How different is the motion of planets against the motion of stars?

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The question asks: How different is the motion of planets against the motion of stars?

Option provided: 'The planets' orbit sometimes loop around, which is called the retrograde motion.' This statement is pointing to the phenomenon where planets appear to reverse direction temporarily against the background of distant stars when viewed from Earth.

To analyze, first recall that stars appear fixed in the sky over short timescales, because they are very far away and move very slowly relative to Earth. In contrast, planets are much closer and orbit the Sun, so their apparent path against the starry background changes more noticeably as both Earth and the planets move.

Branching into why retrograde motion happens: as Earth orbits the Sun faster or slower than a given planet, there are periods where the line of sight to that planet causes it to appear to move backward along its path relative to the stars. This apparent loop or backward motion is a projection effect caused by our vantage point on a rotating and orbiting Earth, not an actual backward travel by the planet through space.

Understanding the broader comparison, stars are generally fixed references over human timescales, while planets show more intricate motion due to orbital dynamics. The key difference is that planets exhibit noticeable changes in direction (including apparent retrograde motion) because they are in orbital motion around the Sun, whereas stars maintain a nearly fixed position in the sky.

If we consider other possible statements that could appear as options, they might include ideas like: (a) planets and stars both move in straight lines at constant speeds relative to each other (which is false, since planetary motion is curved and our vantage changes), (b) retrograde motion is unique to planets and does not occur with stars (true in the sense that stars don’t exhibit this apparent loop for observers on Earth), or (c) retrograde motion is caused by the Sun moving around the galaxy (not the correct primary cause for apparent planetary retrograde).

In sum, the distinguishing concept is that planets show apparent retrograde motion against the fixed star field due to their orbital dynamics and our moving viewpoint, whereas stars remain relatively fixed in the celestial sphere over comparable timescales.

COS SPR 25 class 2 Jan 16 Ch3: The Science of Astronomy

How different is the motion of planets against the motion of stars?

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