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

180'fTrI; rPLANETS.
1nponl thle sunl. MIrcurily never appears fuirthlc fi'om the sun
than 290 (2'8~ 48'), and seldom so flar; and Venus never more
than about 47~ (47 12'). I3othl planets, therefore, appear either
inl tihe west soo after sunscet, or in tle east t a little before sun,rise. In high latittdes, where the twilight is prolongel, Me rcury can seldom be scen with thlel naked eye, and then only at
thec periods of its grleatest elongation.x' Tlhe reason of this will
readily appear froim the following diagram..'Let S r epresent tile sun, ]E the earth, and IMN Merclttlury at
its greatest elongations from the sunl, anld ()ZI) a portion of the
sky. Thell, since we refer all distant bodies to tile same concave sphere of t1he heavens, we should see the sunll at Z land
Aelrcury at 0, when at its greatest eastern elongation, and at P..
wheln at its greatest western clongationl; alld while passing
firom Ai to N through Q, it would appear to describe the are
OfP; and while passing firoln N to N:i, through l'1{, it would
appear to run back across the sun onl the same are. It is
fiurthlcr e vident that it would be visibleC only when at or near
one of its greatest elongations; being at all other times so near
thle suit, as to be lost in his light.
Fig. 60.
30:3. A planet is said to be in COtimCtion with the Sun
wlieti it is seen in the same part of the lheavens with tlte smun,.................................................................... ~..'.........'...'....`.'..`.....................................' Copen'lllus is Nsuaid to have lameulntec, on his dcatlh.i-d, tlhat hwc had never
btc atble to obtain a sight of Mercury; and l)elambre, a great French astronoMlt'r, s'aw it but twice.