ASTRONOMY. 
volutions of Mercury, approaching nearer to a ftate of 
equality the farther we go. The firft period, or that of 
one year, is not fufficiently exadf; the period of fix years 
will fometimes bring on a return of the tranfit at the fame 
node; that of feven years more frequently; that of thir¬ 
teen years ftill more frequently, and fo on. Now there 
was a tranfit of Mercury at its defcending node, in May 
1786; hence, by continually adding 6, 7, 13, 33, 46, &c. 
to it, are found all the years when the tranfit may be ex¬ 
pected to return at that node. In 17S9 there wasatranfit 
at the afcending node, and therefore, by adding the fame 
numbers to that year, we afcertain the years in which the 
tranlits may be expedted to happen at that node. The 
next tranlits of Mercury at the defcending node will hap¬ 
pen in 1799, 1832, 1845, i8 7 8 > jS 9 j i anc '> at *l’ e amend¬ 
ing node, in 1802, 1815, 1822, 1835, 1S48, 1861, 1S68, 
x p 224-7 
1881,1894. For Venus, ; hence- -- 
y P 363-256 
= T3-» *ff-> iVsV* & c - Therefore the periods are 8, ,235, 
713, &c. years. The tranlits at the fame node will there¬ 
fore fometimes return at eight years, but oftener in 235, 
and ftill oftener in 713, See. Nowin 1769 a tranfit of Ve¬ 
nus happened at the defcending node in June, and the next 
tranfits at the fame node will be in the years 2004, 2012, 
-2247, 2255, 2490, 2498, 2733, 2741, and 2984. In 1639 a 
tranfit happened at the afeending node in November, and 
the next tranfits at the fame node will be in the years 1874, 
1882, 2117, 2125,2360,2368, 2603, 2611, 2846, and 2854. 
Thcfe tranfits are found to happen, by continually adding 
the periods, and finding the years when they may be ex¬ 
pedted, and then computing, for each time, the fiiortefl 
geocentric diftance of Venus from the Sun’s centre at the 
time of conjunction, and, if it be- lefs than the femidia- 
meter of the Sun, there will be a tranfit. 
In the tranfit in 1761, the Rev. Mr. Hirft, F. R. S. at 
Madras, obferved a kind of penumbra, or dufky fliade, 
which preceded the firft external contadt two or three fe- 
conds of time, and was fo remarkable, that he was there¬ 
by allured that the contadt was near, which happen¬ 
ed accordingly. In the tranfit in 1769, Dr. Mafkelyne 
was very attentive to obferve if this circumftance took 
place, but he could perceive no fuch effedt. When Ve¬ 
nus was a little moi-e than half immerged into the Sun’s 
difk, he faw its whole circumference completed, by means 
of a vivid, but narrow-, ill-defined, border of light, which 
illuminated that part of its circumference which was off 
the Sun ; but this difappeared about two or three minutes 
before the internal contact. In the tranfit in 1769, Mr. 
Hirft had warning of the approach of Venus to the exter¬ 
nal contadt, by the fudden appearance of a violent cor- 
rufeation, ebullition, or agitation, of the upper edge of the 
Sun, five or fix minutes before the limb of Venus broke 
in upon the Sun. This he thinks might be owing to the 
atmofphere of Venus. He did not, how-ever, obferve any 
kind of penumbra, as in the other tranfit. Some obfer- 
vets perceived, at the firft external contadt, a kind of wa¬ 
tery pointed fliadow, appearing to give a tremulous motion 
to that part of the Sun’s limb. Moft of the obfervers 
took notice of a tremulous motion of the Sun’s limb, which 
rendered the true time of the contadt uncertain to feveral 
feconds. Some aftronomers, at the laft tranfit, obferved 
a luminous crefcent at the times of the ingrefsand egrefs, 
which enlightened that part of Venus’s circumference 
which was off the Sun, fo that the whole circumference 
was vifible. At the internal contadt, the limb of Venus 
feemed, to moft of the obfervers, to be united to the Sun’s 
limb by a black protuberance or ligament, which was not 
broken by the thread of light, till fome feconds after the 
regular circumference of Venus feemed to have coincided 
with the Sun’s. Others obferved that the thread of light 
between the limbs did not break inftantaneoufly, the points 
of the threads darting into each other, and parting again, 
in a quivering manner, feveral times before they finally ad¬ 
hered. Perhaps the beft Way to afcertain the time of the 
Vol. II. No. 76. 
361 
internal contadt, is to judge by the eye, from that part of 
the circumference of Venus which is not difturbed, when 
the regular circumference of Venus would juft touch the 
Sun’s limb. Hence it appears, that Dr. Halley was mifta- 
ken, in fitppofing that the cotitadfs could be obferved to 
a fecond of time ; and accordingly the obfervations made 
by different obfervers, and reduced to the fame meridian, 
differed more than was expedted. The mean of all the 
obfervations, however, have counterbalanced tiiis, and 
rendered the determination of the parallax of the Sun to 
be depended upon to very great accuracy. 
The parallax of the Sun being thus determined, from 
the difference of the times of the tranfits at two places, 
the conclufion will be moft accurate when that difference 
is the greateft pofliblc. The places therefore to be chofen 
for the tw o obfervations fhould be upon oppefite meridians, 
and fuch, that the middle of the tranfit may be when the 
Sun is upon the meridian ; for, under thefe circumftances, 
the ingrefs at one place will be accelerated and the egrefs 
retarded, increafing thereby the time of the tranfit, and 
the ingrefs at the other place will be retarded and the egrefs 
accelerated, by which the time of the tranfit will be dirni- 
nifiied ; the difference therefore of the times of the tranfits 
at the two places will thus become the greateft. As the 
tranfit mu ft be obferved under oppolite meridians, it muft 
happen in the day at one of the places, and at night at the 
other; the place therefore where it happens in the night 
muft be fo near the north or fouth pole, according 
as the declination of the Sun is north or fouth, that 
the ingrefs may be obferved before the Sun fets, and the 
egrefs next morning after it rifes. Hence the tranfits of 
Venus which happen in June are more convenient than 
thofe which happen in December, becaule there is a great 
choice of fituations towards the north pole, which is not 
the cafe towards the fouth. Dr. Halley made a miftake 
by fetting off the axis of the planet’s orbit on the fame fide 
of the ecliptic that the axis of the equator was fituated, 
inftead of the contrary fide. By ufing therefore the diffe¬ 
rence of thefe two angles inftead of their fum, he made 
the difference of t,he times of the tranfit in 1761, feen at 
the Ganges and Port Nelfon (two places recommended by 
him forobferving this tranfit), longer by 29" than it ought, 
as computed by Dr. Hornfby. See the Philofophical Tranfi. 
adtions for 1763. 
Of the EARTH. 
Tellus, the Earth, is charadterifed by © ; and revolves 
next above Venus. Small as it really is when compared to 
fome of the other planets, it is neverthelefs to us of the 
higheft importance: we wifh only to obtain a knowledge 
of others, that we may find out their x-elation to this, and 
from thence learn our connection with the univerfe at large. 
The Earth’s orbit is placed between thofe of Venus and 
Mars, having the orbits of Venus and Mercury within its 
own, or between it and the Sun, in the centre ; and thofe 
of Mars, Jupiter, Saturn, and the Georgian planet, without 
or above it, w hich are therefore called luperior planets, and 
the others the inferior ones. Its diftance from the Sun is 
ninety-fix millions of miles, and it moves at the rate of 
68,856 miles per hour. 
With refpedt to the figure of the Blarth, the ancients had 
various opinions. Some, as Anaximander and Leucippus, 
held it to be cylindrical, or in form of a drum; but the 
moft general opinion was, that it was flat; that the vifible 
horizon was the bounds of the Earth, and the ocean the 
bounds of the horizon; that the heavens and earth above 
this ocean were the whole vifible univerfe; and that all 
below the ocean was hades ; and of this opinion were alfo 
fome of the Chriftian fathers, as Ladtantius, St. Aitguf- 
tine, Sec. Some, however, among the fages of antiquity, 
concluded, that the Earth muft neceffarily be of a fpheri- 
cal figure, becaufe that figure vvas, on many accounts, the 
moft convenient for the Earth, as an habitable w orld ; 
they alfo argued, that this figure was the moft natural, be- 
4 Z caufe 
