# Astronomy - binary magnitude conceptual question

1. Sep 12, 2011

### accountkiller

astronomy - conceptual, trigonometric parallax

1. The problem statement, all variables and given/known data
To determine a star's trigonometric parallax, we need at least 3 measurements of its position relative to much more distanct objects. Explain why this is true. (What else could change its position on the sky?)

2. Relevant equations
Note - this is for galactic astronomy.

3. The attempt at a solution
Well, I understand what triangulation is.. take a measurement of the angle Earth at one end of its elliptical orbit and then six months alter at the other end, so you have the distance between, and with angles can figure out the distance to the star. But this question is asking relative to other distant objects, not Earth... so I'm not sure how to proceed thinking about it. What could change its position in the sky... light obscuration from dust, perhaps?

I'd appreciate any guidance, thanks!

Last edited: Sep 12, 2011
2. Sep 12, 2011

### jncarter

Try drawing a picture for each situation. Draw one where you take measurements relative to just one star, just two stars and then just three stars. If you already know the position of these stars, then you only need to do this once to get the position of the fourth.

What could change it's position in the sky?
Remember that everything in this problem is moving, and each object may be moving in different ways relative to each other.

3. Sep 12, 2011

### accountkiller

Got it. Thank you :)