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How many arcseconds does Sun travel through sky?

  1. Dec 10, 2009 #1
    Hi Everyone,

    I have a simple question:

    How many arcseconds does the sun "travel" through the sky in one Tropical Year?

    Mark
     
  2. jcsd
  3. Dec 10, 2009 #2

    D H

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    Mark, you are trying to resurrect a closed thread. It was closed for a reason.

    From the perspective of an Earth-centered inertial frame, the answer is a tiny bit (about 50 arcseconds) less than 360 degrees.
     
  4. Dec 10, 2009 #3
    Hi D H,

    I'm not resurrecting that thread. I just hope we can come to a consensus on the number.

    I mean all the travel of the sun in a year. So it's got to be a lot.

    Mark
     
  5. Dec 10, 2009 #4

    D H

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    Mark, you are still confusing the Earth's daily rotation with the Earth's orbit. This is the sole source of your confusion. Imagine what things look like from a reference frame that is not rotating with respect to the remote stars and with origin at the solar system barycenter. The International Celestial Reference Frame, for example. The Earth-Moon barycenter follows a nearly elliptical path in this frame. The time it takes for the Earth-Moon system to complete one orbit in this frame is one sidereal year.

    Now flip your point of view to an Earth-centered frame, but keep the same axes. This is an Earth-centered inertial frame (a bit of a misnomer; this is an accelerating but non-rotating frame). From this perspective, it is the Sun that completes one orbit about the Earth in one sidereal year.

    What about a tropical year? The Earth's rotation axis is tilted with respect to the Earth's orbital angular momentum vector. Suppose the only bodies in the universe were the Sun and Earth (i.e., no Moon or Jupiter to confuse things) and suppose the Earth's rotation axis was unchanging. With these simplifying assumptions, the line connecting the Earth and the Sun would lie completely on the ecliptic plane twice a sidereal year -- and those equinoctal points would be fixed points in the solar system barycentric frame. The between successive vernal equinoxes, the tropical year, would be one sidereal year.

    The Earth's rotational axis is not constant; it instead undergoes a slow precession. Because of this precession, a tropical year is a bit shorter (~20.5 minutes) than a sidereal year. Given that a full orbit (one sidereal year) is 360 degrees by definition, 20.5 minutes corresponds to about 50 arcseconds.
     
  6. Dec 10, 2009 #5
    Hi DH,

    I think you misunderstand my intention here. I simply want to find out how many arcseconds the Sun travels through the sky in one year.

    You are the one continuing the conversation from the closed thread.

    Mark
     
  7. Dec 10, 2009 #6

    D H

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    What year (tropical, sidereal, anomalistic), and what reference frame (Earth centered inertial or Earth-centered, Earth-fixed)?
     
  8. Dec 10, 2009 #7
    Hi DH,

    Thank you! I'm sure you're a nice person : )

    So, by tropical year I mean the amount of arcseconds the sun travels from the spot over the vernal equinox to that same spot over the vernal equinox the next year. Of course it doesn't travel, so, apparent travel.

    Mark : )
     
  9. Dec 10, 2009 #8

    D H

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    Dang! Rotation with respect to what frame of reference? Do you understand what this question means?
     
  10. Dec 10, 2009 #9
    Ummm. Assume we are standing on the earth and looking up....all year.
     
  11. Dec 10, 2009 #10

    D H

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    That is not a very useful frame of reference. It is just going to lead you to confusion -- and that is exactly what is going on. Forget that frame. Read post #4.
     
  12. Dec 10, 2009 #11
    Let's approximate...

    365 days * (3,600sec/day) = 13,140,000


    A very boring analysis
     
  13. Dec 10, 2009 #12
    Thank you for responding to my question!

    3,600 arcseconds per day?

    I think it's 1296000 arcseconds per day.

    So then it would be, approximately, 1296000 x 365?

    Mark
     
  14. Dec 10, 2009 #13

    ideasrule

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    Yes, the Sun would travel approximately 1296000 x 365 arcseconds a year if you stood on Earth looking up. (It doesn't actually, but let's clear up one issue at a time.) This does NOT mean the Sun moves 1296000 x 365 arcseconds a year relative to the stars.

    Do you understand the difference? Suppose I put you on a merry-go-round and start spinning it at one revolution per second. To you, it would appear that the Sun is moving 360 degrees per second. This is analogous to the 1296000 x 365 arcseconds the Sun moves a year. However, the Sun doesn't move 360 degrees per second relative to the fixed stars; it moves 360 degrees a year, whether you're on a merry-go-round or on the ground.
     
  15. Dec 10, 2009 #14
    You are correct. That would be a lot of difference.

    I assume you mean 360 degrees per day from the reference frame of standing on one spot of the Earth. Every day, from my reference frame, the sun circles me 360 degrees.

    So, can we say the sun travels 473040000 arcseconds every year from our reference frame? Standing in one spot on the solid earth? I think this is the right answer. We see the sun once, twice, 365 times and back to the same spot, making 473040000 arcseconds of angle.

    Mark
     
  16. Dec 10, 2009 #15
    This is really all that I wanted to come to in this thread: how many arcseconds the sun travels in one year from the reference point of a person standing on one spot of the solid Earth.

    Tropical Year: 473040000 arcseconds

    Tropical Day: 1296000 arcseconds

    I want to be sure about this, so please post if you think it is incorrect.

    Mark
     
  17. Dec 10, 2009 #16

    Vanadium 50

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    I'm afraid you are. Thread closed.
     
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