594 
PLANET. 
We fhall here notice Tome particulars which have oc¬ 
curred to various mathematicians iince the article Astro¬ 
nomy was printed. 
After the Georgian planet had been firft: difcovered by 
Herfchel, (fee vol. ii. p. 391.) aftronomers applied them- 
felves with great aiiiduity to obferve its motions, and to 
compute a table of them : but, its diftance being im- 
menfely great, and its annual motion proportionally 
fmall, many years mull neceftarily elapfe before the ele¬ 
ments of its orbit could be determined to the greatell de¬ 
gree of precifion. It fortunately occurred to Bode, how¬ 
ever, that, as this planet, when feen through a telefcope, 
has the appearance of a ftar of the fifth magnitude, it 
would in all probability have been obferved and entered 
as fuch in the catalogues of different aftronomers. Un¬ 
der this impreflion, he examined the Catalogues of Flam- 
Heed and Mayer, and found that it had really been ob¬ 
served by both thefe aftronomers, though they were not 
aware of its being more than a fimple ftar. The former 
obfervation was made in 1690, and the latter in 1755. 
Le Monnier had alfo feen it three times ; firft in 1764, and 
twice in 1768 : but ftill no idea was yet entertained of its 
planetary nature. The fame planet was again obferved 
ax a ftar by Bradley, on the 3d of December, 1753, when 
its meridian palfage took place at Greenwich, 5I1. 32m. 
34"*8 ; and, fince this obfervation was difcovered by Lin- 
denau in the Catalogue of Bradley, feven others have 
been found in the “ Complete Catalogue of all the Stars 
obferved by Flamfteed,” publilhed by Mifs C. Herfchel, 
the earlieft of which was made in 17125 fo that aftrono- 
mers are thus put in pofteffion of very unexpe&ed re- 
fources for computing the orbit of this diftant planer, and 
for correcting the elements, as firft determined from ob- 
fervations made fubfequent to Herfchel’s difcovery. By 
thefe means, the motion of this body, although it has not 
3'et completed one half of its fidereal revolution (of 83 
years) fince it was found to be a planet, is as accurately 
determined as thofe of the other planets of our fyfteni, 
which have been known from the higheft antiquity. 
We are now enabled alfo to prefent the reader with a 
Table of the Periodical Revolutions and Diftances of the 
Satellites of the Georgium Sidus from its body, exprefled 
in femi-diameters of that planet, and in Englifit miles. 
Satellites. 
Periodic Revolutions. 
Diftances in 
Semidiameters. 
Miles, jj 
Days. 
Hours. 
Mill. 
1 
5 
21 
2 5 
218,750 
2 
18 
17 
I 
16A 
288,750 
3 
IO 
23 
4 
J 9 
332,500 
4 
13 
I I 
5 
22 
385,000 
5 
38 
I 
49 
44 
719,000 
6 
107 
l 6 
40 
88 
I ,540,0001 
great reafon to fuppofe were in faCl the fame comet; its 
appearance in 1793 being after two revolutions, and the 
period being fuppofed 5 years 7I months. The compu¬ 
tations of M. Burckhardt, however, afford ground for 
doubting this identity, though it is by no means certain 
that they are actually two diftinct comets. 
The comet difcovered in the month of September, 1807, 
was the mod beautiful that had been feen for thirty years. 
Its nucleus appeared like a ftar of the firft or fecond mag¬ 
nitude, but its light paler; its tail not very extenfive. 
Its light, however,and that of its nucleus, was very ftriking. 
Its motion one degree per day towards the north, and 
rather more than one degree towards the eaft. It was 
furrounded by a nebulofity of about fix minutes in dia¬ 
meter. This refembled none of the orbits of any comet 
obferved before. It pafled its perihelion on the 19th 
September. Its diameter was 538 miles; its diftance 
from the earth, at the laft obfervation by Dr. Herfchel, 
Feb. 21, 1808, was 275,077,889 miles; and that great 
altronomer fuppofed it to have fhone by its own light, and 
not by light from the fun. 
For an account of the comets of 1811 and 1812, fee the 
article London, vol. xiii. p. 271, 319. 
The comet of 1815 feems to prefent fome faffs lefs un¬ 
certain and more interefting than the preceding ; fince 
it is very probable that this comet has a determinate 
elliptic orbit, and that its revolution is performed in 73 
years. This refult was firft obtained by the computation 
of Meflrs. Lindenau, Nicolai, and Beffel; and fince by a 
fecond computation of the latter aftronomer, founded on 
the bafis of 187 obfervations in different parts of Germany 
and.Italy : the whole being made between the 6th of 
March and the 25th of the following Auguft. The 
French aftronomers had taken only 29 obfervations of 
the fame comet, between the 29th of March and the 29th 
of June; yet by means of thefe, and wholly independent 
of what had been done by the aftronomers above-named, 
M. Nicollet arrived at concluficns very nearly the fame 
as thofe of MM. Lindenau and Befiel ; a very remarkable 
circumftance, confidering that the obfervations were 
entirely diftinfl, and which cannot fail of giving us great 
confidence in the accuracy of both. The elements of 
the orbit of this comet, according to the latter determi¬ 
nation, are as follow : 
Paffage of the perihelion, 26 April, 
reckoned from midnight, at Paris. 
Longitude of perihelion, 
Inclination of orbit - 
Longitude of afcending node 
Excentricity - - - 
Perihelion diftance - 
Semi-axis major - 
Sidereal revolution in years 
Motion diredt 
54.857, mean time* 
- 14.9 0 2' 58" 
- 4 + 3 ° 45 
- 83 26 50 
- 0-9305435 
- - 1.213090 
- 17.46550 
- 72.99110 
The firft occultation of the Herfchel planet by the Moon 
took place on the 6th of Auguft ; this interefting pheno-r 
menon never yet having occurred fince the memorable 
difcovery of this planet, in March 1781. This occulta¬ 
tion had the rare advantage of occurring within a very few 
minutes after the Moon pafled the meridian of London, 
and w-hilft yet fhe remained within the field of view of a 
tranfit telefcope. Indeed, during the clear weather of 
this month (Aug. 1824.) that diftant planet has been for 
fome evenings vifible to the naked eye, not requiring the 
ufe of a telefcope. 
In addition to what has been faid (vol. ii. p. 405) about 
cmnets, we may obferve, that feventeen comets have been 
difcovered fince the year 1789 ; and it will appear afto- 
nifliing, that of ninety-feven comets which have been 
calculated, one only ihould have yet been abfolutely 
known to have returned ; (fee p. 404.) 
Mr. Burckhardt, in the Mem. de l’Acad. des Sciences 
de l’Inftitut, 1818. has dated feveral particulars relative 
to the comets of 1783 and 1793, which there feemed 
The celebrated comet of 1759, predifted by Halley, 
and the only one of which the return has been indifpu- 
tably obferved, has its mean period about 75 years; that 
is to lay, a little greater than the above. M. Delambre 
obferves : “ Halley died at 86 years of age, about 17 years 
before the return of that body which has added fo much 
celebrity to his name; (fee vol. ii. p. 404.) and there is 
no better profpeft of the aftronomers who have given 
this fecond predi&ion living to fee it Verified, fince it 
cannot be expelled again till 1888.” 
Th e newly-difcovered planets have been briefly noticed 
at p. 395 of vol. ii. laft edit. They all lie between the 
orbits of Mars and Jupiter ; and were difcovered in the 
following order. 
Ceres, firft difcovered by Mr. Piazza in Jan. 1801 ; but, 
having been loft through M. Piazzi’s illnels, on the nth 
of February, 1801, after he had obferved it for fix weeks; 
and it never having been feen out of his obfervatory, the 
aftronomers were reduced to the neceflity of endeavouring 
to re-find it by elements of its orbit deduced from his 
obfervations. 
