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

I,fAcMlt2N'rs OF'TIf PIANETIRY oR t131. 227
planie of the coliptic, SA and 1ill partallels in that plane, pointilng to the vernal equinox (wlhichl is considered infilitely distllt), 1P thle plalet, I'0 tlhe perpendicular fi'om it. to thle p)lalt
of tile ecliptic. Let HIEIO, the angular distanice in the plano
Fig. 71................ ---- -—..............
A It
of thle celiptic (from H1 to 0 on the right side of the lines), be
thle gcocentric lonlgitude of the p)lanet; then ASO w\ill be, its
]tcliocentric longitude. Also ItO), the tangiularl distance of tile
p)lalet fi'oml the ccliptic itt a )plalc )erpendicular to it, is tlle
ge{centric latitude, and PSO is thie lheliocentric latitude. Tf}t
plantct's an(lular listtce firom thle sutl, IlES, is also knlownv
fi'oln observationl. I eliee, ill tilhe trialngle SEt', we know St'
antt 8EE an(l the a n;gle 58Et, from N hltich we cant finlld I; nt l a
knowinti og 1' and the angle IP'E:(), in tire riglt-alngled triangle,.
\we calculate XO. Necxt, inl tile triancgle OEIS, N.0 tland ES are
lmlown; also OEhS, foulld by subtracting the planet's longitude
froml the sun's,'. e., IWEO firom it 1tS8 (reckoned to tile riglht
from tI); hence OSE and OS are calculated. If ()SE be addedt
to ASPE, tle supl)lement of IIt]S, vwe hlave A.SO, tle l'lC'leen1tirc 1on yittCe,3 of tlhe plllat. lThe line (S just foulld, is called
tlhe curt ate distance, of tle plfanet firomf the sunll; and this, with
the (wiactt (listance SP, gives lust in thle rigllt-angled trianlgle
PO (), the /hedilcent-qic latitudte PIS0. Th11us, by a f\w tprocsess
il1 platle trigonometry, w\e call do(10 llat is equiivalentl to mlaking
a trtansfer of our position from tlt earth to thle sun.
370. Itvnlltyg' now learned how observations made at thle
eartIt t ay )be converted i tto corrlespt' ondit g obsCrvatiots8 lI.aIde