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Physics Lesson 22.2.6 - Observation of Planetary Motion from Earth
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Welcome to our Physics lesson on Observation of Planetary Motion from Earth, this is the sixth lesson of our suite of physics lessons covering the topic of Sun and Planetary Motion, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Observation of Planetary Motion from Earth
Since all planetary orbits are nearly at the same plane when observed from Earth we can assess that the planets are aligned in a unique arc in space. The plane containing this arc is known as ecliptic plane and it divides the sky in two parts. More specifically, the ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the sun. If we consider the consider only the imaginary line that connects the centre of Sun, Earth and all the other planets of they are all aligned, it is called ecliptics. Obviously, planets are in ceaseless motion even when considered from ecliptic plane viewpoint. The only restriction is that they are seen only at certain parts of the day, when they are found inside the maximum observation angle θ from the Sun as illustrated in the figure below.
Since Mercury and Venus are the only planets inside the Earth's orbit (they are closer to the Sun and their orbit is smaller than the Earth's orbit), they are called inner planets. All the other planets, from Mars to Neptune are called outer planets since their orbits include that of Earth.
The observation angle θ for Mercury is 23° and for Venus it is 47°. Let's clarify what this mean through an example.
Example 2
How many hours a day and at what point can we see Mercury and Venus in the sky?
Solution 2
Given that a complete angle is 360°, this means that Mercury can be observed only 23/360 of the day in the sky while Venus can be observed only 47/360 of the day. When these parts are expressed in hours, we obtain
and
Hence, since this situation is repeated in both sides of the line that connects Earth, the other planet and the Sun, we can see Mercury in the sky twice a day for a maximum duration of about 1.5 h each time (one before sunrise and the other after sunset). For Venus, this duration is a little more than 3 hours each time. We can see Venus (the "morning star") twice a day for 3.13 hours before sunrise and after sunset.
The outer planets can be observed in the sky as well. They can be seen in angles θ2 varying from 0 to 180° on the other side of (away from) the Sun. Obviously, they are observable only during the night as they are in the opposite direction to the Sun. This is illustrated in the image below.
It is evident that the Sun lies on ecliptics and it moves only on it. This motion is the opposite of that of Earth in respect to the Sun (action-reaction principle). Therefore, the duration of this process is 365.2422 days too. Hence, since we observe the Sun moving from East to West, the Sun motion in ecliptics occurs from West to East, as we will see later.
The days of week (in Latin language) are named after the seven celestial bodies known in ancient times: Monday comes from Moon, Tuesday from Mars, Wednesday from Mercury, Thursday from Jupiter, Friday from Venus, Saturday from Saturn and Sunday from Sun.
You have reached the end of Physics lesson 22.2.6 Observation of Planetary Motion from Earth. There are 10 lessons in this physics tutorial covering Sun and Planetary Motion, you can access all the lessons from this tutorial below.
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