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“ 1 ft. By the latitude of the moon one may form a judgment of the longitude ofthe fpot where the obfervation is made. By the latitude of the moon is underfloodits diflance from the ecliptic. The ecliptic is the line of route or orbit of the fun.The moon in her rotation conftantly gets further from this line until at her greateftdiftance from it; afterwards fhe approaches it again until in conjunction with thedragon’s head or dragon’s tail *, when fhe cuts the ecliptic. Now, as the moon, cnreceding from the ecliptic, at the fame time paffes to the weft, fhe mu ft neceffarily havea greater latitude on one part of the globe than the other; and when her latitude isknown, which is afcertained in degrees and minutes by the aftrolabe, her pofition eaft orweft is found, and the number, of degrees and minutes fhe diverges to either quarter.But the longitude of the fpot of obfervation cannot be determined without knowingexactly in what latitude and longitude the moon ought to be at the fame inftant at theplace of departure, at Seville for example: when the latitude and longitude of themoon at Seville in degrees and minutes are known, by comparing them with the lati-tude and longitude of the fpot, the difference will fhew how many hours and minutesthe meridian in which the obferver may be, is diftant from that of Seville ; and thusthe longitude from that city may be afcertained.
“ 2d. The moon furnifhes me with another method of determining the longitudeof any fpot; but for this purpofe muft be known the precife inftant when at Seville the moon is in conjunction with a certain ftar or planet, or when fine is in oppofitionto the fun, or forms an exaCtly afcertained angle with that planet, which I gatherfrom an almanack. Now, as either phenomenon occurs in the eaft before it does in theweft, I difcover my longitude weft by the time that has elapfed between its occurrenceat Seville and the inftant at which it is obferved by me. But if the phenomenon takesplace at the point of obfervation before it happens at Seville , the difference of time pointsout the longitude eaftward. For each hour fifteen degrees of longitude muft be reckoned.
“ No great genius is required to comprehend what I have mentioned. It fhould beknown that the moon has a peculiar movement, oppofite to the general one of theheavens, feeing her courfe is from weft to eaft, and every two hours fhe paffes over adegree and fome minutes. And as the moon is placed in the firft heaven and theftars in the eighth, fhe occafionally eclipfes certain ftars, by pafling between them andus, or intercepts their rays from our obfervance, an event which cannot take place at thefame time to be feen at Seville and at Valentia, which the figure beneath will explainf.”
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* That is to fay, the nodes or orbit of the moon cut the ecliptic,f This figure was wanting in the manufcript, but was eafily fupplied from the text.
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