L O N 



L O N 



able remnants of antiquity, of which enoiifrh remains to en- 

 gage our admiration, and excite an earncll regret for every 

 partible of it that has periihed. It refembles thofe muti- 

 lated ftatues, which are fometimcs dug out of ruins : hmbs 

 are broken off, which it is not in the power of any hving 

 artift to replace, becaufe the fine proportion and delicate 

 finilhing of the trunk excludes all hope of equalling fuch 

 mafterly performances." Smith's tranilation of the trcatife 

 on the Sublime. Moreri. Gibbon. Harwood. 



LONGISSIMUS DoRSi, in Anatomy, a mtifcle of the 

 back. See Doksi. 



LONGITUDE of the Earth, is fometimcs ufcd to de- 

 note its extent from ealt to well, according to the diredlion 

 of the equator. 



By which it ftands contradiftinguifhcd from the latitude 

 of the earth, which denotes its extent from one pole to the 

 other. 



Longitude, in Aflronomy and Geography. The longi- 

 tude of any point of the heavens is the diltaiice of its place, 

 reduced to the ecliptic, from the vernal equinoftial point ; 

 that is, if a great circle pafs through a liar perpendicular to 

 the ecliptic, the arc of the ecliptic intercepted between the 

 interfeftion of this circle and the equinoctial point will be 

 the longitude of the flar. 



The longitude of a place on the furface of the earth, is a 

 portion of the equator intercepted between a meridian paff- 

 ing through the place, and another meridian which paffes 

 through iome principal city or obfervatory aiTumcd as a 

 point of departure, from which the longitudes of other places 

 are taken. The reafon why longitude is fo differently de- 

 fined on the celellial and terreftrial globe, has been already 

 explained under Latitude, to which article the reader is 

 referred. 



The fubjefts of aftronomical invedigation, arifing from 

 different definitions, are fo intimately connetted, that much 

 of the prefent has been already anticipated. Under Right 

 Ascension we have (hewn how, having given the longitude 

 and latitude of a heavenly body, we deduce its right afcen- 

 fion and declination : and under Latitude, a rule has been 

 given far computing the longitude and latitude from the 

 obferved right afcenfion and declination. But though we 

 have fhewn how the quantities are derived reciprocally one 

 from the other, we have referved for this place to explaia 

 tiow they are originally derived from elementary obferva- 

 tions. We are therefore to fuppofe the cafe of a pratlical 

 aftronomer who (hould be defirous of making a catalogue of 

 ftars, and of determining their longitudes and latitudes inde- 

 pendent of previous obfervation, except only fuch as are ab- 

 lolutely neceffary for determining the quantity of preceflion, 

 aberration, nutation, &c. 



The obferver is to be even fiippofed unacquainted with 

 the latitude of his obfervatory, v.'ith the iituation of the 

 equinoSial points, and with the obliquity of the ecliptic. 

 The principles of the method which we mean to explain 

 were familiar to Flamfteed and the aftronomers of that pe- 

 lyod, and are demonllrated in De Lalande's and Vince's Af- 

 tronomy. But the late Dr. MaflvL-lyne was the allronomer 

 who improved and praftifed it with the greateft luccefs in 

 forming his catalogue of the thirty-fix principal ilars, and 

 which would have been much more accurate than any ever 

 known, had the inftrument with which his obfervations were 

 made been as perfeft as thofe o'"Jater conllrutlion. 



As no inftrument now in ufe can give directly the longi- 

 tude or latitude of a ftar, it is neceffary, firft of all, to de- 

 termine the right afcenfions and declinations of thofe liars of 



Vol. XXL 



which we mean to form a catalogue. The method of de- 

 termining the declination has been already explained at great 

 length. (See- Declination.) It is quite independent of 

 the folar theory, and is derived by direel meafurementof the 

 meridian diftance between the objeft and the pole. A mural 

 circle, fuch as that now ere£ling at Greenwich, determin' 8 

 this diftance, without any reference to the zenith; but with 

 a quadrant, and with aftronomical circles of the ufual con- 

 ftruClion, it is either abfolutcly neceffary, or at Icaft conve- 

 nient, to employ the zenith. And in this cafe we determine 

 by one feries of obfervations the diftance of the zenith from 

 the pole, and by another feries the meridional diifance of 

 the zenith from each particular ftar. The firft quantity, 

 called the co-latitude of the place, being apphed to tl.e 

 fecond, or zenith diftance of t!ie ftar, the fum is the polar 

 diilance. It is evident, that all this may be performed 

 without any knowledge of the folar theory, or even without 

 a fingle folar obfervation. 



To determine the right afcenfions of the ftars, we might 

 have affumed (had right afcenfion been otherwife defined) 

 any great circle perpendicular to the equator, and paffing 

 through any given ilar, as a Aquilae, exaclly in the fame 

 manner as we affume an arbitrary meridian for the determi- 

 nation of terreftrial longitudes. But as aftronomers have 

 agreed to affume, as their firll celellial meridian, that which 

 paffes through tlie vernal cquinoitial point, the folar theory 

 neceffarily becomes involved with the fubjeft of our invefti- 

 gation : we are, therefore, under the neceffuy of combining 

 two dillinft objefts of enquiry. In the firft place, it is ne- 

 ceffary to determine exa6tly the relative fituation of the ftar* 

 with refpedl to each other and to the equator ; and next, to 

 place the ecliptic in its true pofition both with refpedt to 

 the equator and to the fixed ftars, and thus determine the 

 fituation of the equinoftial point. To have a clear idea 

 of the whole of this procefs, we ftiould obferve that the two 

 preliminary invelligations are perteftly independent of each 

 other; for the conftellations {as we remarked above) might 

 be truly placed on the celeftial globe without any knowledge 

 of the ecliptic, and the ecliptic, in like manner, might be 

 placed making its proper angle with the equator ; and the 

 declination of the fun and its diftance from the equinoftial 

 point determined at any moment, by a feries of folar obfer- 

 vations condufted without any reference to the fixed ftars, and 

 even without any knowledge of their exiftence. It is by the 

 combination of the refults of thefe feparate invelligations 

 that the intended objedl is accomplifhed. The pradical 

 method of condufting the whole of this operation is as fol- 

 lows : 



In the firft place, we affume the right afcenfion of any 

 given ftar, as for example <c Aquilas, as near the truth as 

 poffible from prior determination, or we may confider it as 

 entirely unknown, and call it zero. This is quite immate- 

 rial, but the former method is the moll ufual. The ftars of 

 the intended catalogue are then obfervcd at the tranCt in- 

 ftrument for a feries of years, with a view to detcrmme their 

 difference of right afcenfion from a Aquilx and from each 

 other. This inveftigation would be much more limple than 

 it is, if the fixed ftars always preferved the fame relative 

 pofition to each other, as the differences of right afcenfion 

 would then remain the fame. But this is not the cafe ; the 

 apparent pofition of each particular ftar is altered by the 

 effefts of aberration, preceffion, folar and lunar nutation. 

 The phenomenon of Aberration has been already explained. 

 That oiPreeijfi-n and Nutation will likewife be minutely de- 

 fcribed under their rcfpedlive titles. At prefent, it is only 

 neceffary to obferve, that the adion of the fuu and moon 

 X % (confidered 



