ASTRONOMY. 
a fecular motion of jf. 6°. 17'. 33”. The fecular equation 
being determined for 100 years, it may be found tor any 
other time, as it was for Saturn, by taking it in propor¬ 
tion to the fquare ot the time. 
The longitude of the Sun alfo requires a fecular equa¬ 
tion of 12' for 2500 years, arifing from the diminution of 
the preceffion of the equinoxes, according to De la Lande. 
Of the GEORGIUM S I D U S. 
The Georgian planet we have already dated to have 
been difcovered by Dr. Herfchel; it is therefore diftin- 
guithed by the initial of his name, with a crofs, deno¬ 
ting the Chriftian planet, or that it was difcovered by the 
Chriftians. Its light is of a bluifti white colour, and its 
brilliancy between that of the Moon and Venus; though 
on account of its immenfe diftance, being the outermolt 
of the fuperior planets, it is but juft vifible to the naked 
eye. Dr. Herfchel firft obfcrved this planet in the even¬ 
ing of March 13, 1781; and, having given notice of it to 
the Royal Society, Dr. Matkelyne inftantly made obfer- 
vations upon it, and wrote to the aftronomers at Paris, de¬ 
claring, with Dr. Herfchel, that he fufpefted it to be a 
planet. M. Lexell was then in England; but he applied him- 
felf to compute the orbit, upon fuppofition that it was a 
comet; he therefore, according to the ufual manner in 
Rich a cafe, fuppofed the orbit to be a parabola, and af- 
fumed feveral perihelion diftances, 6, 8, 10, 12, 14, 16, and 
18, times the Earth’s diftance from the Sun; and found 
that any perihelion diftance between 14 and 18 would an- 
fwer very well to the obfervations. Bofcovich printed a 
Memoir on the fubjeft, in which he (hewed that there were 
four different parabolas in which the body might move, 
and yet the computed places would agree w'ith the obfer¬ 
vations which had then been made. Other aftronomers 
however found, that a circular orbit, whole radius was 
about eighteen times the diftance of the Sun from the 
Earth, would agree better with the obfervations; and this 
confirmed Dr. Malkelyne’s opinion that it was a planet. 
Upon fuppolition, therefore, of a circular orbit, M. de la 
Lande proceeded to inveftigate its magnitude, from the 
following obfervations: 
Time of obfer- 
vation 
Ap. 25,1781,: July 3 1,1781, 
at 9h. 47'. i at 15I1. 33'. 
Dec. 12,1781 
at ioh. 10'. 
Right afcenlion 
obferved 
2f.25°i5'27" ; 3f. i° 7'49" 
3 f. i° 23'31" 
North declina¬ 
tion obf. 
23 35 3+ 
23 4° 25 
23 42 47 
Longitude 
2 2 5 39 n 
3 1 2 7 (3 1 16 28 
Latitude north 
11 36 
12 24 | 14 54 
Nutation in lon¬ 
gitude 
-J-IO 
+ 8 
+7 
Aberration in 
longitude 
+ i 9 
+21 
—18 
Sun’s longitude! 
from the mean, 1 5 58 53 
equinox 
t 
4 9 7 13 ;S 21 21 50 
1 
Log. ot the Sun’sj , 
diftance 1 °' 0 ° 3 i 9 6 
0-006272 
9-992993 
From thefe obfervations, M. de la Lande proceeded 
thus to find the circular orbit. He affumed the radius of 
the orbit, and then calculated the heliocentric piaces of the 
planet at the times of the firft: and laft obfervation ; con- 
fequently the angle defcribed by the planet about the Sun 
in that interval of 213 days 23' was known ; and hence the 
time of the whole revolution was known by proportion, 
upon fuppolition that the orbit was circular. Next, know¬ 
ing the radius of the orbit compared with the mean dif¬ 
tance of the Earth from the Sun, he calculated the perio¬ 
dic time, and the radius of the orbit was found to be 
18,931 times the mean diftanc^of the Earth from the Sun, 
and the duration of the revolution 8237 years. This cir¬ 
cular orbit therefore agreed to the firft and laft obferva¬ 
tions; and, by computing from it the place at the fecond 
obfervation, he found that it differed only five leconds from 
the obferved place, which difference might eafily arife from 
the unavoidable errors in the obfervation. He then cal¬ 
culated thirty-two other obfervations made by Dr. Mafke- 
lyne, Monnier, Meffier, Medium, d’Agelct, Levefque, 
and himfelf, and found they all agreed very well, except 
in April 1781, and July, Auguft, and September, 1782; 
the laft differing more than two minutes. He then pro¬ 
ceeded, as before, to find what radius would anfvverto the 
obfervation on April 25, 1781, and on July 21, 17S2, at 
15b. at Paris, when the longitude obferved was 3f. 4°, 
42'. 31/'. this radius he found to be 18-893, and the perio¬ 
dic time 82-12 years. But, by tiling this radius, he foundT 
the calculations to differ 1' 27" from the place obferved in 
oppofttion in December 1781. This indicated an irregu¬ 
larity in the motion of the planet; but the irregularity 
was too imall, and the obfervations too near together, to 
afford proper data for the inveftigation of the orbit. M. 
de la Lande proceeded to determine the place of the node, 
and inclination of the orbit; but, on account of the fmall 
motion in latitude, great accuracy could not at that time 
be expeffed. The geocentric latitudes obferved on April 
25, and December 12, 1781, were 11'36" and 14''54" 
north, which give the heliocentric latitudes 11 ^59" and 
14'8"; and, the motion in longitude being 2° 46' 3" be¬ 
tween the obfervations, he found the place of the node to 
be 2f. 12 0 . 54', and inclination of the orbit 46'. Again, 
the obferved geocentric latitudes on April 16, 1781, and 
March 26, 1782, were 11'48" and 15'5", and hence the he¬ 
liocentric latitudes were found to be 12' 7"’and 15' io" - p 
and, the motion iri longitude between the obfervations be- 
4 0 7*44", the place of the nodes was found to be 2ft 12 0 . 2', 
and the inclination 44'. He farther obferves, that the pla¬ 
net was ftationary eleven days before Dr. Herfchel firft ob¬ 
ferved it; and therefore, if his obfervations had been made 
eleven days fooner, he would not have perceived any mo¬ 
tion, and the difcovery might, M. de la Lande thinks, have 
been loft. It is probable, however, that, if this had hap¬ 
pened, the difcovery would have been made; for, from 
the Angularity of its appearance, which alone made Dr. 
Herfchel pay attention to it, he would undoubtedly have 
continued to obferve it, till he had difcovered its motion, 
which muft very foon have been perceived- 
It having been found, that the motion did not agree to 
that of any one circle, the next enquiry was to determine 
the ellipfe in which it moved, fuppofing that, like the 
other planets, it revolves in fuch a curve, having the Sun 
in one of its foci. Mr. Robifon, profeffor of natural phi- 
lofophy in the univerfity of Edinburgh, inveftigated the 
elements of the orbit, in the Edinb. Tra-nf. vol.i. 1788; 
we (hall therefore ftate the computations as given by him; 
the method is capable of great accuracy, fo far as the ob- 
fervations are accurate, and may be eafily underftood by 
thofe who are acquainted with only the elementary parts, 
of mathematics and p’nilofophy. The obfervations upon 
which the inveftigation is founded are as follows.- 
True Time at Edinburgh. 
Lons,' 
itvde. 
N. 
Lat. 
Dec. 21, 
178 r, 
17I1 
• 44 '- 
33 " 
3 f. 0° 
■ 52k 
11" 
15 ' 
' 7 '" 
26, 
1782, 
8 
5 6 
56 
3 5 
20 
29 
18 
5*5 
3 1 . 
1 7 S 3, 
0 
46 
24 
3 ’ 9 
5 ° 
5 2 
22 
10 
Jan. 3, 
1 / 85 , 
*7 
28 
56 
3 14 
23' 
2 
25 
40 
Si, 
1786, 
10 
39 
3 i 
3 iS 
57 
5 
28 
52- 
We have here the times of five fucc-effive oppofitions, 
as deduced from obfervations, and the correfponding he¬ 
liocentric longitudes, and latitudes. Hence the longitude- 
of the node-on January 1, 1786, was zf. 12 0 . 48k 45". andi 
inclination of the orbit 46' 26". The place of the node,, 
and the inclination of the orbit, being determined, the 
places of the planet reduced to the orbit will be known, 
and thus we may find the arcs defcribed in the orbit itfelf 
between the above oppofitions. Mr. Robifon next took. 
the 
