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A problem I couldn't solve -- Number of Earth rotations in a year...

  1. Feb 10, 2017 #1
    < Mentor Note -- thread moved to HH from the technical physics forums, so the Homework Help Template is filled out farther down the thread >

    The number of rotations of earth around its own axis in one year as measured by an observer from the sun.
     
    Last edited by a moderator: Feb 10, 2017
  2. jcsd
  3. Feb 10, 2017 #2

    jbriggs444

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    The first step is to clearly define the problem. Rotations are particularly tricky to nail down precisely. Do you mean:

    We draw a line from center of Sun to center of earth and ask "how many times does Earth's prime meridian pass through this line over the course of a solar year"?

    Would the answer to this question differ from "how many solar days are there in a solar year"?
     
  4. Feb 10, 2017 #3

    berkeman

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    Welcome to the PF. :smile:

    Is this question for schoolwork? If so, I can move your thread to the Homework Help forums for you.
     
  5. Feb 10, 2017 #4
    Yes this is for school work sorry I didn't know where to put it I am new to the forum
     
  6. Feb 10, 2017 #5
    Sir the answer to this is to be kept in earth days .....
     
  7. Feb 10, 2017 #6

    berkeman

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    No worries, I'll move it for you. Please copy/paste this Homework Help Template into your next reply, and fill it out based on the questions you have been asked so far (likd the definition of the exact problem. Thanks! :smile:

    1. The problem statement, all variables and given/known data


    2. Relevant equations


    3. The attempt at a solution
     
  8. Feb 10, 2017 #7
    1. Problem statement, all variables and given / known data
    The number of rotations of earth around its own axis in one year as measured by an observer from the sun.
    Options ,1. 365 ,2. 1 ,3. 364 ,4. 366

    2. Relevant equations
    Don't know in what way I have to approach

    3.The attempt at a solution

    I know that sun rotates round it's axis at 24 days/ rotation
    So if the observer is watching earth from sun he can see earth only for 12 days for the other half of the rotation he will be on the opposite side of earth so I think we he doubles the value that he gets for the first 12 days for 24 days....
     
  9. Feb 10, 2017 #8

    berkeman

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    Perfect, thanks! :smile:

    So imagine that you are sitting at the north pole of the Sun. You can see the Earth all year long. Does that change your answer?
     
  10. Feb 10, 2017 #9
    Yes it does ....but looking at the options I am not able to pick the correct one
     
  11. Feb 10, 2017 #10

    berkeman

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    The Earth has spun 365 times in that year with respect to the sun, but what else has happened?
     
  12. Feb 10, 2017 #11
    What if it's a leap year ? They haven't mentioned about what kind of year it is....so...are we supposed to ignore the fact of the year being a leap year?
     
  13. Feb 10, 2017 #12

    berkeman

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    That's a good point, but it seems like they are ignoring that5 subtlety.

    Hint -- would it make a difference it the Earth were spinning the opposite way? Which way does the Earth spin with respect to how it moves around the Sun?
     
  14. Feb 10, 2017 #13
    Earth rotates west to east at its axis and if we observe from sun's and earth's North Pole the Earth would appear to revolve in a counterclockwise direction about the Sun
     
  15. Feb 10, 2017 #14

    jbriggs444

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    What do you think that it means for the earth to rotate once about its axis as measured by an observer from the sun? For instance, does it mean "how many times the Sun observer can look down into noontime London?

    Or, in other words, exactly what measurement is the observer on the sun performing?
     
  16. Feb 10, 2017 #15
    I
    I think he has fixed a point on earth like you mentioned about London...
     
  17. Feb 10, 2017 #16

    jbriggs444

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    If an observer on the sun can look down on London, would an observer in London be able to look up at the sun?

    Does that lead you toward an answer to the question?

    Edit: Ignoring cloud cover -- which, for London, is a pretty big thing to ignore.
     
  18. Feb 10, 2017 #17

    haruspex

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    Leap years are an invention so that a new year always starts at midnight. In reality, using 24 hour days, the number of days in a year is about 365.24... But the options are whole numbers, so the fraction of a day is being ignored.
     
  19. Feb 10, 2017 #18

    CWatters

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    Perhaps consider how many rotations of the earth would be seen from the sun (over the course of a year) if the earth didn't rotate on its axis at all.

    Is it 0, 1 or -1?
     
  20. Feb 10, 2017 #19
    Well if earth didn't rotate the answer will be 0 right ?
     
  21. Feb 10, 2017 #20

    berkeman

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    Not as seen from the Sun...
     
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