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# Parallax method - physics

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Can someone explain this please...I am not sure exactly how it works.
(alternatively posting here would be good
http://www.thestudentroom.co.uk/t114627.html
)
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Basic knowledge:

The earth orbits the sun.
The earth is "always" 1 AU from the sun (a known distance.
The orbit therefore has a radius of 1AU
1 arc second is 1/60 x 1/60 x 1degree

Draw this on a sheet of A4 landscape:

Sun at one end of the paper
A star is located far away from the sun - other end of paper
Draw a pencil line from the star to the sun, label this D
Draw the earth twice, at opposite sides of the sun, so if a line was drawn between the two earths it would be perpendicular to line D
Draw a dashed line from each earth to the star, label these 1 and 2 respectively
Entend the two dashed lines slightly beyond the star and draw stars at the end of these. Label 1 and 2 again
Label the angle between D and line 1, "A"

Ok, so the orbital period of the erath round the sun is a year
This means that the two earths drawn in are 6 months apart
Therefore, are perception of where this star is in the sky changes over these sixth months.
Using trigonometry it can be said that angle A is equal to 1AU divided by D
One parsec is defined by the distance D given when A is equal to 1 arc second.
We cannot measure less than 0.01 arc second, so the upper limit of measurement using star parralax is 100pc (D=1/0.01)
To find a distance in parsec, we say D=1/A
So parralax is seen to be finding the distance to a known object using angles.
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Thank you for your reply. My teacher mentioned approximating tan d with sin d...I am not sure why, or if at all this is relevant.

Also in some exam papers (edexcel) I have seen that parallax method seems to have something to do with the background stars (as light is shown from distant stars in the diagrams in their mark schemes). The method that you suggested (which is the method I thought was right) does not really take background stars into account.
Maybe there is more than one way for the parallax method to work?
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i dont understand all the arc second stuff,
How exactly do you work out the value of the angle, and wich angle are u working out exactly?
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(Original post by perkyDani)
i dont understand all the arc second stuff,
How exactly do you work out the value of the angle, and wich angle are u working out exactly?
isn't arcsecond 1/3600 of a degree? and isn't arc minute 1/60 of a degree?

the parallex method is a way of finding a distance of a distance star; the distance from teh sun to teh earth is r, the angle between the sun, earth and the distance star is theta. The idea of the parallex method is to record theta at time t=0, and time t=6 months. Take averages of the theta, and then tan theta = r/D, where D is the distance of the star from the Sun.

well, i think this is what it is

pk
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(Original post by Phil23)
isn't arcsecond 1/3600 of a degree? and isn't arc minute 1/60 of a degree?

the parallex method is a way of finding a distance of a distance star; the distance from teh sun to teh earth is r, the angle between the sun, earth and the distance star is theta. The idea of the parallex method is to record theta at time t=0, and time t=6 months. Take averages of the theta, and then tan theta = r/D, where D is the distance of the star from the Sun.

well, i think this is what it is

pk
yup... that would all appear to be correct I think. Certainly the angle stuff is right
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look at the letters in this sentence and put your thumb between your eyes and the screen. close one eye then the other. The thumb will appear to move.

Basically, yhe letters and words are distant stars and galaxies, and the tumb is the nearby stay. Your eyes are the Earth's viewpoint in summer and winter.. this makes the bridge of your nose the sun!

Just a nice little intorduction to parallax method.
Updated: May 28, 2005
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