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: http://zebu.uoregon.edu/disted/ph121/parallax.html
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Figure 1.3 Schematic Representation of stellar Parallax. Distant stars act as a fixed reference coordinate system. Nearby stars, when observed 6 months apart, will show a small movement with respect to the background of fixed stars. At position 1, the nearby star would be viewed against a background that contained star B while 6 months later, at position 2, the nearby star would be viewed against a background that contained star A. |
This angle would have a size of 1 arc second (1/3600 of a degree) for a star that had a distance of 1 parsec from the earth. 1 parsec is equal to 3.26 light years.
The nearest star to us has a distance of 4.1 light years so that all parallactic angles are less than 1 arc second for all stars. This means its impossible to measure this effect with the naked eye.
The observational problem in measuring accurate stellar distances is then that atmospheric motions/smearing make positional measurements of stars, at levels of accuracy less than 1 arc second difficult.
Therefore, many measurements of the star are needed to record an accurate parallax. In practice, one usually requires 20 years of measurements of a single star.
If we measure the parallactic angle, then we can directly know the distance to the star. The distance in parsecs is simply
where p is the angle measured in arcseconds. Thus a star that has p = 0.1 would have distance of 1/p = 10 parsecs = 32.6 light years.