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

INFERIOR PLANETS-MERCURY AND VENUS. 1853
the planet in the superior, and to retard it in the inferior conjunction, yet on account of the greater distance, the apparent
motion of the planet is much slower in the superior than in
the inferior conjunction.
306. In passing from either conjunction to the other, an
i/nf eriior planet s8 slationary at a point a little way from the
greatest elongatiorz toward the infe);ior co'njunction.
If the earth, were at rest the stationary points would be at
the greatest elongations A and B; for then the planet would
be moving directly toward or from the earth, and would be
seen for some time in the same place in the heavens; but the
earth itself is moving nearly at right angles to the line of the
planet's motion, and therefore a direct apparent motion is
given to the planet. Hence we need to choose such a position
for the planet that its retrograde movement shall be just sufficient to counteract this. Of course it must be on the arc ACB.
But, as the planet's angular velocity is much greater than the
earth's, it must be near A or B, where the motion is quite oblique to our own, else the retrogradation will be too rapid to
neutralize the direct motion caused by the earth's progress.
The stationary point for MIercury is at an elongation of 15~ or
20~ from the sun, that of Venus at about 29~.
307. Jfercury and Venus exhibit to the telescope phases
6similar to those of the moon.
When on the side of their inferior conjunction, as from A to?B through C (Fig. 61), these planets appear horned, like the
moon in her first and last quarters; and when on the side of
their superior conjunctions, as from B to A through D, they
appear gibbous. At the moment of superior conjunction, the
whole enlightened orb of the planet is turned toward the earth,
and the appearance would be that of the full moon, but the
planet is too near the sun to be commonly visible. All these
changes of figure, resulting from the different positions of the
planet with respect to the sun and earth, will be readily understood by inspecting the diagram (Fig. 61).
The phases show that these bodies are not self-luminous, but
shine only as they reflect to us the light of the sun.; and all
the planets in some way give evidence of the same fact.