Is parallax a "technique to measure the star distances" or a "tiny shift in star's position"?

  • I am not completely sure whether the Parallax is a "technique to measure the star distances" or a "tiny shift in star's position"?

    The a book says two points about Parallax:

    • Astronomers have developed a variety of clever techniques for measuring the vast distances to stars, one is called Parallax.

    • Astronomers can measure a star's position once and then again 6 months later and calculate the apparent change in position. This tiny shift in stars position is called its parallax.

    Apart from the book source, defines Parallax as the way an object’s position or direction seems to change depending on viewing angle.

    You are presenting this as if it's one or the other, but not both. The answer is that both are true. Annual parallax is a tiny shift that varies inversely with distance. This means that the observed shift can be used as a way to measure distance.

    Parallax is something you can see on earth. Eg, if you're traveling by vehicle and look out the window, the nearby trees appear to pass very quickly, the houses further away appear to pass slowly, and the distant mountains more slowly still. Parallax scrolling is used in video games to give the illusion of distance: I think the idea in astronomy is that as the earth moves, the things that appear most stationary must be farthest away, like the distant mountains on Earth.

    BTW, welcome to Astronomy.SE!

  • ProfRob

    ProfRob Correct answer

    4 years ago

    It is both - a small shift of the position of a star on the sky as we see it, and a means of estimating the distance to the star.

    The apparent position (with respect to very distant objects like quasars) changes because our viewing point changes as the Earth moves around the Sun in its orbit. The amount by which the position changes is inversely proportional to is distance.

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