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

. 56 Tf'r OON.
eclipses of the stun, llercfore, happen when the sunll is inl apogee,': and thle moon in perigee.
265i. f/ten thae aoon00t is at her' meawn distance fi'onb the
earth, andlifro, the snit, her shAadow nearly rehaces t]he ea rth's
sti fa ce.
ILet S (Fig. 55) represent thle sun, l) thle moon, and'T tlhe
earthl. Then, tle semi-angle of tlhe cone of thxe meoon's sh1adow,
Fig.65.!~~_~<~..:~........................... I
Dl)Klt will, as in the ea'se of thel earth (Arlt. S47), equal SD:)Ih.'R)1K:1(. of' wlicll SI)l is thle st1s1 alpparent semi-diamneter, as
scell i'oll the 00moon,1 and l1).(C is the sun's horizonttal -arallax
at tlce mnoon. Since, onl account of the great distance of the
stlun colmpared with that of tile moon, the semi-diameter of the
sunll as seen from the moon must evidently be very nearly thef
same as wtIten seenl from tte artll, and since, onl accounllt of t11e
mIllinutcllncss of t.e tmooon's seili-diameter when seeln from the
sunll, the sun's lhorizontal parallax at the moon must l)e very
small, we might, without much error' take tlhe sun's apparent
sei-ditameter fr'om the earthl, as equal to tlhe semli-angle of the
cone of the iroon's shadow; but, for the sake of greater accuracy) let us estimllate tlhe value of the sun's semi-diamieter and
horizontal parallax at tlhe moon.
N\ow, S.I)D.D: Slt':: ST.I: S)::: 400: 399; hlene SI:)
400
"'it 102 Sl.'.02; andf the sutiln's mean semi-diameter
39lt ) i
S'T171 being:16.025, hence S!):.:.1.0025 x1t6.025t:::~:16.035:::: 16'
3".9.
Again, since parallax is inversely as tlhe distance, the sunl's
o'The sun is,aid to be in apogee, lwhen tlhe earth is in aphcelion.
1-'The apparent magnitude of an object being reciprocally as its (listanco from
thle eye. See Note, p. 8t.