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

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1 60..heli e a.rth bciln a globe, thel sull constaltly enligrhtens
the half next to hiimnj- while tlhe other htalf is in (darkiles-s.
The lboundary betw'een te enlighltetned tand thle unlcligh1teled
part, is called the3 cd of'dbuaZ twil ii.a. Whenl, tlh earth is at
one of the equinOxSCs the sun is at the other, and the, circle of
illuminiationl passes throulgl both the poles1. When thie ear'th
r1eael s one of tdie tropics, tihe sulnm beill" at tlc otherll', tOhIe circl
of illumination cuts the earth so as to p1ass 230 28' beyondl thle
neWare'rt, and the same distance shlort, of tle remoter pole.''these
cesults would not ble uniforml were not the earth's axis always
to remaltn1i1 parallel to itself. The following ligtur will illus-trlate
the foregroing statements.'"
Fig. 31.
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S.r.
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o In fact, tihe sun enlightens a little more than half the earth, since, on accounttt of his vast magnitiude, tilhe tangents drawnvl from the sides of tihe st1l1 to
correstlonding sides of the,: earth, converg'e to a pointt befnll the ea'thl, as Yill bo
seen by and by, in thle represertatio- of clipses.'hoe amount of illuninatiot,
also, is ilcretased by refraction,