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

80 TJIlE SUN.
lThe exact declination of the sun at any titme is determlied
from his crtifrrt acdtiude or Zlc-it/It bLst/aet, the latitlude of
the place of observation bein knlown (Art. 37). Thlle in]stanIt
the center of the sunl is on tohe meridian (which instlat is grivel
by tIle transit i-nstrulllmet), we e tlte e (listale of h}is upper
and tlhat of htis lower limb fiom thle zenith h: ]alf thel suilm of tlhe
two observations corrected folr rcfi'action, fives the zenith distanlce of the center. Thlis res\ult is dinillishlcdl r fi parallax
(Art. 8^1:), antd we obtainl tie zenitht distance as it would be if
seen firlom thle center of the eartlh. Tte zenith distance b)eingt
known, tile (cclitnation is readily fotund:by subtlractincg that
distance fromn the latitude. "By tlus taking tt sunl's dtc llation for everl day of th:le yealr at nolt, aind comparing the
results, we learn its motion t/o andJ/5j'o t/1e equator.
1 5 6. To obtalin the motion in right ascCnsion, we observe,
with a tranlsit instrutlcnt, tihe instant when the (-center of tle,
sunil is on tile mcriditan. Our sidereal clock l ive s us thie ri(-rht
tssecnsimt(I ill time (Art. 12 1), wlicl we wlhni easily, if l c
choose, colvcert in to (I erc1 es m eand mnlil.al ll t is Imre
comm)llonli to express riglht ascelsion11 by hours, minull tes, and
stoeeds.'l The di tI'renctes of ri gt t atseension firom day to day
thlroughout the year, give us the sun's atual motion paraclle
to Ic? cttua/t'. lFroll thl daily records of tltcsce two nmotlitls,
at riglt angles to each otli, aliuged in table )':: it is easy to
trace out tthe path of t} t} sun on t l arti(iial dglobe; or to calculate it \ith thelt -t e greatetst )lpr('Ieit io n mt itens of s)]pherical triangles, since tlhe (dccilination uand rin(dl t nsiete8siotl co(nstitute two
sidles of a righltt-t:anglced Tlllpheri('cal triangtle, tile orreslpon(ding are
of the ecliptic, that is, the loh'ngitude, )cing tte tlirdl side (Art,.
1332). By isptect:ing at ta:lldIe o bs t olsrvations, we shlall intlt thlat
the declination attainsl its grelatest N valu-te on the 2d of IDeceanh)er, w lhen it is 2' 27' -t 1" soulth; thl t fi'o mI thlis I ) period- it diminishes daily anl d b)ecoies inothingt on thIe 2tst of MA arch;
that it tthen iInreases toward the northl, nld reaches a sitmilar
miaxi1umt11 at t:hle northern tropic t.bout thte 22d of Jlullle; ald,
iinally, thlat it returll's (ag}ain to tlt: southerntl tropic b)y grada-: Stuch a ttable may ltb follm in Biot's Atstronomy, in )lttanbrc, ltud it ll l)st
collections of Astrointical Tablai3les.