344 ASTRO 
mament, or fphere of the fixed ftars *#, was thought to be 
the boundary next above the orbit of Saturn ; and beyond 
this boundary he conceived there were two folid or cryf- 
talline heavens, to which the fixed ftars were attached. 
This beautiful concave fhell or canopy he fuppofed to be 
inclofed by the prirnum mobile, which, turning round once 
in twenty-four hours, carried with it all the reft. The 
primum mobile he imagined was every way encompafi'ed by 
the empyrean heaven, which he conceived to be of a cu¬ 
bic form, the feat of blifs, and the abode of angels and 
divine fpirits. 
Fig. 3, is the Syftem adorned by Tycho Brahe, in which 
both the Sun and the Earth are made to occupy diftinCt 
centres. The Earth was fuppofed the centre of the orbits 
of the Sun and Moon ; but the Sun was fuppofed to be 
the centre of the orbits of the five planets; fo that the 
Sun, with all the planets, were in this fcheme made to 
turn round the Earth, in order to fave the motion of the 
Earth round its axis once in twenty-four hours. The or¬ 
bit of the fuperior planet Saturn was fuppofed to be ex¬ 
ternally bounded by the fphere of the fixed ftars. The 
orbits of tl'.e feveral planets are diftinguiftied in this figure 
by the fame characters as in the preceding. 
Fig. 3, reprefents the Copernican or Solar Syftem, as 
now uni verbally admitted by the aftronomers in every part 
of the civilifed world ; but by this fyftem is meant that 
portion alone of the univerfe which comprehends the Sun, 
the planets with their moons or fatelites, and the comets. 
The Sun is placed in or near the centre, about whom the 
planets continually move with different degrees of velocity, 
and at different diftances. The firft and neareft to the Sun 
is Mercury, next Venus, then the Earth and Moon; next 
beyond thefe is Mars; after him, Jupiter; then Saturn; 
and above, and far remote from him, the new ly-difcovered 
Georgian planet. Above the obfenre orbit of this planet 
are the flarry heavens, boundlefs and immeafurable. Each 
of thefe planets are diftinguiftied in the Plate by their pro¬ 
per aftroftomical mark or character, as before. 
Mercury and Venus, being within the Earth’s orbit, are 
called inferior planets-, but Mars, Jupiter, Saturn, and the 
Georgiutn Sidus, being without or above the Earth’s or¬ 
bit, are called fuperior planets. The Georgian planet is 
attended by two farellites or moons, Saturn by feven, Ju¬ 
piter by four, and the Earth by one; all of thefe, except¬ 
ing the laft, are invifible to tl'.e naked eye, on account of 
tlte fmallnefs of their lize, and the greatnefs of their dif- 
tance from us. 
Notwithftanding thefe planets uniformly and at all times 
refpett the Sun as the centre of their motion, they do not 
always preferve the fame diftance from him; neither do 
they all move in the fame plane, though every one of them 
revolves in an orbit whole plane if extended would pafs 
through the fun’s centre. Each of them moves in an or¬ 
bit fomewhat elliptical; and' thus fometimes approaches 
nearer, and at other times recedes farther from, the Sun 
than before. This deviation from a circle is called the 
eccentricity of the orbit. But though the orbits of all the 
planets are really elliptical; yet, as the principal pheno¬ 
mena of the Copernican fyftem may be fatisfaCtorily illtif- 
trated, by confidering them as circular, the latter fttppo- 
fition is ufually adopted, in giving a general delineation 
or idea of the difpolition and motion of the heavenly bo¬ 
dies. The orbits of the comets are exceedingly long ovals, 
or ellipfes, as fttewn by part of one in the preceding figure 
of the Copernican fyftem. They are of fuch vaft circum¬ 
ferences, that at one period of their courfe through them, 
they- approach fo near to the Sun, as to be almoft melted 
by his heat, and then go olf again to diftances fo immenfe, 
that they mult be totally deprived both of his heat and light. 
Before we enter upon an inveftigation of the nature and 
properties of the celeftial bodies which form the folar fyf¬ 
tem, we (hall purfue our hiftorical detail fomewhat far¬ 
ther, in order to ftate the recent difeoveries of Dr. Herf- 
chel in the region of the ftarry heavens. Encouraged by 
the fuccefs of his late fpeculations concerning the Geor- 
N O M Y. 
gian planet, and influenced by the extenffve field which 
opened to his view, in confequence of the improvements 
and larger powers he had communicated to the telefcope. 
Dr. Herfchel, with Angular induftry and perfeverance, un¬ 
dertook the laborious talk of making a long fucceflion of 
minute obfervations, in order more clearly to alcertain the 
phenomena of the fidereal heavens ; the progrefs and re- 
fult of which follows in his own words: 
“ Hitherto (fays he) the lidereal heavens have, not ina¬ 
dequately for the purpofe defigned, been reprefented as the 
concave furface of a iphere, in the centre of which the 
eye of the obferver is fuppofed to be placed. It is true, 
the various magnitudes of the fixed ftars even then plainly 
fuggefted to us, and would have better fuited, the idea of 
an expanded firmament of three dimenfions ; but tlie ob¬ 
fervations upon which 1 am now going to enter, ftill farther 
illuftrate and enforce the necellity of confidering the hea¬ 
vens in this point of view. In future therefore we ftiall 
look upon thofe regions into which we penetrate by means 
of telefcopes, as a naturalift regardsa rich extent of ground 
or chain of mountains, containing ftrata varioully inclined 
and directed, as well as confiding of very different mate¬ 
rials. A furface of a globe or map therefore will but ill 
delineate the interior parts of the heavens.” 
With a powerful telefcope of twenty feet focal length, 
and an aperture of eighteen incites, Dr. Herfchel firft be¬ 
gan to furvey the via laElea, and found that it completely 
refolved the whitifti appearance into ftars, which the te¬ 
lefcopes he formerly tiled had not light enough to do. 
The portion he firft obferved was that about the hand and 
club of Orion ; and found therein an aftonifhing multitude 
of ftars, uhofe number he endeavoured to eftimate by 
counting many fields, or fpaces taken in at one view by 
his telefcope, and computing from a mean of thele how 
many might be contained in a given portion of the ntijky- 
way. In the moft vacant place to be met with in that 
neighbourhood he found fixty-three ftars; other fix fields 
contained i to, fixty, feventy, ninety, feventy, and leventy- 
four, ftars ; a mean of all which.gave feventy-nine for the 
number of ftars to each field; and thus he found, that by 
allowing fifteen minutes for the diameter of his field of 
view, a belt of fifteen degrees long and two broad, which 
he had often feen pafs before his telefcope in an hour’s 
time, could not contain lels than 50,000 ftars, large enough 
to be diftinCtly numbered; belides which, he fufpeCted 
twice as many more, which could be feen only now and 
then by faint glimpfes, for want of fufficient light. 
The fuccefs he had within the milky-way foon induced 
him to turn his telefcope to the nebulous parts of the hea¬ 
vens, from which a lift of 103 nebulae had been publilhed 
in the CcnnoiJ/'ance des Temps lor 1783 and 1784, as obferved 
by Meftier and Mechain; fome of which they could re- 
folve, and fome they could not. Dr. Herfchel, however, 
found them all to yield to a Newtonian reflector of twenty 
feet focal diftance and twelve inches aperture ; which plain¬ 
ly difeovered them to be compofed of ftars, or at leaft to 
contain ftars, and to Ihew every other indication pf confift- 
ing of them entirely. “ The nebulae (fays he) are arran¬ 
ged into ftrata, and run on to a great length ; and fome of 
them 1 have been able to purfue, and to guefs pretty well 
at their form and direction. It is probable enough that 
they may (urround the whole ftarry fphere of the heavens, 
not unlike the milky-way, which undoubtedly is nothing 
but a ftratum of fixed (tars. And as this latter immenfe 
ftarry bed is not of equal breadth or luftre in every part, 
nor runs on in one ftraight direction, but is curved, and 
even divided into two ftreams along a very confiderable 
portion of it; we may likewife expect the greateft variety 
in the ftrata of the clutters of ftars and nebulae. One of 
thele nebulous beds is fo rich, that, in palling through a 
feCtion of it in the time of only thirty-fix minutes, I have 
detected no lets than thirty-one nebulae, all distinctly vifi- 
ble upon a fine blue fky. Their fituation and lhape, as 
well as condition, feem to denote the greateft variety ima¬ 
ginable. In another ftratum, or perhaps a different branch 
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