STARS. 



Stars, Motion of the. The fixed ftars have two kinds 

 of apparent motion ; one called the firft, common., or diurnal 

 motion, arifing from the earth's motion round its axis : by 

 this they feem to be carried along with the fphere, or firma- 

 ment, (in which they appear fixed,) round the earth, from 

 eaft to wefl, in the fpace of twenty-four hours. 



The other, called the feconil, or proper motion, is that by 

 which they appear to go backwards from well to eaft, round 

 the poles of the ecliptic, with fuch flownefs, as not to 

 defcribe above a degree of their circle in the fpace of 

 72.,years, or 50 feconds in a year. 



This apparent motion is owing to the recetlion of the 

 equinoftial points, which is 50 feconds of a degree in a year 

 backward, or contrary to the order of the figns of the zodiac. 



Some have imagined, but it is unknown on what grounds, 

 that when they are got round to the points whence they 

 firft departed, nature will have finiftied her courfe, and the 

 ilars having performed their career, the heavens will remain 

 at reft ; unlefs the Being, who iirft gave ihem motion, ap- 

 point them to begin another circuit. 



On the footing of this calculation, the world fhould laft 

 about 30,000 years, according to Ptolemy; 25,816, ac- 

 cording to Tycho ; 25,920, according to Riccioli; and 

 24,800, according to CafTmi. 



In confequence of this fecond motion, the longitude of 

 the ftars will be always increafing. 



Thus, e.gr. the longitude of Cor Leonis was found by 

 Ptolemy, A.D. 138, to be 2° 30'; in 1115, it was ob- 

 ferved by the Perfians to be 17° 30'; in 1364, by Al- 

 phonfus, 70° 40' ; in 1586, by the prince of Htfle, 24" 11'; 

 in 1601, by Tycho, 24° 17'; and in 1690, by Mr. Flam- 

 ffead, 25^ 31' 20" ; whence the proper motion of the ftars, 

 according to the order of the figns in circles parallel to the 

 echptic, is cafily inferred. 



It was Hipparchus who firft fufpefted this motion, upon 

 comparing the obfervations of Timocharis and Ariftyllus 

 with his own. Ptolemy, who lived three centuries after Hip- 

 parchus, demonftrated the fame by undeniable arguments. 



Tycho Brahe makes the iiicre.ife of longitude in a cen- 

 tury l°25'; Copernicus, 1^23' 40" 12'"; Flamftead and 

 Ricciolus, 1° 23' 20" ; BuUialdus, 1° 24' 54" ; Hevelius, 

 1° 24' 46" 50'" ; whence, with Flamftead, the annual increafe 

 of the longitudes of the fixed ftars may be well fixed at 50". 



From thefe data, the increafe of the longitude of a ftar 

 for any given time is eafily had ; and hence the longitude of 

 a ftar for any given year being given, its longitude for any 

 other year is readily found : e.gr. the longitude of Sirius, in 

 Mr. Flamftead's tables, for the year 1690, being 9° 49' i"; 

 its longitude for the year 1724 is found by multiplying 

 the interval of time, u/z. 34 years, by 50" ; the product 

 1700", or 28' 20", added to the given longitude, will give 

 the longitude required, 10° 17' 21". By means of this cor- 

 reftion, applied as the cafe requires, the longitudes of the 

 ftars, tranfcribed from any tables, may be adapted to the 

 prefent or any given time. 



The principal phenomena of the fixed ftars, arifing from 

 their common and proper motion, befides their longitude, 

 are their altitudes, right afcenfions, declinations, occult- 

 ations, culminations, rifings, and fettings. See AltitudEj 

 Ascension, Declination, and Occultation. 



Some have fuppofed that the latitudes of the ftars are in- 

 variable ; but this fuppofition is founded on two affump- 

 tions, which are both controverted among aftronomers ; one 

 is, that the orbit of the earth continues unalterably in the 

 fame plane, and confequeiitly that the ecliptic is invariable, 

 the contrary of which is now very generally allowed. See 

 Ecliptic. 



The other alTumption is, that the ftars are fo fixed as 



to keep their places immoveably. Ptolemy, Tycho, and 

 others, comparing the obfervations of ancient aftronomers 

 with their own, have adopted this opinion. But from the 

 refult of the comparifon of our bett modern obfervations, 

 with fuch as were formerly made with any tolerable degree 

 of exaftnefs, there appears to have been a real change in the 

 pofition of fome of the fixed ftars, with refpedl to each 

 other ; and feveral ftars of the firft magnitude have already 

 been obferved, and others fufpefted to have a proper mo- 

 tion of their own. 



Dr. Halley (Phil. Tranf. N° 355, or Jones's Abr. vol. iv. 

 p. 225.) has obferved, that the IJuIl's eye, Sirius, and Arc- 

 turus, are now found to be about half a degree more 

 foutherly than the ancients reckoned them : that this dif- 

 ference cannot anfe from the errors of the tranfcribers, be- 

 caufe the declination of the ftars, fet down by Ptolemy, as 

 obferved by Timocharis, Hipparchus, and himfelf, fhew 

 their latitudes given by him are fuch as thofe authors in- 

 tended ; 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 ftioulder of Orion has, 

 in Ptolemy, almoft a whole degree more foutherly latitude 

 than at prefent : that an ancient obfervation, made A.D. 

 509, at Athens, as Bullialdus fuppofes, of an appulfe of 

 the moon to the Bull's eye, ftiews that ftar to have had lefs 

 latitude at that time than it now has ; that as to Sirius, it 

 appears by Tycho's obfervations, that he found him 4^ 

 more northerly than he is at this time. All thefe obferva- 

 tions, 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 heaven : this change of 

 place, as Dr. Halley obferves, may ihew itfelf in fo long a 

 time as 1 800 years, though it be entirely imperceptible in 

 the fpace of one fingle century ; and it is likely to be fooneft 

 difcovered in fuch ftars as thofe juft now mentioned, becaufe 

 they are all of the firft magnitude, and may, therefore, 

 probably be fome of the neareft to our folar fyftem. Arc- 

 turus, in particular, aff^ords a ftrong proof of this : for if 

 its prefent declination be compared with its place as deter- 

 mined either by Tycho or Flamftead, the difference will be 

 found to be much greater than what can be fufpefted to 

 arife from the uncertainty of their obfervations. See Arc- 

 TURU.S, and Mr. Hornft^y's Inquiry into the Quantity and 

 Direftion of the proper Motion of Arfturus, Phil. Tranf. 

 vol. Ixiii. part i. p. 93, &c. 



Upon the whole, we have reafon to conclude, that there 

 is not, ftriftly fpeaking, zuy Ji.vetl ftar in the heavens ; and 

 to admit the general motion of all the ftarry fyftems, and 

 confequently of the folar one among tlie reft. 



For an account of Dr. Bradley's obfervations, fee the 

 fequel of this article. 



Dr. Herfchel has alfo lately obferved, that the diftance 

 of the two ftars forming the double ftar y Draconis, Fl. 24 

 and 25, is 54" 48'", and their pofition 44° 19' N. preceding. 

 Whereas, from the right afcenfion and declination of thefe 

 ftars in Flamftead's Catalogue, their diftance, in his time, 

 appears to have been j' ii".4i8, and their pofition 44° 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 proper 

 motion in one or the other of the ftars, or in our folar fyf- 

 tem : moft probably, he fays, neither of the three is at reft. 

 He alfo fufpefts a proper motion in one of the double ftars 

 in Cauda Lyncia media, Fl. 38, and in Ceti, Fl. 68. Phil. 

 Tranf. vol. Ixxii. parti, p. 117. 143. 150. 



It is reafonable to expeft, that other inftances of the like 

 kind mull alfo occur among the great number ef vifible 

 ttars, becaufe their relative pofitions may be altered by 



various 



