4tz 
ASTRONOMY. 
Earth’s motion round its axis: by this they feem to be' 
carried along with the fphere of firmament, in which they 
appear fixed, round the Earth, from eaft to weft, in the 
fpace of twenty-four hours. The other, called the fccond , 
or proper, motion , is that by which they appear to go back¬ 
wards from weft to eaft, round the poles of the ecliptic, 
with an exceeding (low motion, as deferibing a degree of 
their circle only in the fpace of 71^ years, or 503 feconds 
in a year. This apparent motion is owing to the receffion 
of the equinottial points, which is 505 feconds of a de¬ 
gree in a year backward, or contrary to the order of the 
ligns of the zodiac. In confequence of this fecond mo¬ 
tion, the longitude of the ftars will be always increafing. 
Thus, for example, the longitude of Cor Leonis was found 
at different periods to be as follows : viz. 
Year. 
Long. 
By Ptolemy, in 
*38 
to be 2 0 
3 °' 
By the Perlians, in 
11 *5 
- - 17 
30 
By Alphonfus, in 
By Prince of He tie, in 
*364 
- - 20 
40 
1586 
- - 24 
11 
By Tycho, in 
1601 
- - 24 
*7 
By Flamftead, in 
1690 
- - 2 5 
Whence the proper motion of the ftars, according to the 
order of the figns, in circles parallel to the ecliptic, is 
eafily inferred. 
It was Hipparchus who firft fufpefted this motion, upon 
comparing his own obfervations with thofe of Titnocharis 
and Ariftyllus. Ptolemy, who lived three centuries after 
Hipparchus, demonftrated the fame by undeniable argu¬ 
ments. The increafe of longitude in a century, as dated 
by different aftronomers, is as follows: 
By Tycho Brahe - * i° 25' 
Copernicus - - 1 23 
Flamftead and Riccioli - 1 23 
Bullia'.d - - - 1 24 
Hevelius - - 1 24 
Dr. Bradley, &c. - - 1 23 
which is at the rate of 50^ feconds per year, 
data, the increafe in the longitude of a liar for any given 
time is eafily had, and thence its longitude at any time : 
ex. gr. the longitude of Sirius, in Flamftead’s tables, for 
the year 1690, being 9 0 49' i", its longitude for the year 
1S00, is found by multiplying the interval of time, viz. 
no years, by 50$, the product 5537", or i° 32' 17", add- 
-ea to the given longitude - 9 49 1 
o" 
4°i 
20 
54 - 
4<5| 
55 
From thefe 
gives the longitude - - 11 21 18 for the 
year 1800. 
The chief phenomena of the fixed ftars, arifing from 
their common and proper motion, beiides their longitude, 
are their altitudes, right afeenfions, declinations, occupa¬ 
tions, culminations, rifings, and lettings. Some have fup- 
pofed that the latitudes of the ftars are invariable. But 
this fuppofition is founded on two affumptions, which are 
both controverted among aftronomers. The one of thefe 
is, that the orbit of the Earih continues unalterably in 
the fame plane, and conlequently that the ecliptic is inva¬ 
riable ; the contrary of which is now very generally al¬ 
lowed. The other alfumption is, that the-ftars are fo 
fixed as to keep their places immoveably. Ptolemy, Ty¬ 
cho, and others, comparing their obfervations with thofe 
of the ancient aftronomers, have adopted this opinion. But 
from the refult of the comparifon of our beft modern ob¬ 
fervations, with fuch as were formerly made with any to¬ 
lerable degree of exachiefs, there appears to have been a 
real change in the pofition of feme of the fixed ftars, with 
refpeift to each other; and feveral ftars of the firft magni¬ 
tude have already been obferved, and others fufpedted, to 
have a proper motion of their own. 
Dr. Halley (Philof. Tranf. number 355, or Abr. vol. 4. 
p.225) has obferved, that the three following ftars, the 
Buii’s Eye, Sirius, and Arfhirns, are now found to be 
above halt a degree more foutherly than the ancients rec¬ 
koned them : that this difference cannot arife from the 
errors of the tranferibers, becaufe the declinations of the 
ftars, fet down by Ptolemy, as obferved by Timocharis, 
Hipparchus, and himfelf, Ihew their latitudes given by 
him are fuch as thofe authors intended : and it is fcarcely to 
be believed that thofe three obfervers could be deceived 
in fo plain a matter. To this he adds, that the bright ftar 
in the (boulder of Orion has, in Ptolemy, almoft a whole 
degree more foutherly latitude than at prefent: that an 
ancient obfervation, made at Athens in the year 509, as 
Bulliald fuppofes, of an appulfe of the Moon to the Bull’s 
Eye, (hews that ftar to have had lefs latitude at that time 
than it now has: that, as to Sirus, it appears by Tycho’s 
obfervations, that he found him 4| minutes more northerly 
than he is at this time. All thele obfervations, compared 
together, feem to favour an opinion, that fome of the ftars 
have a proper motion of their own, which changes their 
places in the fphere of the heaven: this change of place, 
as Dr. Halley obferves, may (hew itfelf in fo long a time 
as 1800 years, though it be entirely imperceptible in the 
fpace of one (ingle century ; and it is likely fooneft to be 
difeovered in fuch ftars^as thole juft now mentioned ; be¬ 
caufe they are all of the firft magnitude, and may, there¬ 
fore, probably be fome of the neareft to our folar fyftem. 
Arflurus, in particular, affords a ftrong proof of this : 
for if its prefent declination be compared with its place, 
as determined either by Tycho or Flamftead, the difference 
will be found to be much greater than what can be fuf- 
pedted to ari(e from the uncertainty of their obfervations. 
Dr. Herfchel has lately obferved, that the diftance of 
the two ftars forming the double ftar y Draconis, is 54" 
48"', and their pofition 44 0 19' N. preceding. Where¬ 
as, front the right afeenfion and declination of thefe ftars 
in Flamftead’s catalogue, their diftance, in his time, ap¬ 
pears to have been 1' 11-418", and their pofition 44 0 23' N. 
preceding. Hence he infers, that as the difference in the 
diftance of thefe two ftars is fo confiderable, we can 
hardly account for it, otherwife than by admitting a pro¬ 
per motion in one or the other of the ftars, or in our fo¬ 
lar fyftem : mod probably he fays, neither of the three is 
at reft. He alfo fu(pedis a proper motion in one of the 
double ftars, in Cauda Lyncis Media, and in 0 Ceti. Phil. 
Tranf. vol. 72. part 1. p. 117, 143, 150. 
It is reafonabie to expect, that other inftances of the 
like kind rauft alfo occur among the great number of vifi- 
ble liars, becaufe their relative pofitions may be altered by 
various means. For, if our own folar fyftem be conceived 
to change its place with refpedl to abfolute fpace, this 
might, in procefs of time, occalion an apparent change in 
the angular diftances of the fixed ftars ; and in fuch a cafe, 
the places of the neareft ftars being more affedfed than any 
of thofe that are very remote, their relative pofition might 
feem to alter, though the ftars themfelves were really im¬ 
moveable ; and vice verfa, we may furmife, from the ob¬ 
ferved motion of the ftars, that our Sun, with all its pla¬ 
nets and comets, may have a motion towards fome parti¬ 
cular part of the heavens, on account of a greater quantity 
of matter colledted in a number of ftars and their furround¬ 
ing planets there fituated, which may perhaps occalion a 
gravitation of our whole folar fyftem towards it. If this 
furmife (liould have any foundation, it will thew itfelf in 
a feries of fome years; (ince from that motion there will 
arife another kind of hitherto unknown parallax (fuggefted 
by Mr. Michell, Philof. Tranf. vol. 57, p. 252), the in- 
veftigation of which may account for fome part of the 
motions already obferved in fome of the principal ftars 
and, for the purpofeof determining the diredlion and quan¬ 
tity of fuch a motion, accurate obfervations of the diftance 
of ftars, that are near enough to be meafured with a mi¬ 
crometer, and a very high power of telefccpes, may be of 
confiderable ufe, as they will undoubtedly give us tire re¬ 
lative places of thofe ftars to a much greater degree of 
accuracy than they can be had by inftruments or fedtors, 
and thereby nnich fooner enable us to difeover any appa¬ 
rent change in their fituation, eccationed by this new kind 
of fecular or fyftematical parallax, if we may fo exprefs 
