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Homework Help: Would we see the planets move across the sky if Earth stops spinning

  1. Sep 16, 2014 #1
    1. The problem statement, all variables and given/known data

    If the Earth did not spin on its axis, would the other planets still appear to move across the sky? Justify your answer.

    2. Relevant equations


    3. The attempt at a solution

    If the Earth continued to rotate around the sun but NOT on it's own axis then wouldn't we see that there is roughly six months of light and six months of darkness and thus only be able to see the stars for half the year? That being said, since the Earth is still moving in respect to the sun, wouldn't we see the stars/planets (during the six months of darkness) shifting across the sky but not to completion and a lot slower (so not as fast as we do in a 24 hour period but still to some degree)?
  2. jcsd
  3. Sep 16, 2014 #2
    If we assume that our orbit around the sun is a circle of fixed radius (1 AU), then the relative position of a planet would depend on 1) its distance from the sun and 2) the space (temporarily speaking) of our two measurements. Neptune, for instance, is around 30 AU away from our sun and completes an orbit in approx. 165 years. An easy math can help in figuring out the length of the arc that this planet would have moved in a period, say, of 6 months. The same applies for the rest of the bodies of our solar system, and thus closer planets will experiment wider displacements. Remember that this technique (Parallax) is used to measure distance for stars in our own galaxy.
  4. Sep 16, 2014 #3
    Thanks so much for your reply. I understand what you are saying; but, conceptually speaking then would my answer be correct? or I guess, sufficiently correct?
  5. Sep 16, 2014 #4
    If the Earth stops spinning then we would see a 'fixed' starry picture above our heads (at night, our long six-month night), for the stars are pinned far enough not to seem they're moving. Regarding to the planets, they would still be moving across the ecliptic line, because they keep orbiting on the plane of our galaxy. I suppose you're right when you say we would see them as we see now every night, but quite longer. That is, we could see Mercury completing several orbits during our night, but Jupiter (orbit approx. 12 y), for example, just moving some arc. Hope I made it a little bit clear :D
  6. Sep 16, 2014 #5
    Yes, totally clear! thanks! :)
  7. Sep 16, 2014 #6
    Good, have a good day! :)
  8. Sep 16, 2014 #7


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    You can sometimes see Venus in the daytime so I would ignore that issue.
  9. Sep 16, 2014 #8


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    I took me a while to figure out what you meant by "not to completion." I assume you mean the planets won't necessarily rise or set in this scenario.

    There are three different types of motion involved in explaining what we observe in the night sky. There's the rotational motion of the Earth (about its axis), its orbital motion around the Sun, and (the one you seem to have neglected) the orbital motion of the other planets around the Sun.

    As you correctly realized, the planets and stars will no longer rise and set because that apparent motion is due to the rotation of the Earth. What effect does the orbital motion of the other planets have on what we see? What effect does the Earth's orbital motion have on what we see? Will you still observe retrograde motion of Mars?
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