L U N 



L U N 



LUNAR, fomething relating to the moon. 



Lc-sAU Caujlic. See Caustic. 



Lunar Cycle. See CvcLi:. 



LuNAU Dial. See Djal. 



Lunar Eclipfe. See Eclipse. 



LuXAR Horofcope. Sec HoitOSCOPE. 



Lunar Month. See Month. 



Lunar Olfcrvatwin, or Lunar MctljoJ, is the metliod of 

 •finding the lonc;itiide, hy taking tlic diftance betwet-n the 

 TOOon and the fun, or a fixed Itar, which has been already- 

 explained under the article Loxgitudk; but the great irh- 

 portancc of this problem induces us here to give a further 

 and more minute explanation of its principles and opera- 

 tions, and of the different methods tliat have been devifed for 

 •obtaining the folution. 



This method of finding the longitude is the greateft mo- 

 deni improvement in navigation : the idea, however, is not 

 modern, but it has not been applied with any fucccfs until 

 within the lad fifty years. M. de la Lande mentions cer- 

 tain aftronomers who, above two hundred years ago, pro- 

 ,pofed this method, and contended for the honour of the 

 difcovery ; but its prefent ftate of improved and univerfal 

 practice he very iuilly afcribes to Dr Maflielyiie. The 

 difcovery, indeed, feems to claim lefs honour than its fjblc- 

 ■quent improvements ; it is one of thofe things which are 

 obvious in theory, hut ditncult in practice. The moil; an- 

 ■cient n;ethod of finding the longitude was by the lunar 

 eclipfes ; and that of finding it by the lunar diilances is 

 perfeffly ar.alogous : it is therefore highly probable that the 

 method was thought of at a very early period, but the want 

 of projier tables and apparatus prevented its being reduced 

 .to pr.iftice. 



It may be obferved, that, in the moft practical methods 

 of finding the longitude at fea by celellial obfervations, the 

 moon is the chief guide or inftrument ; for the quickneis of 

 her motion renders her peculiarly, well adapted for mea- 

 furing fmall portions of correfpondent time. Now, as llie 

 is feen in the fame part of the heavens nearly at the fame 

 inflant of abfolute time, from all parts of the earth where 

 file is viiible, and as file is continually and feniibly cliang- 

 ing her place, it is evident that if two correfpondent obtervers 

 were to note the precife m.oment of the r refpective times, 

 when fhe is feen in any particular part of the heavens, ihc 

 difference betzveen th-fe times "ojould Jl.^eiu the difference of longi- 

 tude. 



In every method of finding the longitude by the moon, 

 the firfl objeft is to be able to afcertain the "part of the 

 iieavens where (he is : this is ealily feen at the time of her 

 -eclipfes, or at the occultation of a fixed (lar ; and ihefe were 

 naturally the firll methods reforted to, bi!t they occur too 

 feidom for general ufe : 'he moon's place, however, may be 

 marked with equal precifion, by taking her dilhiice from 

 fome fixed objeiif , whofe latitude and longitude are known ; 

 and liars in or near the zodiac are preferred, as the nearer 

 fuch objeifls are to the moan's orbit, the gfreater will be her 

 motion with refpeft to them : and though her motion is not 

 uniform, ytt, during the fliort fpace of time that fhe is near 

 any (tar, (he may be confidered as moving uniformly. 



It has been above obferved, that if two perfons under 

 different meridians were to mark the moon's place, and alfo 

 their relative times of obfervation, they mif;ht thence tell 

 their difference of longitude; but they could not communi- 

 -cate their obfervations fiifBciently foon for practical pur- 

 jjufes; and even admitting the poffibiiity of this, it were 



neceffary that the longitude of one place fliould be known, 

 in order to determine that of the other. Now tlie Nautical 

 Almanac i-^ calculated to fupply all thefe wants. This ad- 

 mirable work may be confidered a perpetual obferver," that 

 communicates univcrfallv and infbuitaneoufly certain celellial 

 appearances, as they take p'ace st Greenwich Obfervatory, 

 Here the niftanccn are given between the moon and the tun, 

 and certain remarkable ftars in or near the zodiac, for every 

 three hours ; and at;y intermediate didance, or time, may be 

 thence found by tlie rule of proportion with fuflicient accu- 

 racy. If, therefore, under any meridian, a lunar dillance 

 be obferved, ihc difference bel'iueen the time of ohfer'vation and ■ 

 the time in the yllmnnac, ivhen the fame di/lance ivas to tale 

 place at Qrcenivich, -will fhetu the Irmgitude. For example, 

 if the obferved dillance between the fun and moon be 50 ' at 

 ei;;ht o'clock, but by the Almanac tlie fame dillance cf e^o' 

 will take place at Greenwich at fix, it is cviileiit that tiie 

 difference between the obferved and computed time is two 

 hours, and therefore the longitude is jo ; and it is alio clear 

 that the longitude is eaft, the time being mofl advanced at. 

 the p!ace of obfervation. 



A method fo apparently fimple mufl have been long fince 

 adopted ; but two difficulties occured : one the want of 

 proper inftruments, which want lias been happily fupplied by 

 the invention and fubfequent iu'.provement of Hadley's 

 quadrant; and the other, correel lunar tables; for the 

 moon, though fo near and fo confpicuous to the earth, has 

 always perplexed allronomcrs more than any other ceklli;J 

 body. The various inequalities of her motions were never 

 properly underilood, until fir Ifaac Newton difcovcred the 

 phyfical laws which govern them ; and from his theory pro- 

 fefTor Mayer formed lunar tables, which have been found 

 fufiiciently correft for nautical praflice, and from which 

 thofe tables in the Nautical Almanac of the lunar diilances 

 had been calculated under the diveclion of Dr, Mi;fl<elyne 

 for many years. In 1806 the French board of longitude 

 pubhfhed new lunar tables, calcu'ated by Du Burgh, fr'.m 

 the theory of La Place and the obfervations of Dr. Mafke- 

 Ivne ; and from thofe tables the lunar diflanecs in the Nautical 

 Almanac of 1815 are computed, and in tlic Almanacs that 

 follow. 



The above two difficulties having been obviated, a third 

 feems fli'l to remain ; and though this is in lome meafurc 

 removed by the application of the Nautical Almanac and Rc- 

 quifite Tables, yet the calculation is (till more tedious lha« 

 might be wiflied ; nor is it poffible to render it much fhortcr, 

 as the problem neceffarily comprehends folutions in two 

 Ipheric triangles : this ariles from the circumftance of the 

 ol/erved diflnnces between tile heavenly bodies not being the 

 true dijlnnces ; for th.e altitude? of thofe bodies are more or 

 lefs affefted both by refraction and parallax ; and though 

 thefe effedts only operate in a vertical direction, yet that 

 which changes the altitude of two bodies muft alfo change 

 their dillance afunder. This is evident from the coiilidera- 

 ti(m, that the altitude of a celellial objeft is an arc of an 

 azimuth circle intercepted between the objeft and the ho- 

 rizon ; and as all azimuth circles incline gradually to each 

 other from the horizon to the zenith, where they meet, it is 

 plain that the more two bodies are apparently ruifed, the 

 lefs will be their apparent dillance afunder, and the con- 

 trary. 



It is well known that the heavenly bodies are raifed by 

 refraftion, arid depreffed by parallax ; and that thefe eflects 

 are greatelt in the horizon, and gradually diminifh tg thp 

 zenith, where they become nothing. , Refraftion depends 1 

 upon altitude alstie, but parallax depends upon bc.th altitudp , 



and ' 



