## Schrodinger 3 years ago The star Wold 359 has a parallax angle of 0.419 arcseconds. Describe how this parallax angle is measured.

1. Schrodinger

*Wolf.

2. Vandreigan

Well, the easiest way to describe it is that the star is photographed throughout the year. The photographs are then compared to find the two that show the greatest change in position with respect to the background stars (the furthest background stars will not have appeared to move at all). From this, you can find an angle. Half of that angle is the parallax angle.

3. Schrodinger

Thanks! It's fine if I don't get it but could you elaborate a little just to see what I can or can't get?

4. Vandreigan

What do you mean, exactly?

5. Schrodinger

I dunno, I assumed by "the easiest way" you meant there was probably a more technical or mathematical way to put it in terms. I've always sucked with understanding parallax so I just kind of see what I can do to get it.

6. Vandreigan

Well, the exact definition is more along the lines of "Using imagery of the star from Earth, while the triangle formed between the star, the sun, and the earth, at two different times of the year (6 months apart), the angular displacement of the star with respect to the background stars can be easily measured. Half of this angular displacement is the parallax angle, which may be used to calculate the distance of the star from the sun"

7. Vandreigan

oops, while the triangle formed by the star, sun and earth is a right triangle, from two different times of the year.

8. Vandreigan

Basically, we use the two pictures that give us the greatest angular displacement of the star with respect to more distant stars (which will appear not to move at all, because they are so far away). These correspond to being at the vertex of the two right triangles.

9. Vandreigan

From this, it's a rather simple matter to measure that angular displacement. Divide it by two, and you have the parallax angle!

10. Vandreigan

I suppose I should ask what level of school is this for? I can make things a little more clear if needed.

11. Schrodinger

High School, senior year Physics. Thanks so much! Right now i'm just reading everything you said, lol.

12. Vandreigan

There's a picture of how to find parallax on wikipedia, which may make things a little more clear. http://en.wikipedia.org/wiki/File:Stellarparallax2.svg

13. Vandreigan

Since it's high school physics, you'll assume the earth's orbit is perfectly circular. So just take one picture. 6 months later take another picture. Measure the angular displacement with respect to the background stars. Divide that by two. Done!

14. Schrodinger

Since the Earth has an elliptical orbit, is our ability to measure the distance (but the parallax angle) of a star affected by where the star lies relative to our orbit? Like, if-oh, lol, just read

15. Vandreigan

It is, but the effect is small. Usually we just do the measurements at certain times to try to squeeze a little more out of this method. It has a limit on it, as you can see by the parallax of the stars you're given. They are in arcseconds, which is an EXTREMELY small angle.

16. Vandreigan

If the star is too far away, it doesn't appear to move at all. We just can't resolve or measure the parallax angles of some stars (most of the stars, in fact).