Expert: Philip Stahl - 8/10/2009

Question
I am looking for a simple explanation of calculating the distance of a star using stellar parallax. Most of the web sites I have found, even Wikipedia, seem to be college level explanations. I need to explain it to a middle school student. Can you either give me a simple explanation or a link to a site that may help. Thank you.

Answer
Hello,

Unfortunately, there is no 'simple means' I am aware of to actually *calculate* ( as opposed to simply showing, demonstrating) stellar parallax. All methods involve geometry if not trigonometry- topics typically left for HS math students.

When I developed the CXC Astronomy syllabus for the Caribbean in 1977-78, the topic of stellar parallax came up. It was decided it was apropos for students at the 4th-5th Form (equivalent to 10th- 11th grade) students but *no younger*. Piaget's levels of concreteness and capacity for abstraction entered into this.

Bottom line: students below that level are simply not ready for computational stellar parallax unless gifted. If gifted, then they will be able to quickly learn the math needed to compute it.

Sorry I cannot be of more assistance.

Post script: if it is only a generic explanation you need, minus computation, simply have the student use his or her finger (say index finger) extended at arm's length. Have the student close the left then right eye while looking at her finger against a fixed background. The finger will appear to move slightly. This demonstrates the basic parallax principle. (Each eye position represents an image taken of a distant "star" which is then seen to appear to reposition against the background)

Computation of it for a specific star is another thing entirely!