Aspects and Phases of the Superior Planets
The aspects and phases of the superior planets differ from those
of the inferior planets because of geometry: their orbits are outside that of
the Earth. These aspects and phases are indicated in the adjacent diagram.
Conjunction and Quadrature
When a superior planet is at quadrature, it is on our celestial
meridian at sunrise or sunset, and when it is at opposition it is high in the sky
near midnight. Unlike for the inferior planets, there is only one kind of conjunction
for superior planets.
Here is an
animation illustrating aspects and phases of the superior planets.
The situation depicted in the top right figure and in the preceding one for inferior planets
is
static, as viewed from the perspective of a
stationary Earth. In reality, the Earth is also moving. A realistic simulation may be
found in this
animation. This animation also introduces the difference between the sidereal and synodic
period for a planet, which we explain in the following section.
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Sidereal and Synodic Periods
|
| Planet |
Sidereal |
Synodic
|
| Mercury |
87.97 d |
115.9 d
|
| Venus |
224.68 d |
583.9 d
|
| Earth |
365.26 d |
-
|
| Mars |
686.95 d |
779.9 d
|
| Jupiter |
11.867 y |
398.9 d
|
| Saturn |
29.461 y |
378.1 d
|
| Uranus |
84.01 y |
369.7 d
|
| Neptune |
164.8 y |
367.5 d
|
| Pluto |
247.7 y |
366.7 d |
|
|
|
|
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Synodic and Sidereal Periods
The true period for a planet to go once around its orbit is called the
sidereal period.
However, we observe the motion of a planet from the Earth, which is itself in motion around the
Sun. We may define a second kind of period that takes the motion of the Earth into account
by asking how long it takes a planet to move on the celestial sphere and return to
the same arrangement relative to the Sun (for example, two successive conjunctions of the same
kind). This is called the synodic period of the planet.
For an inferior planet, the relationship
between the synodic period S, the sidereal period P, and the sidereal period
of the Earth PE is
1/S = 1/P - 1/PE
(inferior)
and the corresponding relationship for a superior planet is
1/S = 1/PE
- 1/P
(superior)
Because of the mathematical form of these formulas, the synodic period is longest for those
planets whose period is closest to that of Earth (Venus and Mars). For example,
the sidereal period of Venus is 224.7 days but its synodic period is 583.9 days. That is, it
takes only about 225 days for Venus to go once around its orbit but it takes about 584 days
for Venus to go from one superior conjunction to the next, as viewed from Earth.
The sidereal and synodic periods for the planets in the Solar System are summarized in
the above right table.
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Turning Saturn into an Inferior Planet
The adjacent figure shows a view of the superior planet Saturn that would be impossible from the Earth. It is
an image of Saturn taken by the Voyager 1 spacecraft after it had passed Saturn and looked back on it from a
distance of about 5 million km.
Because the perspective is now from outside the orbit of Saturn,
we can see the dark back side of Saturn and the planet exhibits a crescent phase.
That is, with respect to the spacecraft
Saturn is now an inferior planet and thus can exhibit a full set of phases.
From Earth we can never see more than a tiny part of the dark side
because Saturn is a superior planet and Earth is always far inside its orbit.
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