ASTRONOMY, 
Vli«n the. eclipfes happen at each node only once, there 
may be fix eclipfes in a year, four of which muff be of 
the Sun, and two of the Moon. But if, as in the laft cafe, 
an eclipfe Ihould happen at the return ot the Earth within 
the lunar ecliptic limits at the fame node a f'econd time 
within the year, there may be fix eclipfes, three of the 
Sun, and three of the Moon. 
There may be feven eclipfes in a year. For twelve lu- 
nations are performed in 354 days, or in eleven days lefs 
time than a common year. If therefore an eclipfe of the 
Sun fhould happen before January ir, and there be, at 
that and the next node, two folar and one lunar eclipfe 
at each node; then the twelfth lunation from the fir ft 
eclipfe will give a new Moon within the year, and (on ac¬ 
count of the retrograde motion of the Moon’s nodes) the 
Earth may be got within the folar ecliptic limits, and there 
may be another folar eclipfe. Hence, when there are fe¬ 
ven eclipfes in a year, five will be of the Sun, and two of 
the Moon. And, as feven eclipfes but feldom happen in a 
year, we may conclude the mean number to be about four. 
The ecliptic limits of the Sun being greater than thofe of 
the Moon, there will be more folar titan lunar eclipfes in 
about the fame proportion as the limit is greater, that is, 
as three to two nearly. But more lunar than folar eclipfes 
are feen at any given place, becaufe a lunar eclipfe is vi- 
fible to a whole hemifphere at once, whereas a folar eclipfe 
is vifible only to a part; and therefore there is a greater 
probability of feeing a lunar than a folar eclipfe. And, 
lince the Moon is as long above the horizon as below, eve¬ 
ry fpe&ator may expetf to fee half the number of lunar 
eclipfes that happen, provided the firmament be clear. 
With refpeft to the periods of eclipfes, if the places of 
the Moon’s nodes were fixed, eclipfes would always hap¬ 
pen nearly at the fame time of the year; but as they have 
a motion of about 3' 11" every day backwards, or con¬ 
trary to the order of the figns, the fucceeding eclipfe mud 
recede likewife ; and in one revolution of the nodes, which 
is completed in eighteen years and 225 days, there would 
always be a regular period of eclipfes, if any complete 
number of lunations were finifired without a fraction. But 
this never happens, for if both the Sun and Moon fhould 
ftart from a line of conjunction with either of the nodes 
in any point of the ecliptic, the Sun would perform 
eighteen annual revolutions and 222 degrees over and above, 
and the Moon 230 lunations and eighty-five degrees of the 
231, by the time the node came round to the fame point 
of the ecliptic again ; fo that the Sun would then be 138 
degrees from the node, and the Moon eighty-five degrees 
from the Sun. But in 223 mean lunations, after the Sun, 
Moon, and nodes, have been once in a line of conjumftion, 
they return fo nearly to the fame ftate again, as that the 
fame node, which was in conjunction with the Sun and 
Moon at the beginning of the firft of thefe lunations, will 
be within 28' 12" of a degree of a line of conjunction with 
the Sun and Moon again, when the laft of thefe lunations 
is completed. And therefore in that time there will be a 
regular period of eclipfes, or return of the fame eclipfe, 
for many ages. In this period, which is called the Chal¬ 
dean Saros, there are xS Julian yrs. nd. 7h. 43'. 20". 
when the laft day of February in ieap-years is four times 
included ; but, when it is five times included, the period 
conlifts of only i8y. iod. 7I1. 43'. 20". Confequently, if 
to the mean time of any eclipfe, either of the Sun or 
Moon, we add 18 Julian yrs. nd. yh. 43'. 20". when the 
laft day of February in leap-years comes in four times, or 
a day lefs when it comes in five times, we (hall have the 
mean time of the return of the fame eclipfe. 
But the falling back of the line of conjunctions or oppo- 
fitions of the Sun and Moon 28' 12" with refpeCt to the 
line of the nodes in every period, will wear it out in pro- 
cefs of time ; and, after that, it will not return again in 
lefs than 12,492 years. Thefe eclipfes of the Sun, which 
happen about the afcending node, and begin to come in at 
the north pole of the Earth, will go a little foutherly at 
each return, till they go quite off the Earth at the fouth 
pole ; and thofe which happen about the defcending node, 
and begin to come in at the fouth pole of the Earth, will 
go a little northerly at each return, till at laft they quite 
leave the Earth at the north pole. To exemplify this mat¬ 
ter, we will firft confider the Sun’s eclipfe (March 21ft old 
ftyle, April 1 ft new ftyle), A.D. 1764, according to its 
mean revolutions, without equating the times, or the Sun’s 
diftance from the node ; and then according to its true 
equated times. 
This eclipfe fell in open fpace at each return, quite 
clear of the Earth, ever fince the creation, till A.D. 129;, 
June 13th old ftyle, at 12I1. 52'. 59". P. M. when the 
Moon’s lliadow firft touched the Earth at the north pole ; 
the Sun being then 70 0 48' 27" from the afcending node. 
In each period fince that time, the Sun lias come 28' 12" 
nearer and nearer the fame node, and the Moon’s lliadow 
has therefore gone more foutherly. In the year 1962, 
July iSthold ftyle, at ioh. 36'. 21". P. M. when the fame 
eclipfe will have returned thirty-eight times, the Sun will 
be only 24' 45"' from the afcending node, and the centre 
of the Moon’s fhadow will fall a little northward of the 
Earth’s centre. At the end of the next following period, 
A.D. 1980, July 28th old ftyle, at iSh. 19'. 41". P. M. 
the Sun will have receded back 3' 27" from the afcending 
node, and the Moon will have a very fniall degree of 
fouthern latitude, which will catife the centre of her fha¬ 
dow to pafs a very fmall matter fouth of the Earth’s cen¬ 
tre. After which, in every following period, the Sun 
will be 28' 12" farther back from the afcending node than- 
in the period laft before ; and the Moon’s fhadow will go 
ftill farther and farther fouthward, until September 12th 
old ftyle, at 23I1. 1 ^ 6 '. 22". P. M. A. D. 2665 ; when the 
eclipfe will have completed its 77th periodical return, 
and will go quite off the Earth at the fouth pole (the Sun 
being then 17 0 55' 22" back from the node), and cannot 
come in at the north pole, fo as to begin the fame courfe 
over again, in lefs than 12,492 years afterwards. And 
fuch will be the cafe of every other eclipfe of the Sun : 
For, as tltere is about eighteen degrees on each fide of 
the node within which there is a poftibility of eclipfes, 
their whole revolution goes through thirty-fix degrees 
about that node, which, taken from 360 degrees, leaves 
remaining 324 degrees for the eclipfes to travel in e.vpan- 
fum. And as this thirty-fix degrees is not gone through 
in lefs than feventy-feven periods, which takes up 1388 
years, the remaining 324 degrees cannot be fo gone through 
in lefs than 12,492 years. For, as 36 is to 13&S, fo is 324 
to 12,492. 
To illuftrate this a little farther, we will examine fome 
of the molt remarkable circumftances of the returns of the 
eclipfe which happened July 14th, 1748, about noon. This 
eclipfe, after traverfing the voids of fpace from the cre¬ 
ation, at laft began to enter the Terra Auftralis Incognita 
about eighty-eight years after the conqueft, which was in 
the laft year of king Stephen’s reign : every Chaldean pe¬ 
riod it has crept more northerly, but was ftill invifible in 
Britain before the year 1622 ; when, on the 30th of April, 
it began to touch the fouth parts of England about two in 
the afternoon ; its central appearance t iling in the Ame¬ 
rican fouth feas, and traverfing Peru and the Amazons 
country, through the Atlantic ocean into Africa, and fet- 
ting in the /Ethiopian continent, not far from the begin¬ 
ning of the Red Sea. Its next vifible period was, after 
three Chaldean revolutions, in 1676, on the firft of June, 
riling central in the Atlantic ocean, palling us about nine 
in the morning, with four digits eclipfed on the under 
limb, and letting in the gulph of Cochinchina in the Eaft 
Indies. It being now near the folftice, this eclipfe was vi¬ 
fible the very next return in 1694, in the evening ; and in 
two periods more, which was in 1730, on the 4th of July, 
was feen about half eclipfed juft after fun-rife, and ob- 
ferved both at Wirtemberg in Germany, and Pekin in 
China, foon after which it went off. 
Eighteen years more afforded us this eclipfe again, which 
fell on the 14th of July, 1748, The next vifible return 
happened 
