370 ASTRO 
length of the days and nights are occafioned by the peti¬ 
tion of the terminator with refpeft to tire axis of the Earth, 
we have now to explain what occafions thefe various pe¬ 
titions; which is the more important, as on thefe depend 
the divertity in the leafons. 
Of the Seasons of the Year. 
In considering this fubject, we dial! find further proofs 
of that divine wifdom which pervades all the works of 
the tmiverfe, and fee, that no other conformation of the 
fyftem could have given i'uch commodious diltributions of 
light and heat, or imparted fertility and plealure to lb 
great a part of the revolving globe. 
The changes in the pofition of the terminator are occa¬ 
fioned, i. By the inclination of the Earth’s axis to the 
plane of the ecliptic, or orbit in which it moves. 2. Be- 
caufe, through the whole of its annual courfe, the axis of 
the Earth preferves its pofition, or continues parallel to 
itfelf; that is, if a line be conceived as drawn parallel to 
the axis, while the Earth is in any one point of its orbit, 
the axis will in every other pofition of the Earth be pa¬ 
rallel to the faid line. It mult be evident, that the paral- 
lelifm of the axis mull occafion confiderable differences. 
By a bare infpection of the globes at A and B, fig. 1, of 
the above-mentioned Plate, we fee, that, when the Earth 
is in one pofition of its orbit, the north pole will be turn¬ 
ed towards the Sun, but in the oppofite part it will be 
turned from him. But the abfence of the Sun’s light pro¬ 
duces a proportionable degree of cold; hence the fealons 
are, in the northern and fouthern parts of the globe, dif- 
tinttly marked by different degrees of heat and cold. It 
it is this annual turning of the poles towards the Sun, that 
occafions the very long days in the northern and fouthern 
parts. It is owing to the fame caufe, that the Sun feems 
to rife higher in the heavens during fummer than in win¬ 
ter; and this alternate linking and riling is perceptible 
over the whole globe. 
If the axis of the Earth were perpendicular to the plane 
of its orbit, the equator and the orbit (orecliptic) would 
coincide;. and, as the Sun is always in the plane of the 
ecliptic, it would in this cafe be always over the equator, 
as fihewn in the globe at C, and the two poles would be in 
the terminator, and there would be no diverfity in the days 
and nights, and but one feafon of the year; but, as this is 
not the cafe, we may fairly infer, that the axis of the 
Earth is not perpendicular to the plane of its orbit. But, 
if the Earth’s axis be inclined to the plane of the ecliptic, 
when the Earth is in the fituation reprefented at A, the 
pole N will be towards the Sun, and the pole S will be 
turned from it; but juft the contrary w ill happen, when 
*he Earth, by going half round the Sun, has arrived at 
the oppofite point in its orbit. Hence the Sun will not al¬ 
ways be in the equator, but at one time of the year it will 
appear nearer to one of the poles, and at the oppofite fea¬ 
fon it will appear nearer to the other. Here then is the 
caufe of the change of feafons; for, when the Sun leaves 
the equator, and approaches to one of the poles, it will 
be fummer on that fide oF the equator, and when the Sun 
departs from thence, and approaches to the other pole, it 
will be winter. 
Thefe ideas may be firengthened, and a clearer notion 
obtained of the effect produced by the inclination of the 
Earth’s axis, by confidering Jig. 2, in the fame Plate ; in 
which the ellipfis is fuppofed to reprefent the Earth’s or¬ 
bit, the eye fomewhat elevated above the plane thereof. 
The Earth is here reprefented in the firlt point of each of 
the twelve figns of the ecliptic, as marked in the figure 
with the twelve correfponding months annexed; I J is the 
north pole of the globe, .P m its axis, round which the 
Earth performs its diurnal revolution from weft to eaft; 
this axis is exhibited as parallel to itfelf in every part of 
the orbit; PCE (hews the angle of its inclination, e the 
pole, f d the axis of the ecliptic, perpendicular to the plane 
of the orbit. In March, when the Earth is in the firfl 
point of Libra, the Sun appears in the oppofite point of 
1 
N O M Y. 
the ecliptic at Aries. In September, when the Earth is 
in the firft point of Aries, the Sun will be in Libra. At 
thefe times the terminator palfes through the poles of the 
world, and divides every parallel into two equal parts, 
(fee C, Jig. 1,) conlequently the nocturnal and diurnal 
arches, or the length of day and night, will be equal in 
all places over the world. 
Conceive the Earth to have moved from Libra to Ca¬ 
pricorn in June, the axis P m preferving its parallelifm by 
this motion, the north pole will have gradually advanced 
into the enlightened hemifphere; fo that the whole nor¬ 
thern polar circle will be therein, -while the fouthern pole 
is intmerged in obfeurity; the northern parts of the world 
will enjoy long days, while they are fhort in the fouthern 
parts. While the Earth is moving from Libra through 
Capricorn to Aries, the north pole remains in the illumi¬ 
nated hemifphere, and will therefore have fix months con¬ 
tinual day. But in the other half year, while the Earth 
is moving from Aries through Cancer to Libra, the north 
pole is turned from the Sun, and therefore in darknefs, 
but the louth pole is in the illuminated hemifphere. When 
the Earth is at Cancer, the fun is at Capricorn; at this fea¬ 
fon the nights to us will as much exceed the days, as the 
days exceeded the nights when the Earth was in the op- 
polite point of her orbit. Hence it is eafy to perceive, 
that the inhabitants of the fouthern hemifphere have the 
fame vicifTitudes with thofe of the northern, though not 
at the fame time, it being winter in one hemifphere when 
it is fummer in the other. During this courfe of the Earth 
through her orbit, there are four days particularly to be 
remarked; thefe aftronomers have diltinguilhed by the 
names of the foljlitial and equinodial days. The folftitial 
days are thofe on which the Sun appears mod to the north¬ 
ward and the fouthward : the equinoctial days are thofe on 
which he appears in the equator, and the days are equal to 
the nights. 
The annual motion of the Earth occafions a daily appa¬ 
rent change in the declination of the Sun. Thus, about 
the 22d of December, when the Earth is in Cancer, the 
Sun will be over the tropic of Capricorn ; and confequent- 
ly, by the Earth’s rotation on its axis, the inhabitants of 
every part of this circle will fucceilively have the Sun in 
their zenith, or, in other words, he will be vertical to 
them that day at noon. About the 21ft of March, the 
Earth is at Libra, and the Sun will then appear in Aries; 
a central folar ray will terminate upon the furface of the 
Earth, in the equator; and therefore the Sun appears to 
be carried round in the celeftial equator, and is fucceflively 
vertical to thofe who live under that circle. About the 
21 11 of June, when the Earth is in Capricorn, a central 
folar ray terminates on the furface of the Earth, in the 
northern tropic, and for that day the Sun appears to be 
carried round in the tropic of Cancer, and is vertical to 
thofe who live under that circle. About the 22d of Sep¬ 
tember, the Earth is in Aries, and the Sun in Libra, and 
the central folar ray again terminates at the equator; con- 
l'equently the Sun again appears in the celeftial equator, 
and is vertical to thofe who live under it. 
We have feen, that, as the Sun moves in the ecliptic 
from the vernal equinox to the tropic of Cancer, it gets to 
the north of the equator, or its declination towards our 
pole increafes. Therefore, from the vernal equinox, when 
the days and nights are equal, till the Sun comes to the 
tropic of Cancer, our days lengthen, and our nights fhort- 
en; but, when the Sun comes Jo the tropic of Cancer, it 
is then in its utmoft northern limit, and returns in the 
ecliptic to the equator again. During this return of the 
Sun, its declination towards our pole decreafes, and con¬ 
lequently the days decreale, and the nights increafe, till 
the Sun is arrived in the equator again, and is in the au¬ 
tumnal equinoctial point, when the days and nights will 
again be equal. As the Sun moves from thence towards 
the tropic of Capricorn, it gets to the fouth of the equa¬ 
tor; or its declination towards the fouth pole increafes. 
Therefore, at that time of year, our days ihorten, and our 
nights 
