Download 7. The Solar System

7.1 Planetary Configurations
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7.1
Planetary Configurations
The apparent motions of the planets are quite complicated, partly because they reflect the motion of the
Earth around the Sun (Fig. 7.3). Normally the planets
move eastward (direct motion, counterclockwise as seen
from the Northern hemisphere) when compared with the
stars. Sometimes the motion reverses to the opposite or
retrograde direction. After a few weeks of retrograde
motion, the direction is changed again, and the planet
continues in the original direction. It is quite understandable that the ancient astronomers had great difficulties
in explaining and modelling such complicated turns and
loops.
Figure 7.4 explains some basic planetary configurations. A superior planet (planet outside the orbit of the
Earth) is said to be in opposition when it is exactly opposite the Sun, i. e. when the Earth is between the planet
and the Sun. When the planet is behind the Sun, it is
in conjunction. In practise, the planet may not be exactly opposite or behind the Sun because the orbits of
the planet and the Earth are not in the same plane. In astronomical almanacs oppositions and conjunctions are
defined in terms of ecliptic longitudes. The longitudes
of a body and the Sun differ by 180◦ at the moment
of opposition; in conjunction the longitudes are equal.
However, the right ascension is used if the other body is
Fig. 7.3. (a) Apparent motion of Mars during the 1995 opposition. (b) Relative positions of the Earth and Mars. The
projection of the Earth–Mars direction on the infinitely distant
celestial sphere results in (a)
Fig. 7.4. Planetary configurations. The angle α (Sun–object–
Earth) is the phase angle and ε (Sun–Earth–object) is the
elongation
not the Sun. Those points at which the apparent motion
of a planet turns toward the opposite direction are called
stationary points. Opposition occurs in the middle of the
retrograde loop.
Inferior planets (Mercury and Venus) are never in
opposition. The configuration occurring when either of
these planets is between the Earth and the Sun is called
inferior conjunction. The conjunction corresponding to
that of a superior planet is called upper conjunction or
superior conjunction. The maximum (eastern or western) elongation, i. e. the angular distance of the planet
from the Sun is 28◦ for Mercury and 47◦ for Venus.
Elongations are called eastern or western, depending on
which side of the Sun the planet is seen. The planet is
an “evening star” and sets after the Sun when it is in
eastern elongation; in western elongation the planet is
seen in the morning sky as a “morning star”.
The synodic period is the time interval between two
successive events (e. g. oppositions). The period which
we used in the previous chapters is the sidereal period, the true time of revolution around the Sun, unique