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Stargazing Trigonometric parallax

  1. Feb 26, 2017 #1
    Hi community, trigparallax.2.jpg

    I get the concept of trig parallax and the apparent shift of nearby stars when viewed against a distant background, by viewing the star in say summer and then winter and it appears to move against the much further away distant background.

    I get what the angle p represents from the diagram above.
    To find the angle p, would the astronomer (in June say) star the telescope point along a line parallel with the sun star axis and then measure how far it has to rotate to point at the star, by alternate angles they have then moved through an angle p which is the same as the angle p labelled in the diagram.
    I was then wondering that if all you had access to was the picture from Jan and July and you could measure the apparent linear movement of the star over that six month period and that you knew what the angular separation of two of the well known stars were in the distant constellation then you could find the angle p that way, or is that what the scientists do anyway?
    thanks as always,
  2. jcsd
  3. Feb 27, 2017 #2


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    I don't understand that sentence, but you don't point telescopes at the sun. Typically you measure the position relative to background stars: You remember the January position, and in June you measure the angle between that and the new position, a telescope also gives you the angular scale of its pictures. With two pictures and the known angular scale you can determine the angle p. If you also know when your images were taken (not necessarily at the two points in the picture...) and where in the sky the object is, you can determine its distance.

    This is a simplified description. Stars move relative to the Sun. With just two images, you can't distinguish parallax from motion, you need at least three observations, better four or more. If you want to achieve a better precision, finding background stars gets more challenging. As an example, the Gaia telescopes measures the parallax of all stars in the Milky Way. It needs background sources outside the galaxy. Background sources can move as well. And so on. For really precise measurements you have to account for many different effects influencing the measurement.
  4. Feb 27, 2017 #3


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    Try looking at my Post #9 in this thread. It shows some actual data measured by the Hipparcos satellite. You can see the superposition of parallax and proper motion. With enough measurements, you can deduce bot the parallax and the proper motion.
  5. Feb 27, 2017 #4
    Thanks so could the distant 'fixed' background be stars in another galaxy or indeed other galaxies for the fixed background? I'd imagine then that for different measurements made over a 12 month period the shift of these very distant entities would seem small compared to the star we're measuring the parallax angle of?
  6. Feb 27, 2017 #5


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    I think (not sure) that they use distant quasars as the stationary reference points. These are so distant that they effectively don't move at all, and are point sources so their positions can be accurately determined.
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