To be able to visualize and thus thoroughly understand the phenomenon of retrograde motion
it is advisable to study the cycles of two groups of planets: the minor planets, those between
the Earth and the Sun; and the major planets, those whose orbits lie outside that of the Earth.
Analyzing the cycle of Mercury, as typical of the orbits of the minor planets, shows this
succession of phenomena:
Superior conjunction, when it passes on the far side of the Sun in direct motion, at which
time it is invisible. Since thereafter it rises after the Sun and remains invisible during the
daylight hours, it becomes visible only after the Sun has set in the west: the Evening Star.
About fifteen days after the Superior conjunction it is at its smallest, a small circular orb.
Greatest Elongation East: Some six months later it reaches the point of the greatest distance
ahead of the Sun in its counterclockwise direct motion in orbit, hence East. At this time it
passes out of its gibbous phase, showing only half of its surface illuminated, yet seemingly
larger and brighter because it comes closer to the Earth.
Enters Retrograde Arc: Some two weeks later it enters the arc over which it will shortly
retrograde.
Maximum Brilliance as Evening Star: Even though reduced to a crescent of illumination it
appears still larger, and with its elevation it remains longer above the horizon and is at its
greatest brilliance.
First Station: Another two weeks and it becomes stationary, in preparation for retrograde (S.
R.) motion. In another two weeks, about six days before the Inferior conjunction, it becomes
a slender crescent.
Inferior Conjunction, when it passes in retrograde between the Earth and the Sun and is lost
from sight in the Sun's rays. This conjunction is shorter in duration. They separate faster
because Mercury's motion is opposite to the apparent motion of the Sun. In another five days
it again becomes visible on the other side of the Sun, the West, when as the Morning Star it
appears before sunrise as a slender crescent, but turned in the opposite direction.