342 ASTRO 
by computation, from thefe principles, it might be ex¬ 
pected. 
The orbit of the Moon is notprecifely in the fame plane 
with that of the earth ; but makes a very fmall angle with 
it. The points of interfedtion of thofe two planes, are 
called the nodes of the Moon. Thefe nodes of the Moon 
are in continual motion, and, in eighteen or nineteen years, 
revolve backwards, from eaft to weft, through all the dif¬ 
ferent points of the ecliptic. For the Moon, after having 
finifhed her periodical revolution, generally interfeCts the 
orbit of the Earth fomewhat behind the point where fhe 
had interfered it before. But, though the motion of the 
nodes is thus generally retrograde, it is not always fo, but 
is fometimes direCt, and fometimes they appear even (fa¬ 
tionary ; the Moon generally interl'edts tire plane of the 
Earth’s orbit behind the point where (he had interfered 
it in her former revolution ; but fhe fometimes interfers it 
before that point, and fometimes in the very fame point. 
It is the (ituation of thole nodes which determines tlie 
times of eclipfes, and their motions had, upon this ac¬ 
count, at all times been particularly attended to by adro- 
nomers. Nothing however had perplexed them more, than 
to account for thefe fo inconfilfent motions, and, at the 
fame time, preferve their fo much fought-fur regularity 
in the revolutions of the Moon. For they had no other 
means of connecting the appearances together, than by 
fuppofing the motions which produced them to be in rea¬ 
lity perferiy regular and equable. The hiftory of adro- 
nomy therefore gives an account of a greater number of 
theories invented for conneCfing together the motions of 
the Moon, than for connecting together thofe of all the 
other heavenly bodies taken together. The theory of gra¬ 
vity connected, in the molt accurate manner, by the dif¬ 
ferent aCtions of the Sun and the Earth, all thole irregu¬ 
lar motions ; and it appears by calculation, that the time, 
the quantity, and the duration, of thofe direCt and retro¬ 
grade motions of the nodes, as well as of their (tationary 
appearances, might be expeCted to be exactly fuch, as 
the obfervations of aftronomers have determined them. 
The fame principle, the attraction of the Sun, which 
thus accounts for the motions of the nodes, connects too 
another very perplexing irregularity in the appearances of 
the Moon ; the perpetual variation in the inclination of 
her orbit to that of the Earth. As the Moon revolves in 
an ellipfe, which has the centre of the earth in one of its 
foci, the longer axis of its orbit is called the line of its ap- 
fides. This line is found, by obfervation, not to be always 
directed towards the fame points of tlie firmament, but to 
revolve forwards from wed to eaft, fo as to pafs through 
til! the points of the ecliptic, and to complete its period 
in about nine years; another irregularity which had very 
much perplexed aftronomers, but which the theory of 
gravity diffidently accounted for. 
The Earth had hitherto been regarded as perfectly glo¬ 
bular, probably for the fame reafon which had made men 
imagine that the orbits of the planets mud neceffarily be per¬ 
fectly circular. But Sir IfaacNewton, from mechanical jirin- 
ciples, concluded that, as the parts of the Earth mud be 
more agitated by her diurnal revolution at the equator than 
jat the poles, they mud neeeOarily be fomewhat elevated at 
the fird, and flattened at the fecond. The obfervation, 
that the ofcillations of pendulums were dower at the equa¬ 
tor than at the poles, feeming to demondrate, that gravity 
was drongerat the poles and weaker at the equator, proved, 
he thought, that the equator, was further from the centre 
than the poles. All the meafures however, which had hi¬ 
therto been made of the Earth, feerned to fliew the con¬ 
trary, that it was drawn out towards the poles, and flat¬ 
tened towards the equator. Newton, however, preferred 
his own mechanical computations to the former meafures 
of geographers and adrqnomers; and in this he was con¬ 
firmed by the obfervations of adronomers on the figure of 
Jupiter, whofe diameter at the pole feems to be to his dia¬ 
meter at the equator, as twelve to thirteen; a much greater 
inequality than could be fuppofed to take place betwixt 
N O M Y. 
the ccrrefpondent diameters of the Earth, but which was 
exaCtly proportioned to the fuperior bulk of Jupiter, and 
the fuperior rapidity with which he performs his diurnal 
revolutions. The obfervations of adronomers at Lap- 
land and Peril have fully confirmed Sir K'aac’s fydem, and 
have not only demondrated, that the figure of the Earth 
is, in general, fuch as he fuppofed it; but that the pro¬ 
portion of its axis to the diameter of its equator is alinod 
precifely fuch as he had computed it. And, of all the 
proofs that have ever been adduced of the diurnal revo¬ 
lution of the Earth, this perhaps is the mod (olid and fa- 
tisfaCfory. 
Hipparchus, by comparing his own obfervations with 
thofe of fome former adronomers, had found that the equi- 
noxial points were not always oppohte to the fame part of 
the heavens, but that they advanced gradually eadward by 
fo dow a motion, as to be fcarcely fenfible in one hundred 
years, and which would require thirty-fix thoufand to 
make a complete revolution of the equinoxes, and to carry 
them fucceflively through all the different points of the 
ecliptic. More accurate obfervations difcovered that this 
procedion of the equinoxes was not fo (low as Hipparchus 
had imagined it, and that'it required fomewhat lefs than 
twenty-fix thoufand years to give them a complete revo¬ 
lution. While the ancient fydem of adronomv, which 
reprefented the earth as the immoveable centre of the uni- 
verfe, took place, this appearance was necedarily accounted 
for, by fuppofing that the firmanent, belides its rapid di¬ 
urnal revolution round the poles of the equator, had like- 
wife a (low periodical one round thofe of the ecliptic. And 
when the (ydem of Hipparchus, was by the fchoolmen 
united with the folid fphere of Aridotle, they placed a 
new crydalline fphere above the firmanent, in order to join 
this motion to the red. In the Copernican fydem, this 
appearance had hitherto been connected with the other 
parts of that hypothefis, by fuppofing a fmall revolution 
in the Earth’s axis from ead to wed. Sir Ifaac Newton 
connected this motion by the fame principle of gravity, by 
which he had united all the others, and diewed how the 
elevation of the parts of the Earth at the equator mud, by 
by the attraction of the Sun, produce the fame retrograde 
motion of the nodes of the ecliptic, which it produced on the 
nodes of the Moon. He computed the quantity of motion 
which could arife from this aCtion of the Sun, and his cal¬ 
culations here too entirely correfponded with the obferva¬ 
tions of adronomers. 
Comets had hitherto, of all the appearances in the hea¬ 
vens, been the lead attended to by adronomers. The 
rarity and incondancy of their appearance, Teemed to fe- 
parate them entirely from the condant, regular, and uni¬ 
form, objects in the heavens, and to make them referable 
more the incondant, tranfitory, and accidental, pheno¬ 
mena of thofe regions that are in the neighbourhood of 
the Earth. Aridotle, Eudoxus, Hipparchus, Ptolemy, 
and Purbach, therefore, had all degraded them below the 
Moon, and ranked them among the meteors of the upper 
regions of the air. The obfervations of Tycho Brahe 
demondrated that they afcended into the celedial regions, 
and were often higher than Venus or the Sun. Des Cartes, 
at random, fuppofed them to be always higher than even 
the orbit of Saturn ; and feems, by tire fuperior elevation 
he thus bedowed upon them, to have been willing to com- 
penfate that unjuft degradation which they had Coffered 
for fo many ages before. The obfervations of fome later 
aftronomers demondrated, that they too revolved about 
the Sun, and might therefore be parts of the folar fydem. 
Newton accordingly applied his mechanical principle of 
gravity to explain the motions of thefe bodies. That they 
defcribed equal areas in equal times, had been difcovered 
by the obfervations of fome later aftronomers ; and New¬ 
ton endeavoured to (hew how from this principle, and 
thofe obfervations, the nature and podtion of their feveral 
orbits might be ascertained, and their periodic times de¬ 
termined, fo as even to predict the returns of them. 
But of all the arguments of the Newtonian philofophy, 
which 
