meridian passing through 
the equinoctial points. 
The solstitial colure is a meridian passing through 
the solstitial points. © 
Circles of celestial longitude are great circles per- 
oor a the ecliptic, and consequently. passing 
i through its poles. io is 
els of _ Parallels of declination ave small circles parallel to 
ination. the equinoctial. = ; , 
' Every point in the celestial sphere between the equi- 
noctial and the poles appears to describe: a parallel of 
Land declination in 24: hours. ‘When partof this parallel is 
oa above, and part’ below the horizon, the former is called 
ies. | the diurnal, and a latter the nocturnal arch. 
|» The tropics of Cancer and Capricorn are parallels of 
aint: pt: A the one’ through the ‘summer sol- 
. stiee 23° 28’ to the north, and the other through the 
winter solstice 239/28’ to the south of the equinoctial. 
They are so called’ from a Greek» word’ signifying to 
turn, because the sun in his annual course appears to 
recede from the equinoctial till he reaches the tropics, 
after which he returns towards it; 9 5 © 
The arctic and antarctic polar circles are parallels of 
declination; 23° 28’ from their eorresponding poles. 
‘Parallels of celestial latitude aye small circles parallel 
to the ecliptic. . 
The declination of a heavenly body is its distance 
_ from the equinoctial measured on a meridian; or it is 
lels of 
the arch of a meridian intercepted between the equi- 
noctial, and a parallel of declination passing through 
the body. It is called north or south according as it is 
to the north or south of the equinoctial. 
The declination of a body cannot exceed 90°. 
The right ascension of a heavenly body is its distance 
from the vernal equinox measured on the equinoctial ; 
or it is the arch of the equinoctial intercepted between 
the vernal equinox and a meridian passing through the 
body, reckoning in the order of the signs. 
4 The latitude of a heavenly body is its distance north 
or south from the ecliptic, measured on a circle of lon- 
gitude ; or it is the arch of a circle of longitude inter- 
cepted between the ecliptic and a parallel of latitude 
ing through the body. 
The longitude of a heavenly body is its distance from 
the vernal equinox measured on the ecliptic; or it is 
the arch of the ecliptic intercepted between the vernal 
equinox, and a circle of longitude passing through the 
The altitude of a heavenly body is its height above 
the horizon measured on a vertical circle; or it is the 
arch of a vertical’ circle in between the horizon 
and the body. The arch of = same circle between 
GEOGRAPHY. 
147 
the’ and the zenith, is called its zenith distance, Mathemati- 
The altitude.and zenith distance are together equal to “! Geogr- 
90°. The former is greatest when the body is on the PY 
meridian of the place, which is then called its meridian Zenith dis- 
altitude, and at equal distances on either side of the tance. 
meridian, the altitude is the same. Meridian 
The azimuth of a body is its distance from the meri- altitude. 
dian of the place measured on the horizon, or it is the 4 ji .uth, 
a of the horizon intercepted between the meridian 
e place, and the vertical circle passing through the 
body. It is called eastern or western azimuth, accord- 
inp as’ the body is on the east or west side of the meri- 
ian. ; 
Avbody is said to rise, culminate, and set when its Rising, &c 
centre-is’in the eastern side of the horizon, themeridian °f 4 'e* 
of the place, and the western side of the horizow re» “°° 
ively. 
|. The casein and western amplitude of a body is the Amplitude. 
arch of the horizon intercepted between. the point 
where the body rises or sets, and the east or west point 
of the horizon. ; 
The zodiac 'is a zone or circular space of the celestial 
sphere, extending about 9° on each side of the ecliptic. 
The preceding definitions will be rendered more in- 
telligible perhaps, by referring to Plate CCLXV. Fig. 1. 
where HRN represents the celestial sphere, and hzrn 
the earth, situated in its centre. Then if Z be the ze- 5. ony: 
nith of a place z, P and S the north and south poles of cep_xy. 
the world, and C the vernal equinox, HR will be the vig. 1. 
horizon, AEQ the equinoctial, EL the ecliptic, and P’ 
its pole; PS the equinoctial colure, HZRN the: solsti- 
tial colure, TL the- tropic of Cancer, and ED the tropic 
of, Capricorn. Also, since the circle HZRN passes 
through Z, P and P’ the poles of the horizon, equinoc- 
tial and ecliptic, it represents a vertical circle, a meri- 
dian, and a circle of celestial longitude: If; therefore, 
A be a given star, the arch RA will be its altitude, and 
ZA its zenith distance to an observer at’ z, or rather at 
the centre of the earth, QA will be its declination, LA 
its latitude, CQ its right ascensién, and CL its longitude. 
In this case, its azimuth is nothing, the star being on 
the meridian of the place. It is hardly necessary to 
observe, that though CL and CQ, as represented in the 
Figure, appear to be straight lines, they are nevertheless 
arches of great circles, the point C being the pole of 
the hemisphere HZRN. The same is true of’ all the 
other straight lines passing through the point C. , 
Having thus explained some of the principal points 
and lines, which geographers have imagined to be tra- 
ced out on the celestial sphere, we shall briefly men- 
tion the different classes and characters of the heavenly 
bodies themselves. These are all comprehended under 
three heads, Fixed Stars, Planets, and Comets. 
The fixed stars are those heavenly bodies, that have 
always the same situation relatively to one another, 
having no other motion than what they derive from the 
apparent diurnal revolution of the celestial sphere from 
east to west. In order to distinguish them more easily 
from one another, they are divided into classes, accord- 
ing to their brilliancy and apparent magnitude, the yag,;. 
brightest and largest being denominated’ stars of thé tudes. 
first itude, the next largest of the second magni- 
tude, and so on to the sixth. Stars less than'those of 
the sixth magnitude, are not in general visible ‘by ‘the 
naked eye, and are called telescopic stars. Besides this Telescopic 
classification, the fixed stars are also divided into groups stars. 
called Constellations, each being named after some ani- Constella- 
mal, or other object, to which the stars, in their arrange- tions. 
ment, bear a fancied resemblance.’ There are twelve 
Zodiac. 
Tilustration 
of the pre- 
ceding de- 
finitions. 
Heavenly 
bodies. 
Fixed_stars. 
