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TITLE:  Using Parallax to Determine Distances

AUTHOR:  Neil Priddy,  Seaside High School,  Seaside, OR

GRADE LEVEL/SUBJECT:  10-12, adaptable to lower grades 

OVERVIEW:  Parallax is not only used by astronomers to determine
the distance of many stars, and other heavenly objects, but is
also one of the ways humans use to determine the distance of
nearby objects.  This activity is designed to give students a
better understanding of what parallax is, and to gain some
experience in using it.  We would already have talked about
parallax in class, and seen some simple examples of it.

OBJECTIVES:  Students will be able to:
  1)  devise a method to calculate the distance of objects from
their data (you can show them how to do this if their math/geometry
skills are not up to it).
  2)	describe how our eyes use parallax to determine the distance of objects.
  3)	make estimates of distances using parallax.

RESOURCES/MATERIALS:  six candles on stools, one light, approx. 8
chairs, meter sticks, a dark room, pencil & paper
For the graph paper, I photocopy a piece of graph paper on acetate,
and cut it into long strips for them.

ACTIVITIES AND PROCEDURES:
  1)  In a dark room (I do it in the gym) set out about 6 candles on
stools.  They should not all be in a row, but fairly close, as below.

+--------------------------------------------------------+
|X                                           .            
|X                               c                       |
|X               c                        c              |
|                      c                               L |
|X          c                       c                    |
|X                                                       |
|X                                                       |
+--------------------------------------------------------+

X = chairs           c = candles        L = light

  2)  Place several (6-8) chairs across one side of the room, and a
light on the other side, I use the "I" in the "EXIT" sign on the other
side of the gym for this light.
  3)  Each student will sit in one chair, and hold a piece of overhead
acetate, with graph lines on it, at arms length  closing one eye.
They measure the distance between each of the candles, and the light,
and record them.
  4)  They move to another chair, and repeat 3).  They need to record
how far apart the chairs are, I measure the chair positions for them.
  5)  The students get into groups of two or three, and figure out how
to use their data to determine how far away each candle is.  They are
to assume the light is an infinite distance away, all lines to the light
are parallel.  Their answers should not be the same as the actual
distances, because the light is not an infinite distance away.
  6)  Compare class results.
QUESTIONS TO DISCUSS:  Did they realize they would have to measure the
lengths of their arms?  Did it matter which chairs they used, or how
long their arms are?  How does parallax help us determine how far away
things are when we look at them?
  7)  They each write up their analysis of their results, and class results.

TYING IT ALL TOGETHER:  The students seem to get more out of the
activity if they have to figure out how to do the calculations
themselves.  But in this class, we have already done some similar
calculations, so they may need some guidance if they haven't done
anything like it before.
Calculations will vary, and this can also be used to look at where
experimental error might come from it this project.
This ties in with the work we do next - locating images created by
divergent lenses. This project is also applicable to astronomy classes.

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