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

114tt~~8~' IU I'"MOON.
tainted l1)y dll~Trawiug tlttangents AOCT and A'O'-. (l:ig. 51.)
Tllr'0oulgho ut thle space inclulc(l between theso tangent's, m()or1'
or less of thle fsun's ligh]t is lnterccpted firom the rnoon 1tby tho
intcrl'osition of the eartl; fior it is evident, tlhat as the moon
moves toward the shalldow, she -would gradually lose the view
of the sun, until, oln enllterillng ti slladow, the sun would )be
entirely hidden firom her.
253. Tlhe semi-anyle of the.Penumbra equeals the stan's semidiameter and horizontal tarallax, orl$ 4 }p.
The angloe A.O'M: (Fi g. 51).:.AE'S::::::: S —'. ] But AES
is the sunll's scmildiamlte'r, and B13AE ia the sun's lhorizontal
parallax, both of whichl quantities are known.
254..'The setmin-a/gle of a section. of the,Penumbra, where
the moon cr'osses it, equals the moon's hovi2zonl ta paral lae, plxus
the sun's, plus the sun's senmi-dia,meter'.
The angle hl',t (Fi.g. 51) E/:Ce]O'ji-ECO'h.'But IEhC'...,
tlhe moonl's horizontal parallax, and fC'::::::-p (Art. 253),.
/.... i-t:p, l)+, all wh ficl are like\ise known qulantities.
lcince, by lCeanls of tlhese few elemenlClts, whichY 1 are known
from observation, wr ascend to a complete knowledge of all tiho
parlticulars necessary to be known respecting t.he mooll's pe2t55. XIn the prececding investigations, w~e have suplposed that
thle cone of the carth's sllhadow is fo'rmed by lines dIrawn fi'om
tile sunll and touclhinl' the eartl's sulface. Bult thlt apparent
diameter of tlhe slidow is folund b1y observation to be soImewlhat
greater than would result fi'om thiLs hypothesis. Tlio fact is
accounted for by supposhin t.hat a portion of tlhe solanr rays
whlliclh graze the cartlh's surfiace are absorbed and extinguislled
b)y the lo)wer str ata of the atmospherec. lllhis amounllts to the,same thing as tlhougl the eartlh were larger than it is, in wllicl
case thle moon's horizontal pairallax would be incre1ased; and
accor(l ingily, in order that tlieory and ol)scirvation may coincitie,
it is fotund necessary to increlas the parallax b)y 1o-.o
256. In a total eclipse of the mloo1n, its d(isk is still visible,
shlitlintg witIt I- dlull red lilht.. T.lhis is (duc to the earthl's at0 tl>~~~