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

nMOTIONS OF'OHEl, ILANNJiETARY SYSTEM. 221.
those of ~Tcrcury and'Venus, alternately direct and retrogradle,
and between tte two thte plale ts are stationary. In this ease,
htowever, tile earth moves faster than the planet, and t1le
lalnet has its opposition, bult no inferior conjunction; wltrcas,an infierior planet has its inferior conjunction, but tno o)pposition. Tlese (diflerencets rendler the aplparent motions of tthe
superior pilancts int slome respects unlike tiose of iTrcuryel and
tlVenus. )n tlhe side of the sun ilost remote from the cartht
the motion of a superlior planet is direct, becaiwse, as is the case
with VC'enus illn el superior colnjultction (see figure 61), tlhe
only effect of the earth's motion is to accelerate it; but when
the planlet is in opposition, tlhe earth is movinlg past it with
greater velocity, and makes the planlt seemto to move backwtard, like the apparent backward motion of a vessel when vwe
overtake it andl )ass rapidly by it in a steamboat.
36 1. Let AXB01) (Fig. 70) represent the earth in difterentc
positions ill its orbit, I: a superior planet as Mars, and NRt an
Fig. 70..I. o
are of thle concave sphere0 of the h1eavens. lFirst, suppose the
planlet to remain at rest in:1M, and let us see what apparellt
lotions it would receive fi'om the real motions of thte earlft.
\hrlici the earth is at 13, it will seeJ thle planlet in thel heaven1s