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

1. 2'1 t'IE MOON.Q
describes less thllan 90D of her orbit while sloe turns just one.
foufrtl? roudc ulto her axis consequetlytt sltowing it a little
of tile furthler side on ttle east limb..Rut in the perigeal part
of ter oirbit, she advances- fitster thlan the melan, and therefore
il ot-f)urtllth of er diurnal rotlation she moves forward more
tltalt 90 in:t ]her orbit, and' presents some suriace beyonld tlhewest linb.
Thle libration. inM lItitui', by which sthe alternately presents
to our view the space about her poles, is caused by the obtliqulty of letr equfator to ]ler orbit. iltr equatol is inclinetd anlout
io ( 31 0'It") t3o the ebiptic, and remainls parallel to itself.
fBut the angle betwceen ]emrt equatort and ortit varties fri om ltbout
31-0 to-, onl account of the motionll of the no1des. Each tol.e
of the tloonl is presented toward the earthl eve|ry 27 da) ys, just
as thtle artll's poles are turned towll ard the sun every year,
ttougltl ill a muclt less degree. (Scc Pi.g. 3.1.)
Te lm oon exhib)its another plhenomenon of this ]ind, called
1te1r drut. ldtibrdbcttaiow, depending ol the (laily rotation of the
st)ectatorl. Stc turns the same fae towatlrd the center of tthe
eartl only, whereas we view lher fi'om th e sumrfitce. \\eltcll sio
is on tele tteridian, we see lhlr disk lnearlly thotulh we
viewed it friom the center of the earth, and hentce int this situationl it is sutl)jcet to little changre; l)ut w\ten ne:tar the horizon,
oure circle of vision ttakes in morloe of the l 1upper limb ttlanll
wtould be pr: esented to a spectatolr at the center of tthe elarth.
HIenlce, firont this cause, we see ai portion of one limb w\iile thli
moonll is risillog, w\rlich is gradually lost sight of, atlt \e seet
poorticll of tlte opposite Ellmb as tit moon deelines towttrd the
-vwest. t will be remarlt ked that neitther of the fotreloing
chlanges imlplies any actual motion in tihn rie moon, but that achl
arises fiom at clhange of position in the spectator relative to tho
220. An inhabitant of the imoon wrould have but one day
and one nighlt duringt the whole lunar ml onth of 29,3 days.
One of its days, tlerefore, is equal to nearly 30 of ours. So
prottractedl an exposure to the S\lin's ra- yss, if the moonl htad all
tttmosphlere like that of tflc earth, Nwould occasion anl excessive,
accumulation of Itcat; and so long an absence of the sum 11u111st
occatsiotn a correspolding degree of cold. Ea:tch day wvould be