An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

164'1'fli: t(ON.o
276. 179e.LuInar method f,fi#nding tle hlolmittu&, at sea, is
in many respects preferable to every othller. It co'nsists ill
measuringr (w\ith a sextant) tlle angular distance betwCCee the
mloon1 andl tle sunll or bet\een the moon and a star, and thell
turning to thec Nautical Almanac,* and finding what time it
was at (Ghlccnwichi when that distanf'ol was thle same. The
1m1oon moves so ra-pidly, that this distance will not )be tlhe saine
except at very nearly the same instant of ab)solute timel. FIor
example, at 9 o'clock, A. M., at a certain place, wle find the
angular distance of the moon and thel sun to be i(2~; and onl
looking into the Nautical Almanac, we find that the time
when this distance was8 the same for the meridian of Greenwich
was I o'clock,;P. n.; whlcle Ie infelr that the longitude of thle
placc is four h1olurs, or GO west.
I'Tihe Nautical Almanac contains the titrue angular distance of
thec 1moon filomll he sun, fi'om the four large planets (\enllus,
M'lars, Jupiter, and Satulrn), and flom nine lriiht fi xed stlars,
for the beginning of every third hour of imean time for the
meridian of (-renwich; and ftle timen corresponding to any
intcrmc ediato distance, may bo found by proportional parts. t
277. It would )be a very simple operation to detelrmine tle
longitude by Lunar ])istances, if thle ploes', as d1esribed ill
thle,preccding article, mwrce all that is nccessary;; ut tthe v\ar ious circumstances of pa1rallax, refitaction, and (lip of tile lhoi0on,1 would differ more or less at tlhe tw\\o )laees, even \ere tile
bodies (whose distanccs w\er\c take) ill view froml both, whlich is
not necessarily tlhe case. The observations, therefotre, rquire
to b)e reduced to the center of ttle eartl1), being cleared of thle
effects of )arallax and refrtaction..1 lenle, thlree oblscervcrs are
necessary ill ord(er to take a lunar distanllc in the itmost exact
manner, Nvriv, two to 1melas0lre tllhe altitudelCS of thle tw\o bodies
respectivelry, at the same time tlat the tlid ti takcs the angutlal r
distancc betw een them. The altitudes of thle twto luminarlies
0 The A1?.tiocd Almaona is published an1lually three or four )years inl aldvan"e,
coltaiitg i all tCCSSti ti tables alnd information, for tile utse of navigators.''1ho
ftglisll Board of LoIngitude have for a long series of yeiars issued sutch a work.
The Aalnerican ]sheris and rt t!iV.tl A'lmattc, which possesses the &tulln gcental
clatracter, was commenced but a fiew years sillCo., Sec Biowditch's Navigator, tenth ed., p). 226.