GEOGRAPHY. 
352 
which is above the horizon, and will daily revolve in 
circles above the horizon; and for the'other half, it 
will be in fome part of T L,- and will perform its daily' 
revolutions in circles below the horizon. 
36. In any place between the poles and the equator, 
one celeftial pole will be elevated, and the other de- 
prefled, at an angle lefs than a right angle; and the 
celeftial equator will make an angle lefs than a right 
angle with the horizon. For, fmce the place is not in 
the equator, it has fome latitude; and fmce it is not at 
either of the poles, its latitude is lefs than 90 degrees : 
whence the poles are elevated, or depreffed, in an angle 
lei's than a right angle ; and confequently the equator, 
which is perpendicular to the axis, makes an angle lefs 
than 90 degrees with the horizon. 
37. Thofe who live on any part of the furface of the 
earth between tlm equator and either pole, are in an ob¬ 
lique fphere, and have all the circles of daily motion 
oblique to their horizon.—Let HO, Plate II. fig. 3, be 
the horizon of a place which lies between the earth’s 
equator and either of its poles; the celeftial equator E Q, 
and all its parallel circles, will be oblique to the hori¬ 
zon, and therefore each of the heavenly bodies, being 
in fome one of thefe circles, will appear to move in a 
path oblique to the horizon. 
38. When the I’lin, in his annual apparent courfe, is 
in the points in which tlie ecliptic cuts the equator, the 
day and night will be of the fame length at all places on 
the furface of the earth; but, when the fun is in any other 
piart of the ecliptic, the days will be ionger as the fun’s 
declination towards the elevated pole increafes, and 
fhorter as its declination towards the depreffed pole in¬ 
creafes. The plane of the horizon HO, Plate II. fig. 3, 
of any place, palTing through T, the centre of the fphere, 
and alfb through tlie centre of the equator, divides the 
equator C L into two equal parts, one half above, and 
the other half below, the horizon. When therefore the 
fun has no declination, or is in tli^e equator, it will ap¬ 
pear in its daily revolution to deferibe the equator CL; 
and, therefore, during one half of the revolution, it will 
be above the horizon, and, during the other half, below 
it. But fuppofe the lun to have its declination towards 
P, the elevated pole, equal to E ?« : its diurnal apparent 
revolution will be .in the circle 7nm, the centre of which 
is in a p:u't of tlie axis above the horizon ; whence the 
plane of the horizon does not pafs througli the centre, 
and confequently the circle nun is divided into two un¬ 
equal parts, tlie greater above the horizon, and the lefs 
below it. Tl'.erefore the fun, deferibing the circle mm, 
■with an uniform velocity, in its apparent diurnal revo¬ 
lution, will beTonger in deferibing the part above the 
horizon, than the part below it. And this difference ma- 
mfeftly increafes, as the circle of the fun’s apparent di¬ 
urnal motion recedes from the equator, that is, as" tlie 
fun’s declination towards P increafes. In like manner, 
it may be fhewn, that the days will be fhorter, as the 
fun’s declination towards the depreffed pole increafes. 
Or thus : I.et A B, Plate II. fig. 2, reprefent tlte plane 
of tlie ecliptic feen edgeways ; S, the fun in the focus of 
the orbit; MO, K L, XY, the earth in different parts 
of its orbit. If FI, the axis of the ecliptic B A, were 
alfo the axis or the earth-, that is, if the planes of the 
equator and ecliptic were coincident, it is manifeft that 
the fun, tlie apparent annual motion of whicli is in the 
plane of the ecliptic, would at all times of the year ap¬ 
pear to move in the circle of the equator, and to be equal¬ 
ly dift..at from tiie poles, and confequently could pro¬ 
duce, by its*apparent motion, no varieties in the length 
of days and nights. But tlie earth’s axis being inclined 
to the plane of its orbit, as p, wlien the earth is at 
MO, t;ie pole P will be towards the fun, and the pole 
^ tur;,ed from it, and tlie reverie when the earth is ar¬ 
rived at X Y, When the earth is in tlie middle ftation 
between B ,.nd />, in either piart of its orbit, both tiie 
p.oies will be in the circle illuminated, as at KL.—In 
the pofition M O, fince the fun muft always illuminate 
one half of the globe, the lightwyill pafs beyond the pole 
P as far as F, and will extend towards the pole p no 
farther tlian I. Confequently, in the diurnal revoluciou 
of the earth round its axis, while tlie earth remains in 
this pofition, all the parts of the globe betw'een F and 
G will be illuminated, and all the parts between I and 
H will be dark. Farther, in this pofition greater por¬ 
tions of thofe parallels which lie between the equator 
and the circle F G, will at any inftant be in the illumi¬ 
nated, than in the dark, heniifphere ; and, on the con¬ 
trary, greater portions of thofe which lie between the 
circle HI and the equator, will at any inftant be in the 
dark, than in the enlightened, hemifphere. Confequent-- 
ly, any given place on the fide of the equator towards P, 
will, in one diurnal rev'olution, be longer in the light 
than in the dark, and tire reverie on the fide fovtards/t. 
The difference between the length ofday-light and night, 
will decrcafe on either fide of the equator, as we approach 
towards it; and at the equator, the illuminated and dark 
portions of the circle being alw'ays equal, the days and 
nights will be of equal length. The contrary to all this 
will take place in the fituation XY. Continual varia¬ 
tions will take place, v/hile the eartli palfes from M O 
to K L, and f rom K L to X Y. But in the fituation K L, 
the illumination extending exaffly to both poles, all the 
parallel circles.are half illuminated, and half dark : con¬ 
fequently, any place upon the globe will, in a diurnal 
revolution, have equal portions of light and darknefs; 
that is, day and night will be every where of equal length. 
This muft happen twice in every annual revolution. 
All bodies, which are on the lame fide of the equator 
with the fpeCfator, continue longer above the horizon 
than below it, and vice verla; and as the orbits of tlie 
moon and planets are inclined to the equator, a variation 
of the times of their continuance above and below the ho¬ 
rizon will take place.—When the fun is very near either 
of the tropics, the days do not appear of different lengths, 
for the circles of apparent diurnal motion are fo near to 
each other, tha.t they cannot be lenfibly dilfinguilhed.—^ 
The diljferent degrees of lieat at different fcafbns of the 
year are owing partly to the different lengths of the days, 
and partly to the difl'erent degrees of obliquity with 
which the rays fall upon the atmofphere at different alti¬ 
tudes of tiie I'un.—See Astronom V, vol.ii. p.368. 
39. When the fun, or any other heavenly body, is in 
the equator, it riles in the eaft, and fets in the weft. For 
it then nfes and fets in the points in which the equator 
cuts the horizon ; that is, becaiife the equator is at riglit 
angles to the meridian, which paifes through tlie north 
and fouth points, in the points of eaft and weft. In north 
latitude, tliofe bodies which have north declination, rife 
between the eaft and iiortli; tliofe which have fouth de- 
clination, rife between the eaft and fouth. 
40. When tlie declination of the fun is towards the 
elevated pole, its nieridian altitude is equal to its decli¬ 
nation added to the elevation of the celeftial equator: 
wlieii its declination is towards the deprell'ed pole, its 
meridian altitude is equal to its declination fubtraCfed 
from the elevation of tiie equator.—Let HO, Plate II. 
fig. 3, be the horizon, T the eartii, P and S the celeftial 
poles, D the zenith, N the nadir, E (Ljhe equator. If 
the fun be at C, having its declination towards the ele¬ 
vated pole P, when it arrives at the meridian P S, its 
meridian altitude C H, is equal to the I'um of C E, its 
declination, and EH, the elevation of the equator. If 
the fun be at I, having its declination towards the de- 
prelfed pole S ; when it arrives at tlie meridian, its alti¬ 
tude I H, is equal to the difference of FT-i, tlie elevation 
of the equator, and E I, .tiie fun’s declination, as appears 
from the figure.—See Astronomy, vol. ii. p. 432. 
41. When the declination of a heavenly body towards 
the elevated pole, is equal to the latitude of any place, 
the body will pals tlirough tlie zenith of tliat place : and 
when its declination towards the depreffed pole is equa'l 
to 
