# Physics and Astronomy Labs/Parallax Geometry used in the parallax lab. The circles represent the vertical rods from a "top" down perspective. Line-of-sight is used to align two pairs of rods, separated by the baseline, b.
Movie showing the ring stands, here used to see if the rods are truly vertical (they are not). 

## Purpose and design

The purpose of this lab is to use parallax to measure the length of a hallway. Four ring stands are required. These are depicted in the movie shown below, although the movie does not depict the use of these stands to calculate parallax. Instead the movie documents the fact that the distance between the stands must be measured at the top, where the eye is used to line the stands up to a distant object. The figure to the right allows one to derive:

${\frac {b}{x-a}}={\frac {c-b}{a}}$ The lab consists of measuring a, b, c, and solving this equation for x.

## Results

If a, b, and c are roughly 2 meters, and the distance, x, is roughly 30 meters, we were able to get the error down to about one percent.

## Feb 2016 effort

a=306cm

b=232cm

c=258cm

After algebra: x = 3036 cm

Compare this with 137 ft using angular size.

## Footnotes and references

1. Movie taken by 13hartc (discusscontribs) 18:34, 11 February 2015 (UTC)