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

60 THRIE E'A RT.,
The sextant must be lheld in suchl a malnner ftlat its plane
slthll )ass tllough tfi plane of the two objhects. It 11111st be
teld, thflreftre, inl a vertical pllane in ttakingl altitudes, and in a
ho1rizontatl plalne in taklingt the horizontal diameters of the sun
and mloon. IlFolding the: instrument inl the true plale of the
two bodies, wlltose anigulal distance is measurcd, is indeed thlo
most difficult part of the operation.
The peculiar value of the sextant consists in thlis, that the
obsetrvations ttaken with it arle not affected b)y any motion in
the obsierver; hence it is the chief instrument used for angular
1merasurements at sea.
131. lttrea mples illustrating the fuse of the &extan t.
Ex. 1. *Alt. 60's lower 11imb,. 49~ 10' 00"
0's semli-diameter,. 0't.5 51l
4i90 2,5' 561"
Subtract ]Refractlon,.. 00 00 49
490 25' 02"
Add arallax,... 00 00 06
Trute alt;itude O's center, 490 25:' 08"
Ex. 2. l7ith the Art.~ificial ]Holrizon.
Altitude of O's uppet r limb above tlt image it thte artificial
horizon, 1000 ()' 47".
True altitude,... 50 0.''23".5
O's stemi-diameter,. 00 15 50.
4:90 45-',33"5
Refraction, 00 00 48.
490 44' 45".5
Parallaxt,...... 00 00 05.
T.rue altitude of 0's center, 9. 9 44' 50".5
ASi-TRONTOMtAlf, PROBLEMTfS.
132. Given, tihe stn's.?ifU/t.Ascetsion, (tnd Declination, to
find2 his Longyitutde antd- the Obliquity ofj thie.Joilptc.
Y*mol's Splerical Trigolnoltry, )p. 136. Vinco's Complete System, vol. i.