| XXX | (485) | XXX |
485 ASTRO N O M Y. regard to the method of calculating and pro” the moon defcribes the line ef, fhe juft touches the ftia- Witheclipfes, we muft refer the reader to the aftronodow, but does not enter into it; when fhe defcribes the jedting mical tables of Mr Fergufon and others. When the line cd, fhe is totally, though not centrally^ immerfed principles are explained, the application and life of the in the fhadow ; and when fhe defcribes the line ab, fhe is a matter of fmall difficulty, and eafily acquired paffes by the node at M in the centre of the fhadow, and tables takes the longeft line poffible, which is a diameter, thro’ by a little pradtice. it: And fuch an eclipfe being both total and central is of the longeft duration, namely, 3 hours 57 minutes 6 Chap. XVII. Of the fixed Stars. feconds from the beginning to the end, if the moon be at her greateft-diftance from the earth; and 3 hours 37 minutes 26 feconds, if fhe be at her leaft diftance. The The ftars are faid to be fixed, becaufe they have reafon of this difference is, that when the moon is far- been generally obferved to keep at the fame diftances theft from the earth, fhe moves floweft; and when near- from each other : their apparent diurnal revolutions being caufed folely by the earth’s turning on its axis. They eft to it, quickeft. The moon’s diameter, as well as the fun’s, is fuppo- appear of a fenfible magnitude to the bare eye, becaufe fed to be divided into twelve equal parts, called digits; the retina is affedted not only by the rays of light which and fo many of thefe parts as are darkened by the earth’s are emitted diredtly from them, but by many thoufands fhadow, fp many digits is the moon eclipfed. All that more, which, falling upon our eye-lids, and upon the. th*e moon is eclipfed above 12 digits, (hew how far the aerial particles about us, are refledted into our eyes fo fhadow of the earth is over the body of the moon, on ftrongly as to excite vibrations not only in thofe points that edge to which fhe is neareft at the middle of the e- of the retina where the real images of the ftars are formclipfe. ed, but alfo in other points at fome diftance round aIt is difficult to obferve exadtly either the beginning bout. This makes us imagine the ftars to be much bigor ending of a lunar eclipfe, even with a good telefcope ; ger than they would appear, if we faw them only by the becaufe the earth’s fhadow is fo faint and ill defined a- few rays which come diredlly from them, fo as to enter bout the edges, that when the moon is either juft touch- our eyes without being intermixed with others. Any ing or leaving it, the-obfcuration of her limb is fcarce one may be fenfible of this, by looking at a ftar of the fenfible ; and therefore the niceft obfervers can hardly be firft magnitude through a long narrow tube; which, certain to four or five feconds of time. But both the though it takes in as much of the fky as would hold beginning and ending of folar eclipfes are vifibly inftan- a thoufand fuch ftars, yet fcarce renders that one vitaneous ; for the moment that the edge of the moon’s fible. difk touches,the fun’s, his roundnefs feems a little broke The more a telefcope magnifies, the lefs is the aperon that part; and the moment fhe leaves it, he appears ture through which the ftar is feen; and confequently perfeftly round again. the fewer rays it admits into the eye. Now fince the In aftronomy, eclipfes of the moon are of great ufe. for ftars appear lefs in a telefcope which magnifies 200 afcertaining the periods of her motions ; efpecially fuch times, than they do to the bare eye, infomuch that they eclipfes as are obferved to be alike in all her circumftan- feem to be only indivifible points, it proves at once that ces, and have long intervals of time between them. In the ftars are at immenfe diftances from us, and that they geography, the longitudes of places are found by eclip- ftiine by their own proper light. If they Atone by borfes : But for this purpofe eclipfes of the moon are more rowed light, they would be as invifible without teleufeful than thofe of the fun, becaufe they are more fre- fcopes as the fatellites of Jupiter are; for thefe fatellites quently vifible, and the fame lunar eclipfe is of equal appear bigger when viewed with a good telefcope titan, largenefs and duration at all places where it is feen. In the large!! fixed ftars do. chronology, both folar and lunar eclipfes ferve to deter- The number of ftars difcoverable, in either hemifphere, mine exactly the time of any paft event: for there are fo by the naked eye, is not above a thoufand. This at many particulars obfervable in every eclipfe, with refpedt firft may appear incredible; becaufe they feem to be withto its quantity, the places where it is vifible (if of the out number: But the deception arifes from our looking fun) and the time of the day or night, that it is impof- confufedly upon them, without reducing them into order. fible there can be two folar eclipfes in the courfe of ma- For, look but ftedfaftly upon a pretty large portion of ny ages which are alike in all circumftances. the Iky, and count the number of ftars in it, and you .From the above explanation of the dodtrine of eclip- will be furprifed to find them fo few. Or, if one confes it is evident, that the darknefs at our Saviour’s cru- fiders how feldom the moon meets with any ftars in her cifixion was fupeniatural. For he fuffered on the day way, although there are as many about her path as in on which the palfover was eaten by the Jews, on which other heavens, he will foon be convinced day it v/as impoffible that the moon’s fhadow could fall that thepartsftarsof arethemuch thinner fown than he was aware on the earth; for the Jews kept the paffover at the time of. The Britifii catalogue, which, befides the ftars viof full moon : Nor does the darknefs in total eclipfes of .fible to the bare eye, includes a great number which the fun laft above four minutes in any place; whereas cannot be feen without the affiftance a telefcope, conthe darknefs at the crucifixion lafted three hours, Matth. tains no more than three thoufaud, inofboth xxviii. 15. and overfpread at leaft all the land of Ju- As we have incomparably more light fromhcmifpheres. the moon dea. than from all the ftars together, it were the greateft abVol, I. Numb, 21. 3 6G furdity
