ASTRONOMY. -m 
the change arifing from the motion of the whole folar fyf- 
tem. And, on the other hand, if oar fydem be at red, 
and any of the (tars really in motion, this might likewife 
vary their apparent politions; and the more fo, the nearer 
they arc to us, or the Iwifter their motions are ; or the 
more .proper the diredtion of the motion is to be rendered 
perceptible by us. Since then the relative places of the 
dars may be changed from fitch a variety of caufes, con- 
ddering the amazing didance at which it is certain fome of 
them are placed, it may require the obfervations of many 
ages to determine the laws of the apparent changes, even 
of a lingle (tar ; much more difficult, therefore, mult it be 
to fettle the laws relating to all the moft remarkable (tars. 
Dr. Ma(kelyne,. in the explanation and ufe of the tables 
which he publiffied with the fird volume of his Obferva¬ 
tions, obferves, that many, if not all the fixed (tars, have 
proper motions among themfelves, the caufes and laws of 
which are at prefent hid in almoft equal obfeurity. From 
comparing his own obfervations at that time with thofe of 
Dr. Bradley, Mr. Flamftead, and Mr. Roemer, lie then 
found the annual proper motion of the following (tars in 
right afeenfion to be, of Sirius —C63'', of Caftor—0-28", 
of Procyon —o-8", of Pollux—0-93", of Regulus, —0-41", 
of Ardturus —1*4" and of «■ Aquilae —J-o-57"; and of Si¬ 
rius in north polar diftance i - 2o", and of Arcturus, 2-01", 
both fouthwards. But fince that time he had continued 
liis obfervations; and, from a catalogue of the mean right 
afeenfions of thirty-fix principal (tars (which he commu¬ 
nicated to Mr. Wolladon, and is found in his work), it 
appears that thirty-five of them have a proper motion in 
right afeenfion. 
In the year 1736, M. Mayer obferved eighty (tars, and 
compared them with the obfervations of Roemer in 1706. 
M. Mayer is of opinion, that (from the goodnefs of the 
inftruments with which the obfervations were made) where 
the dilagreement is at lead 10" or 13", it is a very proba¬ 
ble indication of a proper motion of fucli a dar. He fur¬ 
ther adds, that when the difagreement is fo great as he has 
found it in fome of the (tars, amongd which is Fomahand, 
where the difference was twenty-one feconds in fifty years, 
he has no doubt of a proper motion. Dr. Herfchel, fol¬ 
lowing Mayer’s judgment of his own and Rocmet’s obfer¬ 
vations, has compared the obfervations; and, leaving out 
of his account all thofe dars which did not lhew a difagree¬ 
ment amounting to ten feconds, he found that fifty-fix of 
them had a proper motion ; from whence he attempts to 
deduce the motion of the folar fydem in the following man¬ 
ner. He fird takes the feven dars above-mentioned, whofe 
proper motions had been determined by Dr. Mafkelyne, 
and he finds, that if a point be a (fumed about the 77 0 of 
right afeenfion, and the Sun to move progreffively, it will 
account for all the motions in right afeenfion. And if, in¬ 
dead of fuppoling the Sun to move in the plane of the 
equator, it diould afeend to a point near to y Hfrculis, it 
will account for the obferved change of declination of 
Sirius and Arclurus. In refpedl to the quantity of motion 
o-f each, that mud depend upon their unknown relative 
didances ; he only fpeaks here of the directions of the mo¬ 
tions. He next takes twelve dars from the above fifty-fix, 
whofe proper motions have been determined from a com- 
pirifon of the obfervations of Roemer and Mayer, and 
adds to them Regulus and Cador ; thefe have all a proper 
motion in right afeenfion and declination, except Regulus, 
which hasnonein declination. Of thefe twenty-fe ven motions, 
the fuppofed motion of the folar fydem will fatisfy.twenty- 
two. There are alfo fome remarkable circumdances in 
the quantities of thefe motions. Arfturus and Sirius be¬ 
ing the larged, and therefore probably the neareft, ought 
to have the greated apparent motion, and fo we find they 
have. Alfo Artdurus is better fitnated to have a motion 
in right afeenfion, and it lias the greated motion. Several 
other arguments of the fame kind are alfo found to take 
place. But there is a very remarkable circumdance in re- 
fped to Cador. Cador is a double dar ; now how extra¬ 
ordinary mud appear the concurrence, that two fucli ftars 
Vol. II. No. 79. 
fltould both have a proper motion To exactly alike, that 
they have never been found to vary a (ingle fecond ! 1 bis 
feems to point out the common caufe, the motion of the 
folar fydem. *• 
Dr. Herfchel next takes thirty-two more of the fame 
fifty-fix fiars, and (hews that their motions agree very well 
with his fuppofed motion of the folar fydem. But the 
motions of the other twelve dars cannot be accounted for 
upon this hypothefis. In thefe therefore he fuppofes the 
effedt of the folar motion has been dedroyed and counter! 
afted by their own proper motions. The fame may be 
faid of nineteen dars out of the thirty-two, which only 
agree with the folar motion one way, and are, as to fenfe, 
at red the other. According to the rules of p.hilofophizjng 
therefore, which direct-11s to refer all phenomena to as few 
and (imple principles as are fufficicnt to explain them, Dr. 
Herfchel thinks we ought to admit the motion of tile folar 
fydem. Perhaps, however^ this argument cannot be pro¬ 
perly applied here, becaufe there is no new caufe or prin¬ 
ciple introduced_by fuppoling each dar. to have a proper 
motion. Admitting the dodtrine of univerfal gravitation, 
the fixed dars ought to move as well as the Sun. But 
the Sun’s motion, as here edimated, cannot be owing to 
the adtion of a body upon it, which might give it a rotatory 
motion at the fame time, as M. de la Lande conjectures ; 
becaufe a body ailing on the Sun to give it its rotation 
about its axis, would not, at the fame time, give it that 
progreffive motion. See Dr. Herfchel’s account in the 
Phil. Tranf. 1783. But it will be proper to conlider how 
far this motion of the folar fydem agrees with the proper 
motion of the thirty-five dars determined by Dr. Malke- 
lyne. Now upon fuppofition that the Sun moves, as con- 
jedtured by Dr. Herfchel, that motion will account for 
the motion of twenty of them, fo far as regards their di¬ 
rection ; but the motion of the other fifteen is contrary to 
that which ought to arife from this fuppofition. As fome 
of the dars mud have a proper motion of their own, even 
upon the hypothelis of a folar motion, and which proba¬ 
bly arifes from their mutual attraction, it is very probable 
that they have all a proper motion from the fame caufe, 
but mod of them fo very fmall as not yet to have been 
difoovered. And it might alfo happen, that fuch a mo¬ 
tion might be the fame as that which would arife from the 
motion of the folar fydem. Yet it mud be confeffed, that 
the circumdance of Cador, and the motions both in right 
afeenfion and declination of many of the dars being fuch 
as arife from this hypothefis, with the apparent motion be¬ 
ing greated of thofe dars which are probably neared, form 
a firong argument in its favour. 
When the caufes which affeCt the places cf all the dars 
in general are known ; fuch as the precefiion, aberration, 
Snd nutation, it may be of a fingular ufe to examine nicelv 
the relative (filiations of particular dars, and efpecially of 
thofe of the greated ludre, which, it may be prefumed, 
lie neared to us, and may therefore be fubjeCt to more fen- 
fible changes, either from their own motion, or from that 
of our fydem. And if, at the fame time, the brighter dars 
are compared with each other, we likewife determine the 
relative pofitions of fome of the (mailed that appear near 
them, whofe places can he afeertained with diffident ex- 
aClnefs, we may perhaps be able to judge to what caufe 
the change, if any he obfervable, is owing. The uncer¬ 
tainty that we are at prefent under, with refpedt to the de¬ 
gree of accuracy with which former adronomers could 
obferve, makes 11s unable to determine feveral tilings re¬ 
lating to this (object; but the improvements, which have 
of late years been made in the methods of taking the places 
of the heavenly bodies, are fo great, that a few years may 
hereafter he diffident to fettle fome points, which cannot 
now be fettled, by comparing even the earliefi obferva¬ 
tions with thofe of the prefent age. 
Of the CONSTELLATIONS. 
The divifion of the liars into condellations is very an¬ 
cient j being known to the mod early authors, whethe r 
5 N facrefi 
